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Title: Natural & Artificial Sewage Treatment
Author: Jones, Alfred S., Roechling, H. Alfred
Language: English
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NATURAL & ARTIFICIAL
SEWAGE TREATMENT
BY
LIEUT.-COL. ALFRED S. JONES, V.C.
ASSOC. M. INST. C.E., FELLOW AND MEMBER OF COUNCIL
OF THE SANITARY INSTITUTE, ETC.
AND
H. ALFRED ROECHLING
M. INST. C.E., F.G.S., FELLOW OF THE SANITARY INSTITUTE, ETC.
[Illustration: Printer's Logo]
London
E. & F. N. SPON, LTD., 125 STRAND
New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1902
PREFACE.
The Authors, some time ago, read before different Societies of
professional men, Papers[1] dealing with the Natural and Artificial
Purification of Sewage, and as these were favourably received, the
thought occurred to them that the time might be opportune for making
the information there given available for a wider public.
As, however, a mere republication of the Papers would have been
against the rules of the Societies concerned, the Authors decided to
re-write entirely the subject matter, and to bring it up to date, so
that the present publication is not a mere repetition of their old
Papers clothed in a new garb, but an entirely fresh publication, right
up to date.
The Authors hope that they have given the information in such a form
as to be readily available for District Councillors, Sanitarians, and
all interested in this complicated subject.
When considering natural and artificial sewage treatment, it ought to
be borne in mind that in the natural treatment we have to deal with
one treatment only, and that, in order to bring the results obtained
from artificial processes up to the same standard, the artificial
treatment ought to be supplemented by a treatment for the removal of
nitrates from the effluent, and another for the removal of pathogenic
micro-organisms, which means one treatment in natural, as against
three separate treatments in artificial purification.
In addition to this it must be understood that, owing to the great
losses by evaporation and by growing plants, which are continually at
work on sewage farms, especially during the summer months, when, as a
rule, the flow of water in the brook that takes the effluent is
smallest, the quantity of the effluent from the natural treatment is
probably only from one-half to one-third that resulting from the
artificial treatment, which is a point of very great importance.
If it can be proved to them that Nature is not sure and true enough in
its methods, the Authors are prepared to assist it with methods and
forthcoming, they adhere—in preference to groping in the dark—to
Nature’s own methods, knowing from experience, that when allowed full
scope and fair treatment, it is most sure in all its ways. That will
not prevent them, however, from giving in the future, as they have
done in the past, the question of sewage treatment in all its aspects
their most careful consideration.
ALFRED S. JONES.
H. ALFRED ROECHLING.
LONDON: _September 15, 1902_.
[1] ‘Sewage Treatment: Science with Practice.’ By Colonel A.
S. Jones, V.C., C.E. Read at the International Engineering
Congress at Glasgow, 1901. And ‘The Sewage Question during
the Last Century.’ Read by H. Alfred Roechling, M. Inst.
C.E., F.G.S., F.S.I., etc., on December 2, 1901, before the
Society of Engineers, and awarded the Gold Medal of the
Society.
CONTENTS.
PART I. BY LIEUT.-COLONEL ALFRED S. JONES, V.C.
PAGE
INTRODUCTORY 1
THE CHEMIST DIBDIN DISCARDS CHEMICAL PRECIPITATION
IN FAVOUR OF M. PASTEUR’S AEROBIC ORGANISMS 3
THE CLEANLY AND THE DIRTY (SEPTIC) PROCESSES FOR
SLUDGE REMOVAL 5
THE BEST POSSIBLE MEDIUM FOR AEROBIC ORGANISMS TO
WORK IN 5
TABULAR STATEMENT DERIVED FROM THE LEEDS OFFICIAL
REPORT OF EXPERIMENTS 7
THE WREXHAM SEWAGE FARM 8
THE CAMP FARM, ALDERSHOT, TO WHICH SIX OTHER AREAS
OF SEWAGE WORKS HAVE BEEN ADDED FROM JULY 28, 1902 9
EVIDENCE AND REPORTS OF LORD IDDESLEIGH’S ROYAL
COMMISSION, 1898-1902 15
AUTOMATIC APPLIANCES FOR SEWAGE AND EFFLUENT DISCHARGE 17
SLUDGE TREATMENT 19
CROPPING A SEWAGE FARM 21
IMPORTANCE OF HAVING TIDY CONTOUR CARRIERS ACCURATELY
LEVELLED 23
SUMMARY OF THE EXPERIENCE OF A LIFETIME 23
ANTICIPATION OF A COMING REACTION AGAINST OVER-RIDDEN
“FADS” AND TOO MUCH PRESSURE IN SANITATION 25
PART II. BY H. ALFRED ROECHLING.
I. INTRODUCTORY REMARKS 28
II. THE SEWAGE QUESTION DURING THE LAST CENTURY:
A SHORT RETROSPECT 29
III. THE SUBSOIL:
1. Mechanical structure of soil 41
2. Permeability of soil 42
3. Water capacity of soil 42
4. Water-retentive power of soil 43
5. Capillary movements of water in soil 44
6. Temperature of soil 45
7. Subsoil air 47
8. Movements of water in soil 47
9. Micro-organic life in soil 50
10. Absorbing powers of soil 51
IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION
OF SEWAGE 52
V. Artificial Self-Purification of Sewage:
1. General observations 68
2. Artificial self-purification of sewage in intermittent
contact beds:
_a._ Name of process 70
_b._ Explanation of process 71
_c._ Water capacity of bed, and silting up 73
_d._ Absorbing powers of filling material 78
_e._ Consumption of oxygen by the filling material 79
_f._ Formation of carbonic acid 80
_g._ Nitrogen 80
_h._ Formation of nitric acid 80
3. Artificial self-purification of sewage in septic
tanks:
_a._ Name of septic tank 81
_b._ Covered or open septic tank 81
_c._ Explanation of process 83
_d._ Velocity of flow through tank 85
_e._ Destruction and liquefaction of sludge in
septic tanks 87
_f._ Formation of gas in septic tank 88
_g._ Mixing action of septic tank 89
_h._ Micro-organisms in effluent from septic tank 89
4. Continuous contact beds 89
VI. MANAGEMENT OF PLANTS FOR THE ARTIFICIAL
SELF-PURIFICATIONOF SEWAGE 90
VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED
MATTERS IN TANKS 91
VIII. CONCLUDING REMARKS 93
POSTSCRIPT 95
NATURAL AND ARTIFICIAL
SEWAGE TREATMENT.
BY LIEUT.-COL. ALFRED S. JONES, V.C.
ASSOC. M. INST. C.E., ETC.
[Sidenote: Introductory remarks.]
“How extremely simple it all is!” was the remark of a recent visitor
at a sewage farm—which encourages me to venture on publication of the
most recent discussions on a “problem” complicated by engineers,
chemists, bacteriologists and inventors of systems, who have raised
clouds of dust through which it is difficult for ratepayers and
district councillors to find their way to “the best practical and
available means of sewage disposal.”
I have a belief that publication of all attempts to purify _the whole_
of a town’s sewage, rather than small scale experiments with equations
founded on such data, is the desideratum.
[Sidenote: 1872.]
Having begun in the year 1872, with a pamphlet, “Will a Sewage Farm
Pay?”[2] I desire to proceed with the present one thirty years later,
as my humble contribution to a right understanding of the intelligent
Scavenger’s business.
At the earlier date agriculture was prosperous, and ratepayers of
Exeter were just as confident that sewage farming would bring large
dividends as some of the same city’s councillors are at present not in
the least sceptical that their engineer’s septic system is the true
specific for sewage disposal.
In adhering to land as the natural and best agent, I have had the
support of the Local Government Board with that of many Royal
Commissions, notably the one now sitting, and I have naturally chosen
cases where suitable land was accessible when I desired to demonstrate
the efficiency and simplicity with which the powers of Nature can be
applied for the use and convenience of man.
Nor have I failed to study all “artificial” substitutes for the best
means, wherever difficulties of obtaining suitable land presented
themselves, e.g. my Canvey Island scheme for dealing with the sewage
of London on a relatively small area, and other cases.
Of late years I have welcomed the light thrown on this subject by
bacteriologists, but lamented extravagant statements put forward by
those who fail to see that the previously unrecognised microbes can do
their work, as they have always done it, to most advantage in the
upper layers of any porous land.
[Sidenote: 1902.]
An interim report by Lord Iddesleigh’s Royal Commission has, however,
awakened such theorists to the fact that land is not to be discarded
because it may not bring in a profit or because patentees of systems
find it to their interest to contrast neglected or badly managed
sewage farms with carefully nursed little experimental installations
for artificial treatment of selected samples of sewage.
Recognising the marvellous improvements in arts and manufactures of
all kinds due to steam, chemistry and electricity, the public has
naturally expected similar results from applied science in artificial
sewage treatment, and there has been no lack of study of every
imaginable process during the last thirty years.
[Sidenote: 1884. Lord Bramwell’s Royal Commission establishes
principles.]
But the late Lord Bramwell’s Royal Commission on Metropolitan Sewage
Discharge established two very important points of general
application, namely:—
1. The principle of separation in works of sewerage and drainage; and
2. The fact that the suspended matters in town sewage can be very
effectually removed from its liquid by _simple deposition_ without the
aid of any chemical reagent.
[Sidenote: 1887. The chemist Dibdin discards chemical reagents in
favour of M. Pasteur’s aerobic organisms.]
And Mr. Dibdin three years later began to demonstrate the mistaken
policy of adding lime or any other precipitating agent in any quantity
likely to arrest the natural agency of abundant bacterial life, which
ultimately disposes of all dead and effete organic matter by forming
gases or natural compounds, with more or less offence to human senses,
according to the supply of oxygen and rate at which these bacteria can
carry out their work.
[Sidenote: Leeds and Exeter.]
It was soon found that the bacteria of two classes, aerobe and
anaerobe, abound in sewage, and the latest Leeds experiment with the
continuous or trickling filter show the marvellous rapidity with which
the _aerobic_ microbes at any rate, can accomplish their task where
air and liquid sewage are sufficiently diffused in the pores of a
filter; while Mr. Cameron, C.E., at Exeter has shown rapid evolution
of gases and considerable solution of organic solids by _anaerobic_
microbes in a septic tank.
But the enthusiasm of inventors and their converts has made too much
of the benefit to the human race supposed to be conferred by the
bacterial discovery of M. Pasteur as applied by them to sewage
treatment.
Without detracting from the credit due to the great French savant and
other bacteriologists who have followed up his interesting studies of
ferments for the last fifteen years, the practical man may well ask
how much forwarder have we got in the main and pressing business of
purifying our rivers—as a consequence of clearer knowledge of minute
forms of life?
[Sidenote: Intermittent filtration.]
The late civil engineer Bailey-Denton demonstrated, thirty years ago
at Merthyr Tydvil, the best conditions of intermittent downward
filtration, and his filters there and at Kendal, Abingdon, etc., are
still doing their work efficiently to this day, while the coke, coal,
clinker, burnt ballast, etc., beds, so popular of late, are clogging
up after a few years of more careful treatment than was ever accorded
to an acre of land under sewage.
Anaerobic action has also been proceeding in the old sewers of most
towns and, as it has now been proved that there is no advantage in the
exclusion of air, upon which Mr. Cameron laid so much stress when he
brought his Exeter tank to public notice in 1897, there can be no
novelty except its name attaching to the anaerobic or _septic_ system,
which has thrown many sanitary authorities off their balance of late
years.
The whole modern system of self-cleansing sewers having been only
rendered possible by public recognition of the horrible nuisance
arising from middens, cesspools, and irregularly built sewers of
deposit, it is hard for those concerned in the cleanly disposal of
sewage to be told that because sewage works are usually remote from
populous districts they must there put up with the cesspool nuisance
and fancy its old smell changed by the new name, because a preliminary
stage in the transmutation of sewage has not taken place, as was
formerly the case in the sewerage system of some modern towns, before
arrival at the works.
But in this as in other affairs there is force in the old maxim,
_Medio tutissimus ibis_, and a properly constructed open tank, for
simple deposition of the solids (frequently washed out), arrests most
of the solids and allows fresh liquid sewage, after slight anaerobic
action, to pass on to land or filter bed in a perfectly inoffensive
condition.
[Sidenote: The cleanly and dirty processes for sludge removal.]
As an example of this I have, at Aldershot, a pair of tanks close to a
public high road, one of which fills with sludge and is emptied every
fortnight or so, and as a contrast there is another pair of larger
tanks in a remote quarter of the same farm in use for years as septic
tanks, from which some sludge is drawn off at long intervals,
anaerobic action being allowed its full course as in the Exeter
experiments.
It is interesting to compare the results of these preliminary clean,
and dirty, processes respectively on similar _very fresh_ domestic
sewage which enters the clean depositing, and the septic tanks alike,
and my observations are as follows:—
1. The manurial result in growth of crop _slightly_ greater with the
septic liquid.
2. Labour increased by the greater deposit carried on to the land
under septic liquid.
3. The removal of sludge and washing out the clean tank gives an
hour’s work with very little smell ten yards to leeward of the site,
but drawing off sludge from the septic tank is a very unpleasant
operation, and, at all times, the vicinity of tank and carriers is
malodorous for a radius of at least fifty yards from the septic tanks.
[Sidenote: Loam on sand and gravel the best medium for aerobic
organisms to work in.]
Passing now to the aerobic stage of sewage purification we find it
universally admitted, that a good loam resting on very porous sand or
gravel, affords the best medium for work by the oxygen-loving
nitrifying organisms when they are supplied with constantly moving
liquid sewage, and given intermittent periods for the aeration of the
pores of the soil.
The proportion of sewage to land is of course as variable as the
quality of the land itself, and the best sort of land is rarely
available, while the improvement of natural land is not understood by
the engineer or chemist, who are usually appealed to by sanitary
authorities in their sewage difficulties.
Hence the variety of artificial substitutes of contact beds, costing
from 5000_l._ to 12,000_l._ per acre, which have been proposed of late
years, with the object of purifying a large volume of sewage on a
small area.
[Sidenote: Leeds experiments.]
Mr. Dibdin first startled the world with the formula 1,000,000 gallons
per acre, but that has long been cut down to 200,000 gallons, and the
life of the contact bed has become the subject of serious concern, as
shown in the annexed table of experiment at Leeds.
Others have sought to increase the proportion of sewage to area by
arranging for _continuous_ instead of _intermittent_ application; but
the difficulty of sprinkling so that every part of a bed may be kept
just moist, in order that aeration may be continuous as well as the
dropping sewage, is very great, and increases with every gallon and
foot from the scale of a laboratory experiment to that of a practical
working for a town’s sewage.
There was an article published a few years ago in the Journal Royal
Agricultural Society (England) on “The Making of the Land,” showing
how nearly all the value of agricultural land in England has been
stored up in it by the exertions of our forefathers, through a process
of successive improvements from, in many cases, worthless sand and
clay, to a condition of the greatest fertility; and I often think that
the 12,000_l._ spent at Birmingham or elsewhere on an acre of contact
bed could be expended to better purpose in preparing 100 acres of the
worst land to deal, for any number of years, with as much sewage as
the contact bed may do for a few years. In the one case we know no
limit to the life of the purifier, and that it must be a very short
one in the other case.
TABLE SHOWING THE VARIATIONS IN CAPACITY OF CONTACT BEDS.
-------------------------+--------------------+-------------------+
| No. 1 Rough | No. 3 Rough |
| Contact Bed. | Contact Bed. |
-------------------------+---------+----------+--------+----------+
| Dates. | Gallons. | Dates. | Gallons. |
-------------------------+---------+----------+--------+----------+
Original water capacity }| 1897. | | 1898. | |
after putting }|October 1| 83,300 |Nov. 21 | 51,800 |
in the coke }| | | | |
| 1899. | | 1900. | |
After experiment | May 6 | 22,700 |March 10| 14,700 |
| | | | |
-------------------------+---------+----------+--------+----------+
Duration of each of } | | | | |
above experiments } |19 months| 60,600 | 25½ | 37,100 |
and loss in gallons } | | | mths. | |
| | | | |
Loss in percentage of } | | |
original capacity } | 73 per cent. | 71 per cent. |
-------------------------+--------------------+-------------------+
------------------------+----------------+----------------+-----------------+
| No. 5 Rough | No. 7 Single | No. 8 Single |
| Contact Bed. | Contact Bed. | Contact Bed. |
------------------------+-------+--------+-------+--------+--------+--------+
|Dates. |Gallons.| Dates.|Gallons.| Dates. |Gallons.|
------------------------+-------+--------+-------+--------+--------+--------+
Original water capacity}| 1899. | | 1899. | | 1899. | |
after putting }|Feb. 28| 53,100 | March | 75,000 |March 23| 29,500 |
in the coke }| | | 24 | | | |
| 1900. | | 1900. | | 1900. | |
After experiment |June 1 | 13,200 |October| 21,600 | June 1 | 9,800 |
| | | 20 | | | |
------------------------+-------+--------+-------+--------+--------+--------+
Duration of each of } | | | | | | |
above experiments } | 15 | 39,900 | 7 | 34,100 | 14 | 19,700 |
and loss in gallons } |months | | months| | months | |
| | | | | | |
Loss in percentage of } | | | |
original capacity } | 75 per cent. | 61 per cent. | 67 per cent. |
------------------------+----------------+----------------+-----------------+
_N.B._—The average duration of the above experiments was 14 months,
and average loss of capacity about 70 per cent. original water
capacity in that period.—A. S. J.
[Sidenote: Wrexham sewage farm.]
At Wrexham, in North Wales, I had nineteen years’ management of about
150 acres of good land, with a mixed residential and manufacturing
sewage of some 15,000 population, with large breweries and leather
works. The owner of this land at the termination of lease asked so
exorbitant a price for the improved freehold, that the corporation
decided to sacrifice the sewage works on his land, and to carry out a
scheme of mine for carrying the outfall sewer two miles further to a
site of 200 acres, which they could acquire on reasonable terms in the
year 1889.
During my management there was no trouble about the effluent, although
it was carefully watched by the authorities of the city of Chester,
which takes its water supply from the river Dee, some twelve miles
below my late farm; and the fact that the scheme which took the
Wrexham sewage two miles nearer to the Chester waterworks intake was
carried out _unopposed_ is, I think, strong evidence of well-founded
confidence in the efficiency of land treatment where the public have
the opportunity of observing such results. It is easy to get up a case
with expert evidence against any sewage scheme where the land-owners,
clergy and others have no means of properly informing themselves, and
have a prejudice against sewage which it is very difficult to overcome
except by giving the utmost possible publicity to the truth.
[Sidenote: The Camp Farm, Aldershot.]
Of late years, while working for the War Department, I have found it
expedient to be more reticent, but the Camp Farm restoration has in
one way or another become known to the public, and there can be no
great harm in my now referring to the circumstances as neither martial
law nor a censorship has yet been proclaimed in Hampshire.
When Aldershot Camp was first hutted, soon after the Crimean War, a
certain Colonel Ewart, R.E., had imbibed true ideas of the separate
system through his association with the work of the late Mr. Menzies,
the Deputy Ranger of Windsor Forest, who preached and practised that
system in the drainage of Windsor Castle and the town of Eton at a
time when every other civil engineer scouted the possibility of
keeping rain or subsoil water out of foul sewers—they said it was
essential for flushing their big sewers.
Colonel Ewart, at any rate, impressed his corps, and after about 1866
one began to see the word FOUL painted up over gratings into which the
soldiers were to pour their slops. A civilian, James Blackburn, also a
friend of Menzies, was employed by the War Office to deal with the
camp sewage on about 100 acres of rough heather-covered land close by,
and he, knowing his business, watched what came down the sewers in wet
weather and kept the Royal Engineers up to the Menzies standard.
[Sidenote: Mr. Blackburn’s successful management.]
Together with this initial advantage of having a regular volume of
sewage not much affected by storm water to deal with, Mr. Blackburn
had many drawbacks in the “pan,” as it is usually called, of iron
conglomerate underlying the very irregular surface which was pitted
all over with holes from which gravel or sand had been dug many years
ago; but he persevered until he had got nearly all the area to bear
good crops, when he entered the Camp Farm in competition for the Royal
Agricultural Society’s 100_l._ prize in 1879 for the best managed
sewage farm in the United Kingdom. The Report of the Judges at that
competition is recorded in the Society’s Proceedings 1880, giving full
statistics except financial accounts, which Mr. Blackburn withheld
because he was then in treaty with the War Office for new terms after
fourteen years’ work on the War Department Farm. My impression after
reading the judges’ reports and having seen the farm a year or two
previously to its date, is that, if the condition as to the production
of the financial accounts could have been fulfilled, the first prize
would have been awarded to the Camp Farm instead of jointly to those
of Bedford and to Wrexham.
Mr. Blackburn had built a big wooden shed and sublet it to a man who
bought his ryegrass for some fifty cows (for whose milk there was a
great demand in the camp), so this subtenant made a tempting offer to
the War Office and got a fourteen years’ lease of the whole farm,
while Blackburn retired in disgust.
I wish to write only from knowledge of facts, and will therefore take
up my narrative again in 1895, after an interval of some fifteen
years.
[Sidenote: Neglected state of, in 1895.]
In the month of May 1895, I was called upon to visit the Camp Farm and
report to Mr. Henry Campbell-Bannerman, the Secretary of State for War
at that date.
I found the whole farm in a deplorable condition of neglected
nuisance, stagnant lakes of sewage retained here and there by banks of
earth, buildings and fences in decay, and the greater part of the camp
sewage passing, by pipes laid by its tenant, under a road which forms
the lower boundary of War Department land, to some rough meadows held
by their tenant from civilian owners for the purpose of saving him the
trouble of spreading the sewage over the sloping surface of the War
Department Farm—work which required the use of a land surveyor’s level
and staff.
In the ditches of these flat meadows the sewage could go through the
septic process to its fullest extent as the level of the river
Blackwater kept them nearly full at all times, and the supernatant
liquid could spread over the coarse herbage of these meadows only in
winter floods, with the result of heavy crops of hay, and sewage
disposal conveniently out of sight and outside War Office jurisdiction
when a Royal Engineer officer might come to inspect the Camp Farm from
time to time.
[Sidenote: _British Medical Journal’s_ report.]
But before my visit an active Medical Officer of Health (Dr. Seaton),
taking an interest in the state of the river bounding his county of
Surrey, detected the camp origin of the stagnant sewage, and,
concluding that the meadows must form part of the Camp Farm over the
road, made a serious report about “Government Sewage Marshes,” which
the _British Medical Journal_ took as a text for an article, and the
Thames Conservancy attacked the War Department as soon as their 1894
Act gave them jurisdiction in the matter.
[Sidenote: Temporary abatement of nuisance.]
I was told that the Camp Farm milk and grass had been condemned, and
that the tenant had consequently sold his cows and was to give up the
farm on June 20, 1895; therefore my report was wanted _forthwith_,
but it was only to take account of anything which could be done
_temporarily_ to abate nuisance, as an agreement was pending with the
Aldershot District Council for the removal of the camp sewage outfall
to some site, at least two miles distant from the camp, at which the
District Council was to become solely responsible for its future
disposal, together with their own Aldershot town sewage, and the War
Department to be rated for the purpose like any other householder.
I found the Commanding Royal Engineer then in office fully alive to
the existing nuisance and prepared to support any efforts I might make
to abate it. Accordingly I agreed to become manager in control of such
labour and material as was necessary for immediate temporary
improvement, and being supplied with army horses, and any necessary
buildings, tanks, etc. to be constructed by the Royal Engineers.
[Sidenote: 1897. The War Office resolve on permanent improvement.]
After about two years it became understood that the nuisance could be
permanently remedied on the Camp Farm, as I had said from the first,
and accordingly the draft agreement, which had then been in discussion
for five years, was abandoned. I was asked to prepare a scheme and
estimate for such permanent works as would enable the sewage to be
effectually disposed of on the Camp Farm.
Recollecting that the sewage had to be at once cut off from Dr.
Seaton’s “Sewage Marsh,” and its disposal provided for throughout on
War Department land, it will be observed that the improvement work had
to proceed piecemeal with some extra care and arrangement; but on the
whole I am satisfied that the work has been completed with greater
efficiency and economy than would have been the case if the sewage had
been turned into the river and the whole site handed over to a
contractor for two years in the usual course.
About the same date (end of 1897) about 13 acres of land was handed
over to my management with sewage from the Royal Military and Staff
Colleges at Sandhurst, about 8 miles distant from the Camp Farm, and,
being somewhat better land to begin with, this part now presents a
very pretty example of what a small installation for about 1000
population may accomplish.
But it is worked as part and parcel of the Camp Farm, horses being
sent out to Sandhurst from Monday to Saturday when required.
It is, perhaps, worthy of note that the reform of the Camp Farm was
initiated in 1895 by the Secretary of State for War in a Liberal
Ministry, and that it has weathered for seven years all the storms of
Jingoism and the fashionable crazes for artificial sewage treatment.
[Sidenote: Sir Redvers Buller’s period of command at Aldershot.]
But whatever may be the rights or wrongs of General Sir Redvers
Buller’s quarrel with the Press and the Government, his reputation as
a practical agriculturist is undeniable, and while in command at
Aldershot it was his custom to stroll over the Camp Farm on a Sunday
afternoon, occasionally leaving a message with cowman or bailiff to
warn me of anything he found amiss, for which I was very grateful,
living as I do ten miles away. I am proud, therefore, to be able to
publish the following letter from one who has shown that he is not to
be influenced by complaisance to superior or inferior in expressing or
modifying his opinions, and he writes as follows:—
17 LOWNDES SQUARE, S.W.
_July 14, 1902._
MY DEAR JONES,
I am delighted to hear that you are publishing a book about
sewage treatment.
The sewage farms at Aldershot and the Royal Military College
afford ample proof of what a sensible practical man can do.
But it is not every one who knows what those farms were
before you took charge of them, nor do I think that any one
seeing them now could conceive their previous condition. It
is to that I can testify; you have turned putrid sewage bogs
into fertile fields. You will confer an immense benefit on
the country if, by your book, you can only teach sanitary
authorities generally that the crux of the whole question is
the necessity for practical commonsense measures against
sewage stagnation, and if those measures are taken nature
will do the work of purification without the assistance of
expensive patents or artificial devices.
Yours very truly,
(_Signed_) REDVERS BULLER.
_To_ COL. A. S. JONES, V.C., C.E.
It must not be gathered from the foregoing account that the War Office
authorities are prejudiced in favour of the _natural_ treatment of
sewage, for, like many other sanitary authorities, they have been
bewildered of late years by the numerous forms of “_artificial_”
treatment in vogue, and I know of more than one experimental
installation for barracks where good available land has been
neglected, for I read last summer of ghastly failures among the
bacterial arrangements in some of those.
[Sidenote: Success mainly due to activity of farm bailiff, foremen and
other workers.]
I cannot quit the above account of the vicissitudes of the Camp Farm
in fourteen years’ growth from a sandy waste to a condition which
tempted a tenant to pay a rent of 3_l._ odd per acre in 1880—its
retrogression to its primitive waste during the following fifteen
years—and restoration to its present measure of fertility, without
expressing the belief that Mr. Blackburn’s success and my own have
been mainly due to our good fortune in obtaining the willing services
of excellent intelligent foremen and workers who, one and all, have
taken a real interest in their several tasks.
Mr. Cameron and other engineers may boast of their labour saving (?)
automatic appliances for opening and shutting valves on sewage works,
but practical workers, responsible for dealing with a million gallons
a day and upwards average, in hourly varying flow of town sewage, will
agree with me in hesitation as to placing entire confidence in the
substitution of automatic machines for any large proportion of their
manual labour.
[Sidenote: Education and encouragement of sewage employees advocated.]
I have for many years advocated education of sewage farm managers and
watermen, to be selected from the rapidly decreasing class of
agricultural labourers by the tender of high wages, houses and good
gardens, with other profit-sharing allowances which it will well pay
sanitary authorities to hold out to their sewage employees.
In this sense I am glad to note the recent formation of “_The
Association of Managers of Sewage Disposal Works_,” Secretary, Charles
H. Ball, 5 Fetter Lane, London, E.C., as a Trades Union move from
within well calculated to raise the status of the class of men upon
whose exertions the community must mainly rely if there is to be any
hope of improving the condition of our streams and rivers.
[Sidenote: Evidence and Reports of Lord Iddesleigh’s Royal
Commission.]
Two large Blue Books containing the evidence taken by Lord
Iddesleigh’s Royal Commission have been published since the Interim
Report, and their contents more than warrant the opinion expressed in
the latter; indeed it must surely be admitted that the case for each
of the artificial systems was very fully gone into before that
Commission expressed the guarded conclusion, “We doubt if any land is
entirely useless.”
I do not believe that the surface purification obtained by
distribution over even the densest of clay lands was _effectively_ put
in evidence, and too much weight was given to the difficulty of
increasing the effective top soil on such land; but on the whole I
think that the Interim Report is very satisfactory to the reasonable
advocates of a preference being given to the adoption of a large area
of land, where available, over any artificial treatment on a small
area, other things being equal.
At the time when the Interim Report was issued, however, a very full
and careful examination of a select number of sewage farms was still
in progress, and Appendix 22, with a casual mention by Dr. M’Gowan,
affords the only glimpse to be had in the bulky Blue Books, of any
results of that examination having been as yet adduced in evidence.
The Commission’s officers, to my knowledge, were engaged for many
months in examining, surveying and taking numerous samples of sewage
and effluent at the Camp Farm, and, as they doubtless had equal
opportunities of independent observations on the other selected sewage
farms, the further reports of Lord Iddesleigh’s Royal Commission
cannot fail to be interesting and instructive.
On one point Appendix 22 to the Blue Book abundantly supports an
opinion I have so often expressed, namely, that a good strong loamy
surface is a more efficient purifier of sewage than many feet of
barren sand.
I refer to the curves in Appendix 22, showing the greatly superior
purification effected at Nottingham with the best soil as compared to
that of the sandy one at Aldershot, which, in its natural character,
is about the worst for purification and for producing crops to be
found in England.
My experience, however, all points to the extreme importance of
studying local conditions from the first inception of plans in each
particular case, to their completion with the best available
materials.
But when the engineer has done his best, the sanitary authorities,
having borrowed the funds to pay for the work, will take no further
trouble about its sewage, and will often engage careless ignorant
workpeople at inadequate wages to carry on the hourly varying labour,
on efficient performance of which success depends.
[Sidenote: Automatic appliances for sewage and effluent discharge.]
It may seem idle to complain of boards and their employees showing
little interest in the work of sewage disposal, but it is worse to
pander to their failings by selling them automatic machines under the
pretence that all the thought, and fertility of resource, required for
efficient sanitary sewage disposal can be supplied by ingenious
applications of hydraulics on the principle that sewage is a fluid,
and, as such, will behave like clean water.
Of course, when the aerobic treatment is carried out on a bare level
surface of cinders or coke growing only weeds, the lack of interest is
very excusable, but in the natural system the growth of crops and
contouring a sloping surface with carriers so that every part shall
have its trickling water alternating with dry periods for cutting the
crops or hoeing out weeds, should be a matter of constant interest to
an agricultural worker, and, if he knows his business, good crops and
purity of effluent must go together.
[Sidenote: Managers should have a free hand.]
In order to attain this happy result, a manager must know his business
and be given a free hand, not pestered by members of a committee
(farmers, butchers, gardeners or town tradesmen) coming to give their
advice or orders. The river authority should take samples as often as
they like and send the manager as soon as possible the analyses with
day and hour of sampling as a guide for future working.
He will then have to explain any defect from average purity of
effluent, due to one of the hundred contingencies which may arise in
practice, after he and the river authorities have agreed about what
that average analysis should be for his particular farm or works; and
it will be for the advantage of all parties not to try and enforce a
fixed standard for a whole district, as some river authorities usually
attempt to do, because it is easier to lead than to drive a good
manager, and nothing at all can be done with a bad one.
It must not be supposed that I think river authorities should be easy
going, quite the contrary, but they should trust their inspectors’
reports, and “run in” those sanitary authorities who are careless
about the management of their sewage farms and trying to cut down
working expenses and capital.
In precipitation or other artificial sewage works it is easy to judge
this, but more difficult for any one except the good farm manager to
know whether the land is being made the most of for profit or for
purification; still the rivers authority ought to get to know if they
and their officers take pains.
[Sidenote: Purification and profit.]
It is a common idea that working a sewage farm for profit, and for
purification of the sewage, are two incompatible things, whereas, the
good manager with sufficient working capital (double or more what
would be enough for the same acreage in ordinary agriculture) and a
good market for produce will attain the two together in due proportion
in all ordinary seasons, when a fair allowance has been made him for
the necessary sanitary work.
It is easy to see how the popular idea of incompatibility has arisen
in a case like that above stated of the Camp Farm tenant, eating up
year by year all the fertility stored up in the land during the
previous period, and letting nearly all the sewage run to waste,
because its scientific application would cost much in thought and
labour. In much the same way district councils have been, all over the
country, stinting their labour bills and interfering with their
managers’ purchases and sales in order to make as small a demand on
the rates as they can—each year bringing some change of system—to the
end that nobody is responsible or has any confidence in master or man.
With such a state of things up and down the country the way was
prepared for preachers of microbe agency to say, why should you buy
all that land when a septic tank, a few acres of coke or burnt
ballast, and a patent automatic opener and shutter of valves (which
you see working so nicely with tap water and model at some exhibition)
will give you “no more troublesome sludge,” and a first class effluent
with hardly any labour bill? if you only agitate against that
arbitrary Local Government Board, which insists upon land!
But those gentlemen neglected the fact, that in a few years’ time
their filters would have to be pulled to pieces, washed and put back,
while the land remains as efficient as ever, and a valuable asset, in
some cases saleable at building value, if it becomes desirable to move
the outfall further at some future time.
[Sidenote: Sludge treatment.]
In the above comparison between natural and artificial treatment
reference has been had chiefly to the aerobic branch of the business,
but the anaerobic, breaking down _some_ of the solid organic matter
and the sanitary disposal of the remainder in the state of sewage
sludge (containing fully 90 per cent. of moisture) must not be
overlooked or shirked as beneath the attention of the scientific
bacteriologists and chemists whose analyses of effluents, and often of
what they call crude sewage, are made from the liquid which has passed
through a filter paper in their laboratory before their “oxygen
absorbed” or “ammonia processes” are proceeded with.
On the contrary, I have always maintained that sludge, being the
foulest part of town sewage, ought to receive primary and earnest
attention if we desire to improve the condition of our watercourses.
When town sewage is pumped through a long rising main, it can often be
spread on the land in its really crude state, and if the soil is clay
ploughed up to receive it the sludge is most beneficial to its
texture.
But in every other case we must face the nuisance of extracting the
sludge, and its desiccation in one of the following ways.
1. On a farm at some distance from roads and houses, the cheapest plan
is to form a bank of earth about 18 inches high, enclosing a
rectangular area into which the wet sludge can be run or pumped out of
depositing tanks, and left alone until dry enough for cartage, when it
can be used on the farm or sold to neighbouring farmers for a shilling
or two a load.
2. A wall of farmyard _long_ manure may be used instead of earth, and
trench 5 feet wide dug on each side of the longer sides of the
rectangle, leaving 3 feet of ground between the wall and trench, on
which men can stand to scoop the sludge over the wall when it has
consolidated a little in the trench; the latter is then ready to
receive the sludge from another tank emptying, which is again scooped
over the wall on to a thin coating of farmyard manure, which has been
scattered over the last layer of sludge in the rectangle; and thus in
a year’s time a solid mass of the mixture is raised four or five feet
high, and is in capital order for putting in drills for a crop of
mangold wurtzel.
This is the plan in use at the Camp Farm; it occupies little ground
and smells only like rotten dung does during the few days carting to
the mangold field.
3. Pressing by compressed air forcing a liquid mixture of sludge and
lime into the interstices between cloths supported by vertical iron
plates on a horizontal frame; and such pressing is a very expensive
process, only resorted to when the sewage works are in a confined
populated district where no accumulation of sludge can be tolerated.
[Sidenote: Expert examination of neighbourhood a very necessary
preliminary to any sewage scheme.]
Before any sewage scheme is conceived a very careful survey of the
neighbourhood ought to be made by a person who knows the requisites of
a site for sewage disposal, especially if land irrigation is intended,
because natural advantages of site both for tanks, main carriers,
roads, etc., may make all the difference in the world in expense and
efficiency not only in first cost of works but also in their use
afterwards.
And if land is to be acquired for sewage farming it will be very
desirable to include in the purchase some neighbouring high lying
area, not required for sewage disposal but for growing straw crops to
be used on the farm.
CROPPING A SEWAGE FARM.
[Sidenote: Vegetation of some kind, useful or weeds, _will_ grow from
sewage, and must be frequently removed from land or contact bed.]
This is a matter of vital importance, because when sewage is
_intermittently_ applied to land of any kind or to coke beds,
vegetation of some kind or other must result and must be removed in
order to leave a clear course for the next dose of sewage; the cost of
removal and destruction of weeds will be found very great when contact
beds are tried on any working scale and would be quite prohibitive if
allowed to grow on irrigated land.
Hence we must crowd out the weeds as much as possible by useful plants
which will bring something towards the cost of their removal; and as
that return from perishable greenstuff is dependent upon its immediate
sale or consumption on the farm, the manager must cast about for
demands for his abundant supply; but as both the sunshine (in this
climate) and markets are very capricious factors in the problem, he
has no easy task always to make both ends meet.
Theoretically the town which yields the sewage ought to provide an
abundant demand, but in practice it can rarely be depended upon,
Edinburgh being the only exception, where the Craigentinny sewage
meadows are rented at a very high figure by the cow-keepers of a city
situated in the heart of an arable district.
[Sidenote: Alternative destinations for vegetation thus removed. Milk
(everywhere in demand) or a destructor furnace.]
Fortunately, however, there is always an unlimited demand for milk,
and if he has the means of keeping a herd of cows on the farm, or can
arrange with a neighbouring cow-keeper to take all the grass and roots
he can supply at a low rate, it is about the best course a manager can
adopt.
If he maintains a herd of cows, tied up in good, well ventilated
stables, and has them daily brushed and groomed like horses, they
require no exercise and produce milk in perfection for an average
period of fifteen months from date of purchase after their third or
fourth calving.
Such a herd will consume rye-grass carted from the field from April to
November, and mangolds, kohl-rabi, and rye-grass hay during the
winter, thus securing a uniform demand for produce of the sewage land
throughout the year, and such cows will only require a little cotton
cake and oat straw bedding (of which latter they eat a good deal) to
fit them for sale to the butcher as soon as they become dry.
The advantage of such a steady demand is so great when rye-grass and
mangolds, etc., are indicated as the main crops of the farm, owing to
the large volume of sewage per acre, that the system of cow-keeping is
forced upon managers, however reluctant their sanitary authorities may
be to provide the necessary working capital, unless they can find a
reliable contractor to receive at a fixed price any quantity of grass
and roots the authority may grow and deliver.
[Sidenote: Permanent pasture grazed and for hay available in certain
cases.]
When a town has more land in proportion to its sewage, permanent
pasture may take the place of Italian rye-grass, and, with proper
precautions, a part of the permanent pasture may be grazed; but the
saving of labour, thus supposed to result from letting animals bite
and carry their food, is expended in making up, in a necessarily
imperfect manner, the carriers trodden in by the cattle.
[Sidenote: Importance of neat tidy contour carriers, correctly
levelled.]
And here I would observe that most of the bad odour into which sewage
farming has fallen of late years is distinctly traceable to the common
absence of sufficient regularly contoured and neatly cut distribution
carriers resulting from parsimony about wages bills natural to the
ratepayers’ representatives in Council, and often to the manager’s
dependence on a borough surveyor’s coming to the farm with his level
and staff for great measures, or on his own guesses for smaller works,
instead of using an instrument to peg out every distribution carrier
at the right moment.
[Sidenote: Attract good labour.]
There is another important outlay of capital to be provided for in
every complete sewage scheme, which should embrace sufficient good
labourers’ houses and gardens in order to attract and retain on the
spot the best class of workers.
[Sidenote: Summary of the experience of a lifetime.]
To sum up the general conclusions to which my experience points, and
which I trust may prove useful to district councillors, they are as
follows:—
1. In works of sewerage, limit and regulate, as far as
possible, the volume of sewage by excluding subsoil
water and clean surface water.
2. Where the outfall sewage enters the disposal works
provide a pair of open catch-pits (or grit-chambers), each
twice as wide as, and 2 feet deeper than the sewer, with
sluices allowing the sewage to pass through one pit at a
time in its free course, while the other pit is being dried
and the deposited detritus dug out. The depth below sewer
invert may be more than 2 feet, and length of catch-pit is
immaterial, but I confine its width to twice that of the
sewer in order to conserve sufficient velocity in the
current to carry forward organic matter, paper, etc., and
leave only clean sand and gravel in these catch-pits.
Continuing the course by open channel (of same width as
outfall sewer), it should expand to five or six times its
width, forming the screening chamber, and thence discharge
into the
3. Depositing tanks. These are best formed in concrete with
smooth surface, with a semicircular level weir from which
the liquid overflows into a semicircular collecting open
carrier leading to the aerobic process on land or contact
bed.
The semicircles above referred to are struck from centre of
the inlet to depositing tank with a radius of 50 feet or
more.
The weir level should be at least 1 inch below that of
invert of inlet, and the depth of tank immediately under
this point should be governed by consideration of the
facility of drawing off the sludge by valve at that depth to
the sludge drying beds by gravitation if possible, or pump
if necessary, and from this sludge emptying valve the smooth
concrete bottom of tank slopes up to the semicircular weir
above described.
The bottom and sides of such a tank should be made with the
best Portland cement and finest granite chippings wrought to
a smooth surface, so that the sludge may be easily swept
clean away with a squeegee to its outlet valve, as it is
very necessary to have the tank thoroughly washed after each
emptying if my view of the _clean_ mode of sewage disposal
is to be carried out.
But with the dirty mode, on the contrary, some of the sludge
only should be drawn off and the septic anaerobic action
preserved continuously in the tank itself, whereas I prefer
that action to have its early and less offensive course in
the tank and its completion in a drying bed mixed if
possible with farm-yard manure.
4. The aerobic process. The one essential point in this
final process, whether in land or “contact beds,” is
_sufficient_ aeration (excess as by blowing has no result
commensurate with cost of its introduction), and it can be
attained by intermittence of sewage and rest, or by
continuous passage of sewage through a bed of coarse medium
_kept always just moist in all its atoms_ by a rain-like
dropping on the surface so carefully adjusted as to moisten
all parts and not to form a water-seal in any part of the
bed. Intermittence is easily arranged on any scale of
working, and continuous filtration, on the contrary, is
difficult even for a few thousand gallons a day.
[Sidenote: Anticipation of a coming reaction against “fads” and
overpressure in sanitation.]
Since the above was written our grand old philosopher Herbert Spencer
has published a volume of “Facts and Comments”[3] containing a chapter
on “Sanitation in Theory and Practice,” which points to a coming
reaction against the movement begun, some fifty years ago, by the late
Sir Edwin Chadwick and followed up by many enthusiastic exploiters of
the popular dread of “germs,” which he associated with bad smells.
Of course the professor’s practical acquaintance with Chadwick’s hobby
is, as he says, very limited, and his argument, that because sewage
and manure smells are harmless in the open air of the country, they
should be equally innocuous in a town, falls to the ground when
brought to the test of experience, and I trust that Mr. Spencer will
forgive me for pointing out that sewer-gas, drawn into a dwelling
room, in town or country, through scullery waste pipe or other
connection with a sewer in which the air is of lower temperature than
that of the dwelling room, is really prejudicial to health whether
accompanied or not by disease germs.
And although, as one of the experts to whom Chadwick appealed and
whose moderate testimony was cast aside because it did not come up to
the standard desired by his enthusiasm, I fully endorse Mr. Spencer’s
caution with regard to the mass of Blue Book evidence on sanitation, I
venture to express my regret that the dear old man has had an
unfortunate experience of sewage treatment, and my surprise that so
deep a reasoner should have published his judgment in this chapter
without having taken the pains to extend his acquaintance with sewage
treatment in other places than the single instance of Burton-on-Trent.
In thus despising an unsavoury subject Mr. Spencer is not alone, and I
am sorry to have to say that general indifference is answerable for
the waste of much public health and money, because it need not be
surprising if those following a _despised_ trade are sometimes ready
to take advantage of the prejudice and ignorance of their employers.
In this sense I beg to quote Professor Spencer as follows in
justification of the reflection with which I began the above essay:—
“New sanitary appliances are continually being devised,
sanctioned by authority, and required by surveyors; and
surveyors may have and certainly sometimes do have, personal
interests in pushing the use of them; either as being
shareholders in the companies they are manufactured by or as
receiving percentages on the numbers sold through their
recommendation.”
[2]: Published by Longmans, London, 1874. Third edition
published by R. Potter, Wrexham, 1885.
[3]: ‘Facts and Comments,’ by Herbert Spencer. Williams and
Norgate, London, 1902.
NATURAL AND ARTIFICIAL
SEWAGE TREATMENT.
BY H. ALFRED ROECHLING,
M. INST. C.E., ETC.
I. INTRODUCTORY REMARKS.
At the request of Lieut.-Colonel A. S. Jones, V.C., Assoc. M. Inst.
C.E., who has done yeoman service in this matter, I have great
pleasure in putting down some observations on this old but ever
controversial question of sewage treatment.
Colonel Jones has done more than anyone else living to establish
correct views on sewage farming, and he has lately changed the
Government sewage marshes at Aldershot into a veritable “Garden of
Eden,” watered by the waters from Aldershot Camp, growing healthy
crops, and causing not the slightest nuisance. After many struggles,
even the milk from the dairy cows is now recognised as good and
supplied to the military hospitals. This is an achievement of which
anyone might be proud; and all those who have been over the farm
during the time of the “deluge,” and can now study the order and
system evolved out of chaos by Colonel Jones will testify to this! It
is pleasant to record that the War Office have recognised Colonel
Jones’ work for them by having appointed him quite recently to manage
all the sewage disposal works in the Aldershot district. This will
involve the laying out of irrigation works in eight separate places,
in some of which artificial methods of sewage purification have been
tried and found wanting.
Before commencing with my task proper it may not be out of place to
describe here very shortly the various stages through which the sewage
question has passed during the century just closed. Such a retrospect
is of general interest and may throw some further light upon our
subject; it must of necessity be short, otherwise it would absorb more
time and space than is at my disposal, and any shortcomings in this
respect that the reader may discover, I trust he will kindly put down
to this cause.
“The man in the street” seems year after year more called upon to form
an important element in settling questions even of a scientific
nature, and if what I am going to say should prove of some service to
him my labours will be well repaid.
II. THE SEWAGE QUESTION DURING THE LAST CENTURY.
A SHORT RETROSPECT.
In dealing with the sewage question during the last century, it will
be an advantage to distinguish between the theory and practice of
sewage purification, as such a division of the subject will render it
less complicated and will tend to avoid misconceptions.
Dealing first with the theoretical side of the question, it is very
doubtful whether at the dawn of the century even a working hypothesis
existed to explain the process of sewage irrigation which was then
adopted in one or two instances, notably at Edinburgh, where the town
sewage was very successfully purified on the Craigentinny meadows. It
is more than likely, that at this time instinct took the place of
theory, and that sewage irrigation was an instinctive imitation of
irrigation with river water employed for many centuries in some
eastern countries.
Later on it is on record, that Cagniard de la Tour in France, about
the year 1825, and Schwann in Germany, about the year 1836, expressed
the view, that organised substances—micro-organisms—played some role
in fermentative and putrefactive changes. Almost diametrically opposed
to this were the views authoritatively laid down by the then star in
the chemical horizon, Justus von Liebig, who, about the year 1845,
maintained that these changes were brought about by the dead inert
matter itself—by molecular movements in the same—and not by organised
substances, the presence of which in fermenting or putrefying
substances was purely accidental. So great was Liebig’s authority
then, that many almost blindly adopted his views, and the strife that
commenced around these opposing views was fought with the greatest
bitterness. But the stronghold of old ideas, which were gradually but
surely being supplanted by new ones, could not hold out for ever
against combined attacks, however stoutly it was defended by its
designer, and its final downfall came about the year 1860, when a
young Frenchman, Pasteur, established beyond doubt by his ever
classical researches, that fermentation and putrefaction were, in the
first instance, due to living organisms and not to dead matter.
Pasteur further demonstrated that living organisms were also the cause
of some and probably of all zymotic diseases.
So far, so good! But unfortunately the methods of biological research
employed by M. Pasteur were very cumbersome and left otherwise much to
be desired, so that his discoveries could not be fully utilised and
extended, until in 1882 Robert Koch of Berlin published his new
methods of investigation. This was the signal of raising the
floodgates of biological (bacteriological) research throughout the
world with this result, that the flood waters pent up until then
inundated practically other branches of scientific investigation and
drowned their individual life for some time to come.
During this interval, 1860 to 1882, investigators who wished to study
the organised impurities in sewage had to proceed by indirect methods.
They had no means of ascertaining by direct biological experiment the
number and character of the micro-organisms contained in sewage: all
they could do, was to determine chemically the dangerous nature of the
sewage by the amount and origin of organic matter it contained, which
would probably act as food to the germs; and the greater this amount
was, so it was inferred, the greater would be the number of germs it
harboured and the more dangerous its character.
This was the condition of things at the time the second Rivers
Pollution Commission carried out its investigations, which in many
respects, and rightly too, are still considered standard
investigations. It cannot be surprising, therefore, that, being
without proper means of biological examination, and having to rely
chiefly on chemical methods only, the Commissioners came to the
conclusion that the changes brought about in sewage purification were
due to mechanical and chemical agencies!
It is frequently a matter of the utmost difficulty to ascribe, after
the lapse of half-a-century, a new theory to one special author, as
several investigators may have been trending the same way quite
independently of each other, but may not have been equally successful
in the matter of their publications becoming generally known.
Theories, as a rule, do not drop out of the clouds like meteorites,
they force themselves gradually upon men’s minds and are elaborated by
them until ripe.
Bearing this in mind, and subject to further research, it would appear
as if Alexander Müller had been the first to apply Pasteur’s general
theories as to decomposition, fermentation and putrefaction to the
problem of the self-purification of sewage. He made his experiments in
1869 and published them in 1873. Since that date a very large number
of investigators have been at work on similar lines, and whilst it
would lead too far to deal with them minutely, it ought to be stated
that the results of their labour confirmed the view of living
organisms playing a very important part in the decomposition of
sewage. Among the many names prominent in this respect are those of
Schloesing, Müntz, Hatton, Warrington, Sorby, Winogradsky, Percy
Frankland, Dupré, Emich and Dibdin. That set of researches, however,
which has done more than any other to consolidate the theory of
bio-chemical changes taking place in the self-purification of sewage
are the investigations of the Massachusetts State Board of Health,
which were commenced in November 1887, and are still being continued.
Since 1895 a large number of additional experiments have been made,
which will be dealt with more in detail later on, but speaking
generally they have not materially increased our knowledge of the
processes taking place in sewage purification.
Summarising the remarks on the theoretical aspect of this question,
it may be said that, as to the agencies at work, we know now they are
of a mechanical, chemical and biological nature; but as to the
processes and products brought about by these agencies we know very
little beyond the initial and terminal stages, as will be pointed out
in some of the subsequent observations.
Directing now attention to the practical side of the question, it has
already been stated that the only known sewage treatment at the
commencement of last century was land irrigation. Then about the
middle of the century chemistry seems to have taken the matter in hand
and tried to make a lucrative business out of it. It is on record,
however, that it did not succeed in this attempt, and the financial
loss which this endeavour has caused is a dismal subject to
investigate.
There is before my mind’s eye the case of a gallant officer of His
Majesty’s land forces who, after having reached very near the summit
of his career, retired and employed his time in trying to make a
fortune out of sewage. So enamoured was he of the subject, that—so the
story goes—he commuted his pension to have all the more ready money;
but fortune did not smile on him, and his last days were spent under
the lengthening shadows of the sorrow of financial difficulties,
having practically lost all he possessed.
The emphatic verdict of the first Sewage Commission of 1857, the first
and second Rivers Pollution Commission, and, indeed, of all other
authoritative investigations, was in favour of land treatment; and it
cannot, therefore, be surprising to find that the Local Government
Board insisted, save in exceptional cases, that “any scheme of sewage
disposal, for which money is to be borrowed with their sanction,
should provide for the application of the sewage or effluent to an
adequate area of suitable land before it is discharged into a stream.”
Indeed, had this body taken any different view and neglected the
findings of practically all authoritative inquiries, it would have
been singularly deficient in the discharge of its duties to the
ratepayers of this country.
But the best of land cannot go on for ever doing its duty if by
systematic neglect and ignorance the essential conditions for
successful purification are year after year violated; and the great
pity is that the Local Government Board, after deciding in favour of
land treatment, did not systematically superintend this operation. It
may not have had the power, but it is quite evident that had it done
so, things would not have drifted from bad to worse, until local
authorities, driven to despair by the apparent failure of land and not
discerning the right cause, refused altogether to be ruled by what
seemed to them a very unfair and absurd restriction.
It was at this time that Mr. Dibdin, who, on behalf of the London
County Council, had been carrying out a set of valuable experiments,
came forward with his application of well known theories to sewage
operations on a large scale. As I pointed out at the time, Mr.
Dibdin’s experiments proved beyond a doubt that the application of
sewage to suitable land was right in principle and that the failures
were brought about by the non-observance of the rules laid down by
this gentleman—that, in fact, sewage irrigation was the only natural
method of sewage purification and that all the other methods were
artificial. I described land treatment as the natural self-purification
of sewage and the oxidation or contact bed system as the artificial
self-purification of sewage.
But the swift current of public opinion had set very strongly against
sewage farms, and nothing but the contact bed treatment would do. A
large number of experimental plants on this system grew up like
mushrooms all over the country, and the waves of enthusiasm seemed at
one time to engulf even the Local Government Board itself with its
“antiquated notions,” until Parliament came to the rescue and
appointed on May 7, 1898, a new Royal Commission to study the question
of sewage purification.
This Commission consists of nine members,[4] i.e. six professional men
and three laymen. Of the professional men, one is a biologist, one a
chemist, two are medical men in administrative positions, and two are
engineers likewise in administrative positions. Of the laymen two are
members of special boards for the prevention of the pollution of
rivers.
So far the Commissioners have issued an Interim Report dated July 12,
1901, a volume of evidence and a volume of appendices. Quite lately,
it is stated, they have issued a further Interim Report, to which are
attached separate reports on some special subjects by their officers,
but this report has not yet come to hand.[5]
At the time of their first Interim Report, July 12, 1901, the
Commissioners had held altogether thirty-five sittings, the first of
which was on June 22, 1898, and the last on May 22, 1901. The period
thus covered is nearly two years, and out of the thirty-five sittings
thirty took place in London, and five in the provinces, viz. at Leeds,
Ripon, Manchester, Accrington and Reigate.
On these occasions, all in all, fifty-eight witnesses were examined,
who may be grouped as follows:
1 Zoologist
1 Botanist
2 Laymen
3 Bacteriologists
5 Lawyers
7 Medical men
11 Patentees
14 Chemists
14 Engineers
58 witnesses in all.
Out of this number twenty-five were officials, viz. five lawyers, six
medical men, six chemists and eight engineers. Four officials were
further managers of artificial sewage purification works, but not one
single manager of natural purification works, i.e. a sewage farm
manager, was called, the term “sewage farm manager” being used here to
indicate an official whose sole duty it is to manage a sewage farm.
The entire absence of this latter class of official is so striking
that it cannot be due to accident, but must be the outcome of a
settled policy not to reopen questions conclusively settled by
previous inquiries.
Another point that strikes the observer is that the Commission only
called one zoologist and one botanist, as it is to these scientists
that belongs in the first instance the question of studying the fauna
and flora of sewage before the subject is taken up by other branches
of natural science.
Speaking on the whole, the evidence taken by the Commissioners forms
very interesting reading, and ought to be carefully studied by those
who have to deal with the subject. When now and again opinions are
expressed, which seem directly opposed to each other, it must be borne
in mind that here, as in other things human, unanimity of opinion,
though much desired, is apparently unobtainable.
To understand the conclusions fully, at which the Commissioners in
their Interim Report have arrived, it ought to be pointed out that
they had either to accept the recommendations in favour of land passed
by all previous Royal Commissions and authoritative inquiries, or they
had to show by incontestable evidence that their predecessors had made
grievous mistakes, and where!
Of these two courses, the present Commissioners have adopted, no doubt
for very good reasons of their own, the first, and they have started
therefore, in the conclusions to which they have come, at the point
where previous inquiries had left off, viz. that land treatment is a
very proper method of sewage purification.
But before referring more in particular to their observations on land
treatment, it will be necessary to point out that the Commissioners
evidently divide all methods of sewage purification into two main
classes, viz. natural and artificial methods. Into the former they
only place land treatment, whilst they call all other methods
artificial.
This division seems to have given a great deal of offence to all those
who have expressed decided and frequently very one-sided views in
favour of the “bacterial” treatment of sewage; but on closer
examination it cannot be denied that the Commissioners were quite
right in forming this view, as the following remarks will show.
For main divisions of all methods of sewage treatment two factors seem
to be of primary importance, viz. the agencies which bring about this
purification, and the way in which these agencies are employed. Now,
it will not be denied that all agencies are natural ones, whether the
process employed is a purely chemical one, a purely “bacterial” one,
land treatment pure and simple, or a combination of these, and, at the
present time no such thing as an artificial agency is known; indeed,
it is perhaps not too much to say that there cannot be such a thing
as an artificial agency. Hence it is impossible to divide sewage
purification methods in this respect by the agencies employed, and one
is bound to fall back upon the way in which these agencies are
employed. Here it is no longer open to argument whether a chemical
process or the contact bed system—oxidation bed system—is artificial,
or whether the land treatment is natural! For who would deny that
masonry or concrete tanks and the materials contained in the same are
artificial products—i.e. products formed by man—and that land is a
natural product—i.e. formed by nature—and that further the soil is the
natural home of bacteria. Hence it must be perfectly clear, even to a
casual observer, that the line of demarcation drawn by the
Commissioners between all known systems of sewage purification is a
correct and legitimate one, and that all objections to such a division
are based on misconceptions.
Concerning land treatment, the Commissioners observe, “We doubt if any
land is entirely useless,” but further on they observe that peat and
stiff clay lands are generally unsuitable for the purification of
sewage. Concerning peat, nobody acquainted with the subject would
probably differ from their conclusions owing to the great amount of
moisture contained in this material; but as to clay soils, the
Commissioners when making this statement must have known that there
are several successful sewage farms on this kind of land in existence,
such as the sewage farms at South Norwood, Wimbledon, Warwick and
Leicester, not to mention others. In the case of Leicester, although
the land is a very dense boulder clay, the Corporation of this town
have just purchased the freehold of the farm for about 160,000_l._
Dealing with the artificial processes from a chemical point of view,
the Commissioners are of opinion that it is practicable to produce by
these processes alone, either from sewage or from certain mixtures of
sewage and trade refuse, effluents which might be discharged without
fear of creating a nuisance, and that in consequence the Local
Government Board would be justified in modifying, under proper
safeguards, the present rule as regards the application of sewage to
land.
The artificial processes referred to in the observations appear to be
the following:—
Closed septic tanks and contact beds.
Open septic tanks and contact beds.
Chemical treatment, subsidence[6] tanks and contact beds.
Subsidence tanks and contact beds.
Contact beds alone.
Closed septic tank followed by continuous filtration.
Open septic tank followed by continuous filtration.
Chemical treatment, subsidence tanks, and continuous filtration.
Subsidence tanks followed by continuous filtration.
Continuous filtration alone.
The Commissioners do not say what these safeguards are, in fact they
state that no general rules concerning them can be laid down, and that
in the case of these artificial processes it is necessary to consider
every case on its own merits.
The next point dealt with is the bacteriological quality of effluents,
and here the Commissioners observe: “We find that, while in the case
of effluents from land of a kind suitable for the purification of
sewage there are fewer micro-organisms than in the effluents from most
artificial processes, yet both classes of effluents usually contain
large numbers of organisms, many of which appear to be of intestinal
derivation, and some of which are of a kind liable under certain
circumstances at least to give rise to disease.”
No particulars of effluents from sewage farms are given, and later on
it will be shown that this conclusion of the Commissioners is not in
accord with the results published up to now and available concerning
the bacterial purity of effluents from land treatment.
The report concludes with some remarks on rivers pollution. The
Commissioners state that it is of the utmost importance to provide the
simplest possible means for adequately protecting all rivers, and they
think that this subject is of such grave importance “as to demand the
creation of a separate Commission or a new department of the Local
Government Board, which shall be a supreme Rivers Authority, dealing
with matters relating to rivers and their purification, and which,
when appeal is made to them, shall have power to take action in cases
where the local authorities have failed to do so.”
Summing up the observations on the practice of sewage treatment, it
may be said that as a result of their extended inquiries, the present
Royal Commissioners have at the end of the century re-established land
in its position as the first and only natural method of sewage
purification, beside which they have recognised artificial
(biological) treatments as being under proper safeguards admissible
for the purification of sewage.
Before concluding this portion of the observations, it is necessary to
mention the valuable work done by Mr. Scott-Moncrieff and Mr. Cameron,
who, contemporaneous with Mr. Dibdin, but quite independently, had
experimented with sewage and evolved their own artificial methods of
sewage treatment.
These remarks must suffice for the more historic portion of the
subject, viz. the progress of sewage purification during the last
century, and it is time now to direct attention first to natural and
afterwards to artificial sewage treatments.
[4] Two of these have since retired.
[5] This report has just been issued (August 18, 1902), and
although the special reports it contains are of the greatest
interest, it is not necessary to refer to it again in these
observations.
[6] The expression “subsidence tanks” is intended to denote
tanks which are used in such way that little or no septic
action is produced.
III. THE SUBSOIL.
[Sidenote: General remarks on subsoil and its properties.]
Before dealing more in detail with the processes taking place in the
pores of the subsoil of sewage farms, it may not be out of place to
make here a few general observations on the mechanical structure of
soil, its permeability, water capacity, retentive power, the capillary
movements in the same, its temperature, the subsoil air, the movement
of water in and through the same, the micro-organic life in soil, and
its absorbing powers.
1. MECHANICAL STRUCTURE OF SOIL.
[Sidenote: Size of grain and pores.]
Here is of interest the size of the grains or particles composing the
soil, the size of the pores and their collective capacity.
According to the character of the soil, its grains or particles will
vary from very large in coarse gravel to very fine in fine sand and
clay.
[Sidenote: Variable size of pores.]
[Sidenote: Surface attraction.]
The size of the pores will vary as the size of its grains from large
to small, but frequently a certain kind of soil will contain a mixture
of large and small pores. The finer the pores the more energetic will,
as a rule, be the surface attraction of the grains composing the soil.
[Sidenote: Pore-volume.]
[Sidenote: With particles of equal size pore-volume amounts to about
38 per cent. of the total space, and sinks down to 10 or 15 per cent.
with particles of unequal size.]
[Sidenote: With equally sized particles the pore-volume is the same
whether the particles are small or large.]
The collective capacity of the pores or the pore-volume mainly depends
on the equal or unequal sizes of the particles. When the same are of
equal size the pore-volume amounts to about 38 per cent. of the total
space occupied by the soil, but when this is not the case it may sink
to as low as from 10 to 15 per cent. of this space. With equally sized
particles the pore-volume is the same whether the individual particles
are large or small. In nature it will be the exception to find all the
particles of equal size, such a condition of things prevails only when
careful sorting by sifting or riddling has taken place, and in the
majority of cases the larger pores will be partly filled up by the
smaller particles of the soil.
2. PERMEABILITY OF SOIL.
[Sidenote: Permeability depends first on the size of the pores, and
secondly on the pore-volume.]
The permeability of a soil for the passage of air and water depends,
in the first instance, on the size of the pores, and is further to
some extent influenced by the pore-volume.
[Sidenote: Effect of large and small pores.]
Soil with large pores will offer but little resistance to the passage
of air and water, but when the pores are small these movements will be
greatly impeded.
[Sidenote: Permeability is proportional to the fourth power of the
pore-diameter.]
It has been ascertained that the permeability of soils is proportional
to the fourth power of the diameter of the pores, so that it decreases
very rapidly with the diminishing size of the pores.
[Sidenote: In frozen soil permeability decreases rapidly.]
In subsoil with small pores all movements of air practically cease
when it is half full of water, and in frozen soil the decrease of the
permeability is still more marked.
3. WATER CAPACITY OF SOIL.
[Sidenote: Water capacity is equal to the pore-volume.]
[Sidenote: Air can never be wholly driven out of the pores.]
The water capacity of a soil is that quantity of water which can be
stored in its pores; it is therefore equal to the pore-volume. For
very accurate measurements allowance must be made for a small amount
of air, which even after filling remains in the pores and cannot be
dislodged, but for practical purposes this can be overlooked.
[Sidenote: 1 cubic yard of soil with particles of equal size will hold
about 85 gallons of water.]
As has already been stated, the pore-volume of a soil consisting of
equal particles throughout, amounts to about 38 per cent. of the space
occupied by it, and 1 cubic yard of such a soil—whether we have to
deal with coarse gravel or fine sand—will hold about 85 gallons of
water.
4. WATER-RETENTIVE POWER OF SOIL.
[Sidenote: The water-retentive power of soil is a percentage of its
water capacity.]
The water-retentive power of a soil is expressed by that quantity of
water which can be retained by it; it will always be a percentage or
portion of the water capacity of this soil.
[Sidenote: Soil with a large pore-volume and a large percentage of
fine pores retains more water than soil with a small pore-volume and
large pores.]
[Sidenote: Clean gravel retains about 10 gal. and clean sand about 70
gal.]
Soil with a large pore-volume and with a large percentage of fine
pores will retain more water than soil with a small pore-volume and
few fine pores. Clean gravel will retain about 12 per cent. of its
water capacity, i.e. 10 gallons per cubic yard, whereas fine sand may
retain as much as 84 per cent. of its water capacity, i.e. about 70
gallons per cubic yard.
[Sidenote: Organically polluted soil retains more water than clean
soil.]
This will explain why a polluted subsoil containing a large amount of
organic substances will retain more water than the same soil in a
clean condition.
[Sidenote: The retentive power of a soil is due to its surface
attractions.]
The retentive power of a soil is due to the surface attraction of its
particles, and when the space between them is small, or when, in other
words, the pores are small, this attractive power will be all the
greater.
[Sidenote: When, after the limit of the retentive power has been
reached, of water are poured upon the soil, a portion of the
previously stored water is driven out, and its place in the pores
taken up by the fresh supply.]
It is further of interest to observe here, that if after the limit of
the retentive power has been reached further quantities of water are
poured upon the soil, the water retained in the lower layers will
commence to drain away. This means that the water freshly poured upon
the soil will drive out a portion of the water previously stored in
the pores. It is important to bear this in mind when dealing with
polluted water, as owing to this action the water penetrating into
deeper layers will to some extent at least have become purified in the
upper layers.
5. CAPILLARY MOVEMENTS OF WATER IN SOIL.
[Sidenote: Capillary attraction causes an upward movement of the
water.]
Through capillary attraction an ascending movement of the water is
caused in direct opposition to the laws of gravity, and the height to
which water will thus ascend depends mainly on the smallness of the
pores; large pores do not assist in this movement. As the same,
however, extends over the whole pore-volume the quantity of water thus
raised may exceed the water-retentive power of soil.
[Sidenote: Capillary attraction also causes lateral and downward
movements.]
In addition to the upward movement brought about by capillary
attraction, this power is also continually at work in a lateral and
downward direction; but for the present purposes only the upward
movement will be noticed.
[Sidenote: Time occupied by upward movement. Height reached by upward
movement.]
In observing the upward movement, it is interesting to notice the time
occupied by it and the total height reached. As to the time occupied,
it has been established that the upward movement in gravel and coarse
sand is much quicker than in fine and loamy sand, but the heights
attained are reversed. For whereas the height in a material consisting
of coarse or large pores amounts to from 2 inches to 4 inches; a
height of about 4 feet after thirty to thirty-five days has been
recorded in fine or loamy sand; in peaty soil one observer states that
the upward movement of the water may reach a height of 20 feet.
6. TEMPERATURE OF SOIL.
[Sidenote: Three principal sources of heat.]
The earth’s crust receives its supply of heat from three principal
sources, viz.:
1. From the sun through its rays;
2. From the interior of the earth through conduction; and
3. From various physical and chemical processes which take place in it
and create heat.
[Sidenote: Heat through sun’s rays.]
[Sidenote: Dark soils absorb more heat than light-coloured soils.]
[Sidenote: Capacity for heat is greater in damp and fine-grained
soils.]
[Sidenote: Evaporation and condensation of aqueous vapour produce the
greatest effect in fine-grained soils.]
Dealing with the upper layers of the crust, it may be said that,
besides the intensity of the sun’s rays, the temperature also depends
on a variety of properties possessed by various kinds of soil, amongst
which latter may be mentioned the absorption of heat, which is much
greater in dark than in light-coloured soils; the heat conductivity
and the capacity for heat, which lead to higher temperatures in damp
and fine-grained soils; and finally the evaporation and condensation
of aqueous vapour, which tend to prevent extremes of heat and cold and
which likewise produce the greatest effects in fine-grained soils.
[Sidenote: A fine-grained damp soil does not get so hot, but retains
the heat better.]
It follows from these observations that a coarse-grained, dark
coloured and dry soil will show the highest and lowest temperatures,
whereas a fine-grained damp soil does not get so hot but retains the
heat better.
[Sidenote: The temperature of the surface of the soil may exceed that
of the air.]
It ought to be pointed out in this place that a variety of
circumstances may bring about very high temperatures on the surface of
the ground which considerably exceed the average temperatures of the
air at the same time.
[Sidenote: Laws regulating the subsoil temperatures.]
Concerning the laws that have been deduced from careful and long
continued observations of subsoil temperatures, it will not be
necessary at this point to deal minutely with them; it must on the
contrary suffice to summarise only the more important ones.
With the distance from the surface of the ground,
1. The differences of temperature become less,
2. The temperatures are retarded, and
3. The variations of short durations gradually disappear.
[Sidenote: Subsoil temperatures 18 in. below surface.]
[Sidenote: Subsoil temperatures at depths of 4 ft. 6 in. and 9 ft.]
At a depth of 18 inches below the surface the daily fluctuations are
hardly observable, the temperature differences of various days become
obscured, the differences between the monthly mean temperatures are
less by several degrees, and the yearly fluctuation amounts only to
about 10° C. At a depth of 4 feet 6 inches the latter is only 4° C.,
and at a depth of 9 feet it is only 1°C.
[Sidenote: Subsoil temperatures at depths from 9 ft. to 33 ft.]
Between 9 and 33 feet, according to the yearly mean of the surface,
the yearly fluctuation ceases and the temperature remains the same
throughout the year.
Below this point an increase of temperature is observable towards the
earth’s centre, which amounts to about 1° C. for every 40 feet.
[Sidenote: Retardation of temperatures with increase in depth.]
Concerning the retardation of the temperatures with an increase in
depth below the surface, it is interesting to point out that this,
according to Fodor, amounts to about three weeks for every yard, so
that the yearly maximum at a depth of 1 yard will take place in
August, at a depth of 2 yards in the beginning of September, and at a
depth of 4 yards in October. This is on the assumption that the
maximum temperature of the atmospheric air is reached in July.
[Sidenote: Frost depth about 3 ft.]
The depth to which frost under ordinary conditions penetrates is about
3 feet, but there are cases on record where water pipes at depths of
from 4 to 5 feet have been frozen up during long continued severe
frost.
7. SUBSOIL AIR.
[Sidenote: Subsoil air is saturated with aqueous vapour and contains
large quantities of carbonic acid.]
The pores of soil are either partly or wholly filled with air, which
as a rule is saturated with aqueous vapour. This air consists very
largely of carbonic acid (from 0·2 to 14 per cent., on an average from
2 to 3 per cent.) and to a small extent of oxygen, which has been used
up for the formation of carbonic acid. It also contains traces of
ammonia and gases of decomposition.
The movements of subsoil air need not be considered here, and beyond
these few general observations it will not be necessary to deal with
the subject.
8. MOVEMENTS OF WATER IN SOIL.
[Sidenote: Strata above level of subsoil water.]
Two main strata may here be distinguished in subsoil, one above the
level of the subsoil water and one below this level. The latter strata
do not interest us, and those above the level of the subsoil[7] water
may again be subdivided into three zones, which in descending order
are as follows:—
The evaporation zone;
The passage zone; and
The capillary zone.
[Sidenote: One-third of the rain-water evaporates. One-third flows off
the surface. One-third percolates.]
All these three zones must be passed by the water in its descent from
the surface of the ground to the subsoil water level, and the quantity
of water retained by them will depend on their state of dryness.
Speaking quite generally and within wide limits, one-third of the
rain-water flows off the surface, one-third evaporates, and one-third
percolates into the subsoil.
[Sidenote: Evaporation zone.]
The evaporation zone reaches from the surface of the soil to that
point below, which marks the extent of the drying influence of the
atmospheric air. In the same the quantity of water stored in the pores
may at times sink below the retentive power of the soil, i.e. below
that quantity which can be retained in the pores owing to the
mechanical powers of adhesion, etc. When it has become very dry
through evaporation and other causes the zone, especially when it
extends some way down, may retain large quantities of water. In a
depth of 10 inches, 1 square yard of soil, with fine pores, may retain
about 10 gallons of water, and as a rainfall of ½ inch produces only
2·3 gallons per square yard, it is clear that subsoil of this nature
may retain a number of successive showers. During the height of summer
fine porous soil may become so dry that practically no water finds its
way into deeper zones; in this state the evaporation zone can be
compared to a large sponge.
[Sidenote: Passage zone.]
The next zone traversed by the water in its downward movement is the
passage zone, which lies beyond the drying influence of atmospheric
air. When too far removed from the level of the subsoil water, its
pores will not be completely filled with water, but will only contain
that amount which is due to the retentive powers of the soil. By
direct measurement it has been found that on an average a cubic yard
of fine porous soil will retain from 30 to 80 gallons of water, and it
can easily be calculated that in a layer from 1 to 2 yards in
thickness the rainfall of a whole year may be retained. The passage
zone, especially if it is of considerable thickness, represents a very
large storage reservoir.
[Sidenote: Capillary zone.]
The last zone before the level of the subsoil water is reached is the
capillary zone, in which the pores are partially or wholly filled by
the upward movement—due to capillary attraction—from the subsoil
water. The extent of this filling will depend on the size of the pores.
[Sidenote: Springs.]
When the descending water has finally reached the subsoil water it
either comes to a standstill altogether on the impervious layer or
moves along the same, if the latter is not horizontal, until it may
eventually leave the subsoil again by issuing therefrom in the form of
visible or invisible springs.
[Sidenote: Rate of downward movement governed by pores.]
The rate of movement of any liquid—rain-water, sewage or other
polluting liquid—is largely governed by the size of the pores. Where
these are large, as for instance in coarse gravel, the descent of the
water will be comparatively rapid, but when they are small it may take
a very long time before the water reaches the level of the subsoil
water, and in that case it will have undergone material changes as
regards its chemical or bacterial composition.
[Sidenote: With a high level of subsoil water zones become
indistinguishable.]
With a high level of subsoil water the zones may become
indistinguishable, one zone reaching into the other, with the result
that the whole of the soil becomes very wet.
When subsoil has been artificially drained the amount of water
reaching the subsoil water below the general level of the drains will
depend on the size of the latter and the distance between them. In
such a case the downward movement of the water through undrained soil,
previously described, may be further interfered with through the
ventilation of the subsoil by drains, and the drying up action caused
thereby.
[7] The term subsoil water is here used to denote that
portion of the water in the pores of the soil, which is
either at rest on or moves along the inclined plane of an
impervious layer.
9. THE MICRO-ORGANIC LIFE IN SOIL.
[Sidenote: Soil probably original home of micro-organisms.]
[Sidenote: Distribution of micro-organisms in soil.]
The soil is probably the original home of all micro-organisms, from
which they have emigrated into other media. It contains vast numbers,
and, according to some observers, 1 ccm. may hold 100,000 germs. By
far the greater number is found on or near the surface, and in lower
layers the numbers gradually diminish, until at last a depth is
reached, which depends on local conditions, where the soil is
perfectly sterile. The aerobes live near the surface and carry on
their work in this region, whereas the anaerobes are at work lower
down in the soil.
[Sidenote: Cycle of micro-organic activity during the year.]
The picture of the cycle of micro-organic activity in the upper layers
of the soil during the various seasons of the year is probably the
following. In winter, especially during that period when frost and ice
bind the earth, micro-organic life is apparently at its lowest ebb,
and may in some very cold climates come to a standstill altogether,
when micro-organisms may be said to hold their vegetative winter
sleep. With the return of life and the awakening of nature in
spring—especially with the approach of higher temperatures and the
formation of moisture—micro-organic activity once more makes itself
felt all round. During the summer months it is exposed to some
injurious influences such as the heating and drying up of the upper
layers of the soil, but, still gradually increasing, micro-organisms
reach the climax of their activity during the autumnal rains, to
remain in this state until with the advent of the cold season their
activity gradually declines again.
[Sidenote: Micro-organic life in layers from 3 ft. to 6 ft. in depth.]
In the lower layers of the soil, down to 3 feet and 6 feet,
micro-organisms are more protected against the injurious influences of
the atmosphere, sunlight and drying up, but the want of oxygen,
together with the greater difficulty of removing such products as
carbonic acid, has an injurious influence. As the temperature in these
layers is considerably more uniform, it may be inferred that the
micro-organic activity is there of a more uniform kind, less
influenced by sudden changes, probably also less intense, but without
pronounced periods of rest.
[Sidenote: Micro-organisms probably quickly perish in depths greater
than 6 ft.]
In depths greater than 6 feet micro-organisms probably perish very
quickly owing to unfavourable conditions, and if found their presence
must be explained by emigration from higher layers, not by actual
growth at these depths.
On sewage farms the micro-organic activity is without doubt greatly
modified, and proceeds all the year round at a more uniform rate than
on ordinary land, as the sewage always contains the necessary warmth
and moisture so beneficial for it.
10. THE ABSORBING POWERS OF SOIL.
[Sidenote: Absorbing powers due to surface attraction of the particles
of the soil.]
[Sidenote: The finer the pores the greater the absorption.]
The absorbing powers of soil are due to the surface attraction of its
particles or grains, and these, as has already been pointed out, will
be all the greater the finer the pores are; they extend on the one
hand to aqueous and other vapours and gases, and on the other to
matters in solution.
[Sidenote: 1 cub. yd. of coarse gravel may contain 50 sq. yds. of
surface and 1 cub. yd. of fine sand 9200 sq. yds.]
That the attractive force of the surface of the particles is pretty
considerable will be at once apparent when it is stated that 1 cubic
yard of coarse gravel may contain about 140,000 grains with a combined
surface of 50 square yards, and 1 cubic yard of fine sand 40 million
grains with a combined surface of 9200 square yards, which is a little
under 2 acres.
[Sidenote: Deodorising action of soil absorption of gases.]
Concerning the absorption by soil of aqueous vapour and gases (apart
from condensation through a fall in temperature), dry soil with fine
pores acts most energetically. The almost instantaneous deodorisation
of foul-smelling gases, such as are formed by decomposing fæcal
matters (earth closet) or coal gas, through a thin layer of fine dry
soil is well known, and is to be explained in this way.
[Sidenote: Absorption of dissolved substances by soil.]
More interesting still, and also more important, is the absorption of
dissolved substances by soil. In this way is to be explained the
decolorising effect and the retention of dissolved polluting
substances such as are contained in sewage. In the same way soil has
the power of destroying such poisons as strychnine, nicotine, coniine,
etc., and the experiments of Falk and others go to show that ptomaines
and toxines are likewise retained and rendered harmless by it. This
absorbing power of soil is of the utmost importance in agriculture,
and without it soil could not possess purifying powers for polluting
liquids. It is quite true that in this process of purification other
factors play an important part, but they could not come into play if
this absorption did not exist.
The absorbing powers of soil are in some way dependent on the presence
of micro-organisms and air, and in the absence of these they will soon
come to a standstill.
IV. SELF-PURIFYING POWERS OF SOIL. NATURAL SELF-PURIFICATION OF
SEWAGE.
[Sidenote: Self-purifying powers of soil.]
After these preliminary remarks it becomes necessary now to examine
into the self-purifying powers of soil with special reference to
sewage farms. Generally speaking, the term “self-purifying powers
powers of soil” comprises all those processes which go on on the
surface and in the pores of the soil of sewage farms, and by which
polluting liquids such as sewage become purified as these take place
under natural conditions and in a natural medium, the process of land
treatment of sewage is called—see previous observations—“the natural
self-purification of sewage.”
[Sidenote: Self-purifying powers vary with local conditions.]
[Sidenote: Soil best suited for sewage farms.]
It should be stated at the outset that the self-purifying powers of
soil will depend largely on the soil itself and the local conditions
under which they come into play, so that observations made in one
locality will not be immediately applicable to others without making
full allowance for the differences; this will be clear from the
preliminary remarks as to the character and properties of soils made
in the previous pages. As will be pointed out more in detail later on,
a subsoil that combines great permeability for air with high retaining
and absorbing powers, is best suited for sewage farms.
Let us now consider what becomes of water, sewage or any other
polluting liquid containing organic substances after it has been
poured out upon the surface of the ground, and for this purpose we
will assume a subsoil of a suitable character and in fair condition
for work with proper under-drainage.
[Sidenote: Retention of liquid by pores of soil.]
The liquid thus poured out upon the surface will sooner or later
disappear in the soil, and will at first be retained in the pores of
the zone of evaporation, which may be said to extend to the level of
the under-drains. This retention is due to the retentive powers of
soil.
[Sidenote: Suspended matters retained on the surface, soil acts like a
sieve.]
[Sidenote: Coating of surface of the land.]
[Sidenote: Removal of suspended matters generally an advantage.]
Portion of the suspended matters will be retained on the surface and
the rest will be strained out in a mechanical manner in the pores, the
soil acting as a sieve more or less fine according to its character. If
the suspended matters are present in very large quantities it may
happen that they will gradually form a coat on the surface of the land
and choke the pores to the exclusion of air, and as this is a thing to
be avoided in sewage farming it is in most cases advisable to remove
them out of the liquid before it is poured upon the land.
[Sidenote: The more finely divided the suspended matters are, the
lighter the work of the land.]
Even where such a removal has taken place there will still be left a
certain portion of the suspended matters, and if these are in a finely
divided state, such as is probably the result of their passage through
fine strainers or pump valves, the work of the land will be
considerably lightened.
[Sidenote: Micro-organisms screened out in a mechanical way.]
The micro-organisms contained in the liquid will be to a large extent
screened out in a mechanical way with the suspended matters and
deposited on the surface and in the upper layers of the soil.
[Sidenote: Retention of matters in solution after removal out of the
liquid is due to physical and chemical agencies.]
The matters in solution will partly, after removal out of the liquid,
be retained by the absorbing powers of the soil in the pores, a
process that is due to physical and chemical agencies.
[Sidenote: Absorbing powers gradually ripen.]
It is well known that land which is being treated with sewage for the
first time does not purify sewage so well as land that has been under
systematic treatment for some time, and this is probably due to the
absorbing powers, which gradually ripen until they have reached their
maximum of efficiency. This process of gradual improvement seems to be
due to the formation of a slimy coating round each particle of soil,
which growth does not only assist mechanical filtration, but also
possesses high powers of absorbing oxygen.
[Sidenote: Depths to which polluting substances may penetrate into
soil.]
The depth to which polluting substances may penetrate into soil will
probably differ in each case, but the following factors may be said to
influence it, viz. the velocity of the downward flow, the nature and
degree of the polluting liquid, and the character of the soil. Where,
therefore, the powers of the soil are over-taxed the polluting
substances may reach the level of the underdrains and pass out through
them, in which case the effluent will be but little better than the
raw liquid. It must be the aim of careful management to avoid this.
[Sidenote: Process of decomposition of organic matters stored in soil
during periods of rest.]
The polluting substances of an organic nature thus stored in the pores
undergo here—and that probably chiefly during periods of rest—a
process of decomposition or disintegration, which goes on until the
whole of the organic matter has been converted into stable mineral
forms.
[Sidenote: Explanation of the term “self-purifying power of soil.”]
This process of retention, absorption and decomposition of organic
impurities is called “the self-purifying power of soil.”
[Sidenote: After conversion substances are removed out of the soil by
the plants, by the subsoil air and subsoil water.]
The substances thus converted do not remain in the pores, but they are
removed either by the plants, for which they act as food, or by the
currents of subsoil air, or by the subsoil water, and as the removal
of fertilising substances by the subsoil water indicates a waste it
must be the aim of a careful management to utilise them as much as
ever possible for the benefit of the plants.
[Sidenote: Process of digestion. “Sewage sick.”]
The whole of these intricate and very complicated changes may be
likened to the process of digestion in animals, and when these
digestive powers are overtaxed signs of sickness may be noticed as the
inevitable result, which increase until, in sewage phraseology, the
land becomes “sewage sick.” In this condition it remains until the
flow of the polluting liquid is stopped, when after a period of
rest—recreative period—the digestive powers gradually return and begin
to do their work afresh.
[Sidenote: Action of lime.]
When the soil of a sewage farm has got into this state, owing to
having received heavy doses of sewage, the application of lime has
proved very beneficial by accelerating the process of nitrification,
and in this respect interesting experiments have been made on the
Berlin sewage farms. The action of lime is said to be a twofold one.
1. It quickly attacks and splits up the organic matters and
accelerates afterwards their decomposition and their utilisation by
plants; and
2. It neutralises the excess of acid in the soil, and causes the
latter to part with its carbonic acid.
[Sidenote: Decomposition proceeds quickest at or near the surface.]
The process of decomposition proceeds as a rule at a much quicker rate
on the surface and in the upper layers of the soil, where, as already
mentioned, the number of micro-organisms is greatest.
[Sidenote: When carefully worked there is no time limit to the
purifying powers of the soil.]
It has been maintained that the soil of sewage farms will after a
while silt up and cease to purify sewage, but the results obtained
with carefully managed farms clearly disprove this, and under these
conditions there appears to be no limit as to time to the purifying
power of soil.
[Sidenote: Depth of soil necessary for purification.]
Concerning the depth of soil—evaporation zone—that is necessary for
the successful retention, absorption and decomposition of sewage, no
generally applicable rule can be laid down, as this will depend on a
variety of factors, amongst which may be mentioned: the character and
thickness of the top soil (humus), the nature and cultivation of the
top soil; the character of the subsoil—its permeability for air and
its retaining and absorbing powers; the surface slopes of the land and
the level of the subsoil water.
[Sidenote: Greater depths than 4 ft. will be rarely necessary.]
On some farms a depth of 3 feet on an average has proved sufficient,
and on others the drains have been laid at depths ranging between 3
and 6 feet, but very special reasons ought to be shown for all depths
over 4 feet.
[Sidenote: Soil best suited for sewage farms.]
Whilst practically no soil is entirely useless for sewage farming,
with the exception perhaps of peat, owing to the quantity of moisture
it contains, a soil that combines great permeability for air with high
retaining and absorbing powers—such as a loamy sand with fairly large
grains—is probably the best.
[Sidenote: Clay soil not unsuitable for sewage farms, but it
necessitates a greater area of land.]
It has been maintained that clay, owing to its impervious character,
is totally unsuitable for sewage farming, but the experience of such
farms as South Norwood, Wimbledon, Warwick and Leicester disproves
this. It is true, however, that as the purifying powers of the soil
are restricted in a vertical sense to the upper layers, it may become
necessary in places to extend the area of the farm beyond what would
be necessary with a more pervious soil.
[Sidenote: Changes observed in the heavy clay land at Leicester since
sewage treatment was commenced.]
It may not be without interest to draw attention here to some of the
changes that have taken place on the Leicester sewage farm since the
land has received regular dressings of sewage. When I was engaged in
laying it out in 1888 my powers of locomotion over the land were
greatly impeded during wet seasons by the inordinate amount of clay
that adhered to the boots; but when engaged again for some
considerable time on the land during the winter 1900 to 1901 this
unpleasant peculiarity had completely disappeared even on land that
had recently been sewaged. Through the action of the sewage the very
dense clay had been disintegrated and become so pliable that, when
trod upon, it crumbled to pieces. The colour of the soil had been
changed from a yellowish-brown to a greyish-black, and altogether the
land had been greatly improved by the application of the sewage.
[Sidenote: Movement of liquid through the passage and capillary zones
to the impervious layer.]
If more sewage is poured upon the land than the effluent drains can
deal with—and here it may be well to bear in mind that on sewage farms
in our climate on a broad average throughout the year about one-third
of the total quantity is lost by evaporation—the excess will pass down
between the drains from the evaporation to the passage zone, and if
the flow of the sewage is not discontinued the downward movement in
the passage zone may be continued until, after having traversed the
capillary zone, the level of the subsoil water is reached.
[Sidenote: Length of downward movement of water may be very great.]
What length of time may elapse before this level is reached will
entirely depend on local circumstances, but it will be clear from the
preliminary remarks that the completion of this downward movement may
in places and under certain conditions take a very long time.
[Sidenote: Displacement of sewage held by the pores of the land by the
fresh discharge of sewage upon the surface of the land.]
In connection with this it is of importance to point out that not the
fresh sewage which is poured on the surface of the land will at once
pass into the lower layers, but a portion of the old sewage, which up
to then was stored in the pores and is now displaced by the fresh
discharge, so that the fresh raw sewage is retained and only purified
sewage allowed to escape into deeper layers, which means that in its
downward movement all sewage undergoes purification. Were this not the
case the raw sewage might reach the effluent drains.
It appears time now to examine somewhat more closely the processes of
decomposition and the products elaborated therein.
[Sidenote: Factors that influence the process of decomposition.]
Concerning the factors which have a favourable influence upon this
process, some of them, such as permeability for air, high retentive
and absorbing powers, have already been mentioned, and to these can be
added moisture and warmth, the latter of which are always present in
sewage.
[Sidenote: Advantages of a systematic underdrainage.]
One word here concerning the systematic under-drainage of the subsoil.
Its chief function is, of course, the carrying away of the effluent
water and by doing so to prevent the formation of a swamp, but after
the land has done its work, and during so-called periods of rest, the
under-drains act as ventilators of the subsoil and thus make it
artificially more permeable for air, with the result that a drying-up
action is set up and oxygen supplied for micro-organic life. For the
purpose of improving the ventilation of the soil it may become
advisable in places to connect the upper ends of the drains with a
short upcast shaft. The mouths of the drains should always discharge
above water so as to allow of a free circulation of air.
[Sidenote: Micro-organisms that carry on the work of splitting up and
converting organic compounds.]
The work of splitting up and converting the organic compounds is
primarily carried out by micro-organisms such as yeast fungi, mould
fungi, algæ, protozoa and even by higher forms of life such as
earthworms and insects.
To what extent in addition to these other agencies take part in this
bio-chemical process is not yet fully elucidated.
[Sidenote: Decomposition and putrefaction most complicated processes.]
Fischer in his interesting book, ‘The Structure and Functions of
Bacteria,’ observes (page 99): “The decomposition of dead animal
bodies, of vegetable tissues, or of substances like stable manure, is
far from being a simple putrefactive process. Side by side with the
disintegration of nitrogenous bodies there are going on a number of
fermentative changes by which non-nitrogenous compounds are being
broken up, besides nitrification and other bio-chemical processes. For
this reason it is always difficult and often impossible to determine
the respective parts played by the different species of bacteria.…
“The phenomena of putrefaction are so complicated that we do not know
all of the compounds that arise during the process.… Very careful
chemical investigations on pure cultures will be necessary before the
chaos of phenomena presented by putrefactive bacteria can be arranged
in something like order.
[Sidenote: In the decomposition of proteids five or rather six stages
may be distinguished.]
“Proteids are split up by putrefaction into a large number of simpler
compounds both nitrogenous and non-nitrogenous. The substances thus
produced are precisely similar to those resulting from the artificial
decomposition of proteids by fusion with caustic potash or boiling
with hydrochloric acid or barium hydrate. Five groups may be
distinguished:
[Sidenote: Albumoses and peptones.]
“1. Albumoses and peptones: soluble diffusible bodies closely
resembling albumen. They are produced by the action on albumen of
bacterial enzymes, similar to the enzymes (pepsin and pancreatin)
which give rise to peptones in the digestive tract of man.
[Sidenote: Aromatic compounds.]
“2. Aromatic compounds; among others indol and skatol, which give the
characteristic odour to human excrement; also some non-nitrogenous
substances such as phenol, phenylacetic acid, and phenylpropionic
acid.
[Sidenote: Amido compounds.]
“3. Amido compounds, all nitrogenous: leucin, tyrosin, aspartic acid,
glycocol.
[Sidenote: Fatty and aromatic acids.]
“4. Fatty and aromatic acids, all non-nitrogenous and therefore having
no part in the circulation of nitrogen; acetic, butyric, succinic and
valerianic acids.
[Sidenote: Inorganic end-products of putrefaction.]
“5. Inorganic end-products of putrefaction: free nitrogen, ammonia,
free hydrogen, methane, carbonic acid, methylmercaptan, sulphuretted
hydrogen. It is probable also, but not certain, that phosphuretted
hydrogen is formed and is oxidised at once by the free oxygen of the
atmosphere.
[Sidenote: Ptomaines.]
“Most of these substances are formed also by the chemical decomposition
of proteids, but there is a sixth group which may be termed specific
putrefactive products. These are the so-called ptomaines or
putrefactive alkaloids.”
Some of these bodies are either not poisonous or only poisonous in
large doses, whilst others, derived from putrid foods of various kinds
(sausages, cheese), are highly toxic (ptomatropine, tyrotoxine).
[Sidenote: Definition of some terms.]
Concerning the work done by micro-organisms, it may not be out of
place here to define the meaning of certain terms, and to direct
attention at the same time to the modifications in the results brought
about by the presence or absence of air during the various stages of
the process.
The terms mineralisation, disintegration, oxidation, hydrolysis,
bacteriolysis, nitrification, decomposition, eremacausis,
putrefaction, fermentation, etc., have by many been used somewhat
promiscuously, and this has led to a good deal of confusion,
bewilderment and misconception. The cause of this has been undoubtedly
our small amount of knowledge concerning this process and the changes
brought about therein, but this would appear to be no reason why
complication should be made worse. For the purposes of these remarks
the undermentioned terms shall have the following meaning.
[Sidenote: Mineralisation.]
The term “mineralisation” is used for describing the whole process of
the disintegration and conversion of organic into mineral matter, and
no distinction shall be made between organic matter containing
nitrogenous and organic matter containing carbonaceous substances.
[Sidenote: Aerobic fermentation or decomposition.]
When this process of mineralisation is carried on in the presence of
sufficient quantities of air it is called “aerobic fermentation,” or
“decomposition,” which is generally characterised by the absence of
strong smells. The process may then be called one of complete
oxidation.
[Sidenote: Anaerobic fermentation or putrefaction.]
Where, however, the mineralisation proceeds in the absence of air the
process is called “anaerobic fermentation,” or “putrefaction,” and it
is then that very pronounced foul smells are emitted. The process may
then be called one of incomplete oxidation.
[Sidenote: Obligatory aerobes and anaerobes.]
[Sidenote: Facultative anaerobes.]
That class of micro-organisms which can only live in the presence of
oxygen is called “obligatory aerobes,” and that which can only exist
in the absence of this gas “obligatory anaerobes.” Between these two
is the group of “facultative anaerobes,” which, while growing best
with a plentiful supply of oxygen, are nevertheless able to exist with
a very small amount, and even with none at all, although in this case
their vitality is often much impaired.
[Sidenote: Organic matters are first split up and then converted into
mineral substances.]
In the process of mineralisation two stages may be distinguished, viz.
the first or disintegration stage, and the second or oxidation stage,
i.e. the organic substances are first split up and afterwards
converted into inorganic ones; and frequently these processes are
taking place side by side and not after each other.
[Sidenote: The splitting up of organic substances is frequently
carried out in the presence of air.]
It has been maintained—probably with a view to justifying the
necessity of a septic tank—that the preliminary process of splitting
up is best carried out in the absence of oxygen, but sufficient proof
does not appear to have been advanced in support of this statement,
and in some cases at any rate it is evidently carried out quite
satisfactorily in the presence of air.
Concerning the presence or absence of oxygen, Fischer observes as
follows:—
“The effects of the presence of oxygen are somewhat better
understood. If air have free access, putrefaction
(decomposition) may go on without any odour at all, the
evil-smelling gases (NH{3} and SH{2}, for example)
being oxidised at once to form nitrates and sulphates.
Aerobic bacteria, too, such as the nitre and sulphur
bacteria, bring about this mineralisation of organic
nitrogen. Moreover, when air is circulating freely, there is
no accumulation of intermediate products such as skatol or
indol. It occurs on the surface of manure heaps, on the
outer surfaces of carcases, and in well ventilated soil.
“In anaerobic decomposition (putrefaction proper), as in
anaerobic fermentation, the organic molecules are at first
only partly disintegrated, intermediate products such as
leucine, tyrosine, skatol and indol being formed. In the
absence of air these accumulate, and hence it is that
putrefaction going on in the mud of ponds and ditches, or
inside carcases, is accompanied by such evil odours.
“Although the details of the process vary considerably,
according to the presence or absence of air, the ultimate
products of decomposition and putrefaction are in both cases
the same: namely, free nitrogen, free hydrogen, ammonia,
methane, carbonic acid and sulphuretted hydrogen. These are
also the end-results of the disintegration of the human
body.
“After the organic nitrogen of decomposing substances has
been converted into ammonia, and to a small extent into free
nitrogen, the latter can at once be utilised by the
root-nodule organisms and other bacteria in the soil, but
the ammonia must undergo two further changes and combine
with a base to form a nitric salt before it is available for
plant life. These two changes are brought about by bacteria,
which convert the ammonia first into nitrous and then into
nitric acid; this process has been called ‘nitrification.’”
It will be clear from the foregoing remarks that the process of
mineralisation is a very complicated one, which under favourable
conditions, for instance in the pores of an open soil, may come to an
end fairly quickly, but which under very unfavourable conditions—such
as the interior of large heaps of refuse—may last many years.
[Sidenote: Chemical purification on sewage farms.]
Concerning the chemical purification effected on sewage farms, i.e.
the purification of the sewage as revealed by chemical analysis, it
has been put on record over and over again, and is now fully and
universally understood, that suitable land well managed is capable of
changing even the foulest sewage to a perfectly clear water devoid of
smell and danger, so that this point need not be laboured here. For
instance, on the Berlin sewage farms the degree of purification
attained has averaged for a period of 20 years 97 per cent., and on
the farm at Gennevilliers—one of the Paris sewage farms—the effluent
is so sparkling, bright and clear that the inhabitants drink it in
preference to other available water.
[Sidenote: Micro-organic purity of effluent from sewage farms.]
But in reference to the purity of the effluent as to the products of
micro-organic activity and pathogenic micro-organisms, it will be
necessary to make a few observations with a view to remove
misconceptions that have from time to time been put forward.
[Sidenote: Ptomaines have not been found in effluents from well
managed sewage farms.]
The question whether the specific products of putrefaction, i.e. the
putrefactive alkaloids “ptomaines and toxines,” are capable of doing
further mischief by escaping with the effluent into the stream, may be
answered as follows. These substances are fortunately very unstable,
and the experiments conducted by Falk and others seem further to
indicate that soil is capable of retaining them and of rendering them
harmless. At any rate there is no well authenticated case on record
of these bodies having wrought mischief on sewage farms. (See here
also the remarks made on pages 51 and 52 under the heading “The
Absorbing Powers of Soil.”)
[Sidenote: Pathogenic germs on sewage farms.]
It has further been maintained that the presence of pathogenic
organisms on sewage farms might in two ways lead to mischief, viz.
either by transmission through air or by transmission through water.
The pathogenic organisms after spreading over the land might rise into
the air through the movements of the atmosphere and then be carried
about by it, or they might escape through the land and be conveyed
with the effluent into the stream or river that takes the latter.
[Sidenote: Pasteur’s fears as to mischief likely to be brought about
by pathogenic micro-organisms on sewage farms not borne out by facts.]
In connection with this point it may not be without interest to
mention here that even the late M. Pasteur at one time of his career
considered the wholesale spreading of disease germs on sewage farms
might prove highly injurious to the public health of the
neighbourhood. As he himself admitted, he based his fears on purely
theoretical considerations and opposed, for this reason, the extension
of the sewage farms in the neighbourhood of Paris. But when, later on,
he was made acquainted with the results observed on the Berlin farms,
he tacitly modified his views and ceased to oppose the extension of
the Paris farms.
[Sidenote: No well-authenticated case is on record where a sewage farm
has acted as the focus of a local outbreak of typhoid fever.]
Indeed, search as I might, I have not been able to discover one single
instance where a sewage farm has acted as the focus of a local
outbreak. On the contrary, during one or two small epidemics of
typhoid fever in Berlin, no case of this complaint has been observed
on the sewage farms of that city.
[Sidenote: Experience on the Berlin farms.]
Concerning the escape of pathogenic micro-organisms into streams and
rivers, no case is on record where such a thing has actually occurred:
indeed, the very painstaking investigations on the Berlin farms have
led to negative results.
[Sidenote: Observations made at the Freiburg sewage farm.]
Another sewage farm, that of Freiburg in Baden, has likewise been made
the subject of careful and long-continued investigation by Dr. Korn,
who, for the twelve months ending August 1897, made no less than 165
elaborate chemical and bacteriological examinations. Summing up his
observations on the presence of bacteria in the effluents from subsoil
drains, he remarks:
“Apart from the few exceptional cases of high numbers, generally
speaking my experiments show that the number of germs in the
subsoil drain effluents is relatively small, and even omitting
these experiments, in which a dilution with subsoil water must
have taken place, the number of micro-organisms is still so small
that the effects of filtration through soil are clearly
perceptible. In addition to this—and this is of considerable
importance in forming a judgment—it must be borne in mind that
the bacteria in sewage are principally derived from the
intestines, whereas in the subsoil drain effluents the
inhabitants of the intestines are either not present at all or
only in very small numbers compared with the number of soil and
water bacteria, which are always present. Out of 165 examinations
I only succeeded in 18 cases in proving the presence of bacterium
coli.”
[Sidenote: Bacterium coli no longer a true criterion of sewage
pollution.]
[Sidenote: Dr. Weissenfels’ conclusions.]
It may be convenient to point out in this place that bacterium coli
can no longer be looked upon as a typical inhabitant of the human
intestines after the very elaborate investigations carried out by Dr.
Weissenfels, who arrived at the following conclusions:
1. The so-called bacterium coli can be cultivated from almost every
kind of water, and its presence can be demonstrated in nearly every
case, provided a sufficient volume of water is utilised.
2. It is not possible by the result of the experiments upon animals to
decide whether the bacterium coli was cultivated from a pure or
infected water, and the discovery of a virulent bacterium coli in any
sample of water cannot, therefore, be regarded as a criterion that
such water has been polluted with fæcal bacteria.
After these remarks, it would seem quite possible that the bacterium
coli discovered in eighteen cases by Dr. Korn in the Freiburg
effluents was not derived from sewage at all but from the ordinary
subsoil water of the land.
[Sidenote: The possibility of further mischief by pathogenic
micro-organisms on sewage farms is exceedingly remote, if it exists at
all.]
Bearing these observations in mind, it is quite clear, therefore, that
neither theoretical investigations, as available up to now, nor
practical results, support the theory that pathogenic micro-organisms
may do mischief on sewage farms, and one is forced to conclude that
this possibility—if it exists at all—after systematic treatment on
land is an exceedingly remote one.
[Sidenote: Sewage farms reduce the quantity of final effluent.]
Before concluding these remarks on the natural purification of sewage
it is necessary to draw attention to another considerable advantage
which it possesses over artificial sewage treatments, and that is the
reduction in quantity of the effluent, which at times is very
considerable, whereas in the artificial methods such a reduction is
comparatively small.
[Sidenote: Loss of liquid by evaporation and by plant life.]
Spread over a large area of land, well cropped, evaporation is very
active—especially during the summer months, when the flow of water in
the brook that takes the effluent is as a rule at its lowest; and, in
addition to this, the growing plants further abstract a considerable
amount of the liquid that finds its way into the soil, so that the
quantity of the effluent may not be more than from 30 to 50 per cent.
of the total quantity that was poured over the land. In the artificial
treatment the evaporation is considerably smaller, and as plants are
altogether absent the quantity of the effluent is probably about 90
per cent. and more of the total quantity of the raw sewage. This is a
point of very considerable importance so far as the influence of the
effluent upon the water in the stream that takes the same is
concerned.
Although the subject of natural purification is by no means exhausted,
it is now time to direct attention to artificial methods.
V. ARTIFICIAL SELF-PURIFICATION OF SEWAGE.
1. GENERAL OBSERVATIONS.
[Sidenote: Enumeration of more important experiments.]
A great many experiments have been made during the last ten years with
artificial processes for the self-purification of sewage, and amongst
the more important the following may be mentioned:
London experiments.
Sutton „
Exeter „
Manchester „
Leeds „
Sheffield „
Leicester „
York „
Hamburg „
[Sidenote: Experiments have not been conducted on uniform lines.]
A casual observer might, therefore, consider himself justified in
thinking that all these experiments had added a great deal to our
knowledge of the intricate changes taking place in these processes,
but such a conclusion would not be justified in reality. For beyond
settling questions of local importance by chemical analysis, the
experiments, owing to a variety of causes, have not materially
enhanced the stores of our information, indeed not unfrequently the
results obtained are apparently contradictory and bewildering.
An experiment must be looked upon as a question addressed to nature,
and the answer will depend on the way the question has been put. If
this way differs in every case it must be clear that the answer, too,
will differ in every case, and it is this absence of uniformity which
greatly reduces the general value of these experiments.
These remarks must not be misunderstood to convey the impression as if
the experiments had not been conducted with care and skill! Far from
it! Some of them have been made with the greatest skill and care and
with the very evident desire to arrive at correct conclusions, and it
is only when they are placed side by side with other experiments, with
a view to deducing from them general conclusions concerning the
processes at work, that great difficulties are experienced. The result
of each experiment is governed by a large number of factors, which by
slightly different manipulations may attain in this ever-fluctuating
process different weights, so that the results may be contradictory,
and it is only by arranging these factors on a common basis, as it
were, and by addressing the questions to nature in the same systematic
and uniform way, that good general results may be expected.
It is well known, for instance, that in some cases septic tanks have
not given good results, whilst in others they have worked very well;
again, continuous filtration has failed in some experiments, whilst in
others, notably in the York experiments, it has given good results.
If, therefore, in future the mistake of the past is to be avoided, it
will be necessary to settle on a common line of action in all
experiments.
[Sidenote: Attempt to evolve general theory.]
In spite of all the difficulties which beset such a task, an attempt
will be made in the following observations to evolve some general
theory concerning the processes at work in the artificial
self-purification of sewage. Such a theory, it is quite clear, cannot
be complete in the present state of our knowledge, and it is sincerely
hoped that the many and serious gaps will be filled up by later
investigations.
For convenience of reference the different forms of the process, such
as are now employed, shall be dealt with separately, commencing with
contact or oxidation beds.
2. ARTIFICIAL SELF-PURIFICATION OF SEWAGE IN INTERMITTENT CONTACT
BEDS.
At the outset it may not be out of place to make a few remarks
concerning the various names given to this form of application. The
term “intermittent contact bed” is here used to distinguish this kind
of bed from the “continuous contact bed,” frequently called
“continuous filtration.”
[Sidenote: Names of process misleading.]
(_a_) _Name of Process._—This process has frequently been called
“biological process,” “bacteriological process,” “contact bed system”
or “oxidation bed system,” but all these terms do not appear to define
it sufficiently, as they do not cover the whole, but only phases or
stages in the same; hence, they do not seem appropriate.
[Sidenote: Biological process.]
The name “biological process” is decidedly misleading, for besides
biological agencies there are also at work physical (mechanical) and
chemical ones.
[Sidenote: Bacteriological process.]
The term “bacteriological or bacterial process” is likewise erroneous,
for besides bacteria a number of other micro-organisms participate in
it—such as yeast fungi, mould fungi, algæ, protozoa, and even higher
forms of life, such as earthworms and insects.
[Sidenote: Contact bed system.]
[Sidenote: Oxidation bed system.]
The expressions “contact bed system” or “oxidation bed system” are in
so far inappropriate as they describe only portions of the process but
not the whole. The term “contact bed” describes the first stage, and
the term “oxidation bed” portion of the second stage only.
[Sidenote: Term most suitable.]
The term which seems most suitable of all is “artificial
self-purification in contact beds,” as it includes every phase of this
lengthy process applied in an artificial form; the term “natural
self-purification” being applied to land treatment of sewage, as it is
the only method in which the self-purifying powers are employed under
natural conditions.
[Sidenote: Working operations.]
(_b_) _Explanation of Process._—The cycle of operations commences with
the filling of the bed, and during the same the sewage comes gradually
in contact with the filling material. When the bed is full, the inflow
is stopped and the sewage allowed to remain in contact with the
material for some time. The bed is then emptied, and a period of rest
is given it before the filling is commenced again.
[Sidenote: Purification of sewage in full bed due to absorbing powers
of filling material and only to a small extent due to activity of
micro-organisms.]
It has been held, that while the sewage is in the contact bed it
undergoes a very rapid process of decomposition by bacteria, but it
must be evident, that as the sewage—including filling—remains only for
about two hours in the bed, the micro-organisms would have to work at
an express rate. This fact alone is apt to make this theory very
doubtful, but apart from it, it has been proved by experiments that
the by far greater amount of purification—whilst the sewage is in the
beds—is due to the absorbing powers of the filling material, which are
derived from the surface attraction of its component particles.
[Sidenote: Retention of suspended matters by bed.]
[Sidenote: Absorbing powers of filling material.]
The filling material retains in its upper layers the suspended
matters, which it strains out of the sewage in a purely mechanical
manner, much after the fashion of a screen, and when the bed is filled
its absorbing powers come into play, which cause the removal of the
dissolved matters out of the liquid and their retention on the surface
of the particles. This latter process is probably a chemico-physical
one assisted by the micro-organic life in the sewage.
[Sidenote: Decomposition of organic substances by micro-organisms when
bed is empty.]
It is only after the bed has been emptied that the real activity of
the vast number of micro-organisms commences, which is directed
towards converting the organic substances into mineral ones. This
process of splitting up, decomposing, disintegrating and mineralising
organic waste products is an exceedingly complex one, which ever
fluctuates according to the prevailing conditions, and which does not
come to an end until finally stable mineral forms are reached. In the
presence of a plentiful supply of oxygen, the process proceeds as a
rule at a more rapid rate, and the intermediate forms produced are
less complex than in the comparative or total absence of this gas;
hence the progress of the process is largely determined by it. The
amount of oxygen necessary for bacterial activity is partly
abstracted, and with extraordinary energy, from the atmospheric air in
the pores of the filling material, and a portion of the substances
formed, such as carbonic acid and nitrogen—in gas form—escape into the
atmosphere, whilst the remaining portions are washed out of the bed
with other products, such as nitric acid, by the effluent.
Further remarks upon this process of mineralisation have been made in
connection with the subject of natural self-purification of sewage,
and these may be referred to here.
[Sidenote: Effluent from bed practically raw sewage as far as its
bacterial contents are concerned.]
The effect of the bed upon the bacterial flora of sewage is, as was to
be expected, but very slight, and it is on record now that, as far as
the micro-organic life is concerned, the effluent is to all intents
and purposes raw sewage.
[Sidenote: Silting up of bed.]
Some of the substances contained in raw sewage remain in the bed, no
matter how carefully the sewage has been previously strained, and
these, in combination with the slimy surface coating of the component
particles, the accumulation of mineralised substances in the pores,
the consolidation of the bed, the disintegration of the filling
material, and the liquid retained, lead gradually but surely to the
silting or sludging up of the bed.
[Sidenote: Theoretical original water capacity of bed.]
(_c_) _Water Capacity of Bed and Silting up._—The theoretical water
capacity of the bed, previous to commencing operations, is the
aggregate of the cubical space occupied by the pores or small passages
between the particles forming the filling material, and the pores of
the filling material itself; but in practice a certain amount of this
space is occupied by air, which it is impossible to dislodge
altogether in filling. The aggregate of the cubical space of the pores
may be called the pore-volume.
It is difficult to lay down general rules as to what the original
water capacity of a bed should be expressed in per cent. of the space
occupied by the filling material, but speaking within fairly wide
limits the following is somewhat near the truth.
[Sidenote: Original water capacity with spherical particles of uniform
size.]
When the particles forming the filling material are fairly spherical
and of equal size, the original water capacity of a bed amounts to
about 38 per cent. of the space occupied by the filling material; but
as in practice it is difficult to obtain spherical particles of
uniform size, the original water capacity is found to range from 35 to
45 per cent. of this space.
[Sidenote: Original water capacity with particles of different sizes.]
When, however, the particles are of materially different sizes, and
when the smaller ones fill up the spaces between the larger ones, the
original water capacity may sink down to as low as from 5 to 10 per
cent. of the space occupied by the filling material.
[Sidenote: Size of particles of filling material does, under certain
conditions, not affect original water capacity of bed.]
It has been further demonstrated that the water capacity of a bed is
not affected by the size of the particles, provided the latter are
spherical and of uniform size. In other words, the water capacity of
two beds filled with material of different sizes is the same, provided
the particles are spherical and of uniform size throughout each bed.
[Sidenote: Silting up of bed during regular work.]
[Sidenote: Rapid initial decrease of capacity.]
[Sidenote: Consolidation of bed.]
This original water capacity is, however, not maintained in regular
work, as has been pointed out already. Basing the observations on
regular work only, the original capacity decreases at first, after a
new bed has been started or after an old reconstructed bed has been
taken in hand, rapidly for some time and afterwards more slowly.
Graphically expressed, this decrease is not represented by a straight
line but approaches more nearly a parabolic curve. This initial rapid
decrease is chiefly due to the consolidation of the bed.
[Sidenote: Disintegration of filling material.]
In connection with the movements in the bed tending towards its
consolidation, it is also clear that the continual filling and
emptying operations cause the smaller particles to be washed out of
their original position and to be placed in the larger passages
between the filling material, and if this process is assisted by the
gradual disintegration of the particles composing the filling
material, it is clear that the pores must become smaller and smaller
in time, i.e. choked.
From these observations it follows that the filling material should be
a hard substance, which will only to a limited extent be subject to
this crumbling away process.
But besides these there are, as has already been pointed out, other
silting up agencies at work.
[Sidenote: Water-retentive power of filling material.]
Of the total quantity of sewage which has entered the bed a small
portion will always remain in it owing to the water retaining power of
the material. This power has sometimes been called “minimum water
capacity,” but as this name is liable to be misunderstood, it is
better to adopt here the term “water-retentive power” of material.
The quantity of the sewage retained by the bed varies with the
material and pore-volume, and is due to adhesion and capillary
attraction. The greater the pore-volume, and the greater the
percentage of fine pores, the greater is the quantity thus retained.
Clean gravel retains about 12 per cent. and fine sand about 84 per
cent. of its water capacity—i.e. expressed per cubic yard of filling
material, one cubic yard of clean gravel will retain about 10 gallons
and one cubic yard of fine sand about 70 gallons of water.
Through draining a bed for several hours through evaporation and other
atmospheric influence, a portion of the sewage retained is lost, but
the quantity so lost will vary continually with the circumstances
under which the bed is worked.
The water-retentive power of the filling material does not decrease
with the working of the bed, but increases, which in a large measure
is probably due to the slimy coat which forms round the surface of the
component particles, and to which reference is made in the following
paragraph.
[Sidenote: Slimy surface coating of component particles.]
A further silting-up agency is the slimy surface coating of the
particles of the filling material. This accumulation is well known to
all who have had to do with intermittent contact beds, and has been
described as spongy bacterial growth. The Manchester report for the
year ending March 27, 1901, contains on page 62 the following passage:
“This (spongy bacterial growth) is at once the cause of increased
efficiency in the bed and loss of capacity. On examining the material
of a contact bed in active condition, every piece is seen to be coated
over with a slimy growth. If this is removed it soon dries to a stiff
jelly, which can be cut with a knife. Under the microscope masses of
bacteria and zoogloea will be found to be present.”
[Sidenote: Accumulations of decomposed substances in the pores.]
In addition to this slimy surface-coating of the particles, there are
also found in the pores, especially in the upper layers of the filling
material—and in fine beds more so than in a coarse bed—accumulations
which are “akin to humus or garden soil.” They contain to a limited
extent only putrescible substances, and appear to be the remains of
organic matter decomposed by the activity of micro-organisms.
[Sidenote: Periods of rest will not permanently restore portion of the
original water capacity of the bed.]
It was formerly maintained with considerable persistency that periods
of rest would permanently restore to a systematically worked bed a
portion of its lost water capacity, but such a contention has been
proved to be wrong. It is quite true that immediately after periods of
rest an increase of the water capacity is very noticeable, which is
probably due to drying up processes within the bed during the rest,
but such an increase is not permanent and is lost again more or less
quickly; it is therefore only temporary and not permanent.
Where, however, a bed has not been systematically worked, i.e. where
it has been worked at a greater rate than is suitable, and where in
consequence of this a large quantity of undecomposed substances is
stored in it, a period of rest may permanently restore a portion of
the lost capacity; but this is due to the mineralisation of these
undecomposed organic substances during the rest.
It follows from these remarks, as has been stated above, that when the
organic substances are regularly decomposed during systematic work a
period of rest cannot materially affect the water capacity, and that
where a considerable permanent restoration of the water capacity takes
place the bed has not been properly worked.
[Sidenote: Decrease of capacity is accompanied to some extent by
increase of efficiency and _vice versa_.]
It would, however, be incorrect to assume that the silting up of the
bed affects its efficiency besides reducing the capacity. On the
contrary! To some extent decrease of capacity is accompanied by
increase of efficiency and _vice versa_!
[Sidenote: Higher capacity of beds in summer than in winter.]
At this point it ought to be stated that in the Manchester experiments
(see page 61 of the report for the year ending 27th March, 1901) a
higher average capacity is maintained during the summer than during
the winter, which is no doubt due to the greater activity of the
micro-organisms during the warm weather of the year.
[Sidenote: Raking of beds not advantageous.]
The raking of the surface does not materially affect the capacity of
the bed, and it is better to scrape off the matters retained on the
surface than to rake them into the body of the bed.
[Sidenote: Renovation of filling material either partially or wholly.]
It will be clear from these observations that, no matter how carefully
the bed has been worked, sooner or later a time will come when the
decrease of capacity becomes so pronounced as to render it impossible
any longer to treat the daily flow of sewage with the available plant;
and when this point has been reached a renovation, either partially or
wholly, of the filling material becomes an inevitable necessity.
[Sidenote: Minimum capacity of beds to be provided for.]
To provide for this at the outset, and thus avoid the difficulties of
reduced capacity, it seems advisable to lay down, when designing the
works, a minimum capacity, which will just allow the daily volume of
sewage to be treated by the plant, and which when reached will
necessitate the cleansing of the bed.
The idea, formerly frequently expressed, that the filling material
when rationally worked need not be renewed or renovated, can no longer
be maintained and is outside the reach of practical possibilities.
[Sidenote: Underdrainage of intermittent contact beds.]
At this place a word or two about the under drainage of intermittent
contact beds may not be out of place. It is of the greatest importance
that all drains should work well, and that the entrance of the sewage
into them should not lead to disturbance in the filling material,
especially should the tearing of portions of the filling material into
the drain pipes be avoided.
By carefully arranging the position, number, size and fall of the
master drains and branch drains, it is possible to reduce the
resistance so as to allow of a fairly even flow of sewage through all
the drains, and to prevent a great rush of water through the drains
near the outlet end.
[Sidenote: The presence of lime is of no consequence.]
In passing it may not be out of place to point out that the view,
formerly expressed, that an admixture of lime in some form would prove
advantageous to the purification of sewage, is not supported by the
experience gained.
[Sidenote: Absorbing effect increases with the time of contact.]
(_d_) _Absorbing Powers of Filling Material._—The absorbing effect of
any filling material seems to increase with the time of contact.
[Sidenote: Absorbing powers increase until bed has become ripe.]
It ought further to be pointed out that the absorbing powers of the
filling material gradually increase until the bed has become ripe.
This fact was formerly stated to be due to the development of the
proper micro-organisms within the bed, but it would seem to be
chiefly due to the slimy surface coating of the particles of the
filling material, or spongy bacterial growth, as it has frequently
been called, which does not only assist mechanical filtration but also
possesses high powers of absorbing oxygen.
[Sidenote: Absorbing powers soon cease in the absence of
micro-organisms and air.]
But it cannot be open to doubt that the absorbing powers of the
filling material are dependent in some way or other on the presence of
micro-organisms, for Dunbar has shown that in the absence of
micro-organisms and without periods of aeration these powers soon
cease.
[Sidenote: Oxygen is absorbed from air in the pores with great
energy.]
(_e_) _Consumption of Oxygen by the Filling Material._—The oxygen
necessary for the proper work of an intermittent contact bed is
abstracted with great energy from the atmospheric air, with which the
pores become filled during periods of rest. Through diffusion, and
through the vacuum created by the processes of absorption, further
quantities of oxygen are taken from the atmospheric air, even under
difficult conditions, and, as pointed out in the Manchester report for
the year ending 27th March, 1901, “there is, therefore, little need to
force air into a bed.”
[Sidenote: The oxygen taken up during aeration is not imparted to the
sewage at the next filling and does not escape in the effluent.]
The oxygen thus taken up is not imparted in gas form to the sewage
during the next filling, and the effluents from intermittent contact
beds are not saturated with oxygen. Dunbar states that the effluents
of a satisfactorily worked bed frequently only contain one cubic
centimetre of free oxygen per litre. Clowes reports a similar result
in his third report on the London experiments.
[Sidenote: The greatest quantity of oxygen is consumed during the
oxidation of the products formed by micro-organisms.]
There can be no doubt that by far the greatest quantity of oxygen is
consumed during the process of oxidation of the products formed by
micro-organisms from putrescible organic substances.
[Sidenote: Consumption of oxygen and formation of carbonic acid not
solely due to biological agencies.]
(_f_) _Formation of Carbonic Acid._—Dunbar has shown by his
experiments that the consumption of oxygen and the formation of
carbonic acid is not solely due to biological agencies, but is to some
extent the result of physico-chemical processes.
[Sidenote: More free carbonic acid contained in the effluent than in
the raw sewage.]
[Sidenote: By far the greatest portion of carbonic acid escapes into
the air.]
He further reports that in his experiments the effluents contained on
an average 100 milligram per litre more free carbonic acid than the
raw sewage, and that the quantity contained in the effluents
represents only a small portion of the total amount of carbonic acid
formed during the whole process. The by far greatest portion of
carbonic acid escapes into the air. Concerning the air in the pores of
the filling material during periods of aeration, Dunbar states that it
contains sometimes not less than from 6 to 10 per cent. carbonic acid.
[Sidenote: Nitrogen escapes in gas form into the air.]
(_g_) _Nitrogen._—It is quite clear from all experiments that a
considerable amount of the total nitrogen contained in raw sewage is
abstracted by the filling material of intermittent contact beds, and
it is interesting to ascertain what becomes of it! Does it accumulate
in the bed? In that case, one has a right to assume that the
satisfactory work of the bed would gradually cease! As this is,
however, not the case, and as, further, the sludge formed in the bed,
whether it be fairly fresh or very stale, only contains a very small
amount of total nitrogen, we must surmise that the nitrogen after its
retention by the bed escapes in gas form—like the carbonic acid—into
the atmosphere.
[Sidenote: The presence of nitric acid is not an unfailing guide for
determining the satisfactory character of the effluent.]
(_h_) _The Formation of Nitric Acid._—Concerning the presence of
nitric acid in the effluents from intermittent contact beds, Dunbar is
of opinion that it offers certain means for forming an opinion of the
processes taking place in the same, but that it is only in a
subordinate sense an indication of the degree of purification attained
and must not be taken as an unfailing guide for determining the
satisfactory character of the effluent.
[Sidenote: Nitrifying bacteria always present in town’s sewage.]
[Sidenote: Nitric acid is formed very rapidly, but only during periods
of rest.]
Nitrifying bacteria are always present in ordinary town’s sewage, but
it would appear that other micro-organisms besides Winogradsky’s
bacteria assist in the process of nitrification. Nitric acid is formed
very rapidly, but only during periods of rest, and besides aeration
other less powerful influences are at work.
[Sidenote: Reduction of nitric acid when bed is filled from bottom
with an upward flow.]
It is further interesting to note that, according to Dunbar, the
greater portion of nitric acid which has been formed during periods of
aeration becomes completely reduced in a very short time, when the bed
is filled with an upward flow from the bottom, and that only a small
portion remains in the form of nitrous acid.
3. ARTIFICIAL SELF-PURIFICATION OF SEWAGE IN SEPTIC TANKS.
[Sidenote: Septic tanks only used in combination with contact beds.]
Although it has never been claimed, and is further not open to doubt,
that a septic tank alone and unaided by subsequent treatment in
intermittent or continuous contact beds does not sufficiently purify
the sewage, in these remarks the work of the septic tank only will be
considered, as the treatment in contact beds will be dealt with
separately.
[Sidenote: Septic tank a suitable name.]
(_a_) _Name of Septic Tank._—A good many names have been suggested by
different observers—such as “anaerobic fermentation tank,” “putrefying
tank,” “liquefying tank,” “cess-pit,” etc.—but there appears to be no
reason why the name “septic tank” should not be adhered to, as it
describes sufficiently correctly the work done by the tank, which is
chiefly of a septic nature.
(_b_) _Covered or Open Septic Tank._—Before dealing with the processes
taking place in a septic tank, it will not be out of place to consider
here, shortly, whether a closed septic tank confers advantages over an
open septic tank sufficiently great to justify the considerably
greater expenditure necessitated by its construction.
It is well known, that at Exeter in the first experimental
installation of this process, the septic tank was covered in by an
arched roof; but subsequent experiments made elsewhere do not seem to
support the theory then advanced, that such a tank should be a closed
one. This is chiefly due to the thick skin which, after a few months’
work, forms on the surface of closed or open tanks, and which
according to locality and season may reach a thickness of from 1 to 2
feet; it is maintained then that this cheap natural cover does away
with the expensive artificial cover.
In the report on the treatment of the Manchester sewage, by Messrs.
Baldwin Latham, Percy F. Frankland and W. H. Perkin, it is stated on
page 54, amongst the conclusions and recommendations, as follows:—“The
anaerobic or septic process is found to take place as effectively in
an open tank as in a closed one.” This conclusion does not appear to
have been modified by the experiments made subsequent to the issue of
this report.
In the Leeds experiment a similar result was obtained.
[Sidenote: Closed septic tanks possess generally speaking no
advantages over open ones.]
Whilst it would, therefore, appear to be correct to say, generally,
that closed septic tanks afford no material advantages over open ones,
so far as the purification of the sewage is concerned, they may become
necessary in special cases, when the smells emanating from open ones
might create nuisances in crowded neighbourhoods.
The following remarks refer, therefore, equally to open as well as to
closed septic tanks, and no distinction will be made between them.
[Sidenote: The work in the septic tank is chiefly done by obligatory
anaerobes.]
(_c_) _Explanation of Process._—Although the processes taking place in
septic tanks are at present but imperfectly understood, they may be
said to be in the main due to anaerobic micro-organisms, i.e. due to
such micro-organisms which carry on their life’s work in the absence
of oxygen. They split up or peptonise the organic compounds in the
absence of air, and the group of changes brought about by them has
been termed “anaerobic fermentation” or “putrefaction.” During the
same, it is claimed that a considerable amount of the sludge retained
in the tank is liquefied or destroyed, and that the rest becomes so
changed as to be denser than ordinary sludge, and to contain less
moisture.
[Sidenote: Dissolved matters entering and leaving the septic tank.]
Concerning the amount and nature of the dissolved matters entering and
leaving an open septic tank, the following is taken from the
Manchester report for the year ending March 27, 1901:—
[Sidenote: Manchester observations.]
“A series of determinations have been made of the amount of
dissolved matter entering and leaving the tank, by evaporating
known volumes of the sewage and effluent after filtration through
paper and weighing the solid residue.
“An average of six determinations (confirmed by similar
observations in connection with the closed septic tank) gave the
following results:—
----------------------------------+--------------------------------
Raw Sewage. | Open Septic Tank Effluent.
Dissolved matter, grains | Dissolved matter, grains
per gallon. | per gallon.
------------+--------------+-------+----------+-------------+-------
| Organic and | | | Organic and |
Mineral. | Volatile. | Total.| Mineral. | Volatile. | Total.
-----------+--------------+-------+----------+-------------+-------
33·0 | 33·0 | 66·0 | 30·8 | 25·0 | 55·8
| | | | |
| | | Reduction, in per cent.
| | | 6·67 | 24·24 | 15·45
-----------+--------------+-------+----------+-------------+-------
“A certain amount of loss of ammonia, as ammonium carbonate, will
take place on evaporation in both cases, and this will probably
be greater with septic tank effluent.
“An examination of the residue obtained by evaporating large
quantities of open septic tank effluent (filtered through paper),
shows that the mineral matters largely consist of iron oxide,
from the decomposition of organic compounds of iron, and calcium
sulphate. Among the volatile constituents have been detected
ammonium carbonate, mercaptan-like compounds of very offensive
smell, acetic and butyric acids. No evidence of the presence of
amines could be found in the residue on evaporation, but by
distilling large volumes of the liquid and carefully analysing
the platinum salts obtained from the distillate, the presence of
amines is indicated.
“Research in this direction is being continued; careful
comparison especially will be made of the products obtained by
evaporation and distillation of crude sewage and septic tank
effluent respectively.
“The evidence, however, points to a breaking down of albuminoid
and cellulose matter in the septic tank into simpler and to some
extent volatile compounds. The reactions are probably hydrolytic
in character, ammonia, amines, carbonic acid, water, and possibly
alcohol, being produced.
“A further quantity of organic matter also disappears as methane,
nitrogen and hydrogen.”
[Sidenote: Must aerobic fermentation in all cases be preceded by
anaerobic fermentation?]
It will be clear from the foregoing, that the changes going on in a
septic tank are entirely different from those brought about in contact
beds, and the question whether a septic tank is a necessity for the
subsequent contact bed treatment, or whether it is a distinct
disadvantage, can only be definitely settled when we know whether
aerobic fermentation, i.e. decomposition, must in all cases be
preceded by anaerobic fermentation, i.e. putrefaction, and to what
extent, or whether such a succession of changes is not necessary.
[Sidenote: At Manchester contact beds accustomed to septic tank
effluent did not at once purify raw sewage.]
It is interesting to note in connection with this point, that during
the Manchester experiments it was established that contact beds, which
have become accustomed to septic tank effluent, will not at once
purify comparatively fresh sewage.
(_d_) _Velocity of Flow through Tank._—The velocity of flow through
the septic tank is of great importance, as on it depends the size of
the installation.
It seems to have become a habit to express this velocity by the length
of the sojourn of the sewage in the septic tank—for instance, “the
flow of sewage through the tank was such that it would fill it in
twenty-four hours”; but as all tanks vary in size, and as in
consequence the distance which has to be traversed by the sewage from
the entrance to the exit in twenty-four hours is different in nearly
every case, such a habit is, to say the very least, very misleading.
It will not be disputed that the deposition of the suspended solids in
sewage is dependent on the rate of movement of the liquid, and that in
a quickly moving liquid there will be less deposition than in a very
slowly travelling liquid.
-----------------+----------+---------+-----------+-------------
Town. | Length. | Width. | Depth. | Contents.
-----------------+----------+---------+-----------+-------------
| | | |
| feet. | feet. | ft. in. | gallons.
Manchester tanks | 300 | 100 | 6 0 | 1,125,000
| | | |
Leeds “ | 100 | 60 | 7 7 | 250,000
-----------------+----------+---------+-----------+-------------
Bearing this in mind it will not be without interest to examine the
velocities employed during the Manchester and Leeds experiments. The
tanks employed in these have the dimensions given in the table on the
preceding page.
Now assuming that each tank is to be filled once in twenty-four hours
we obtain the following velocities:
Manchester (300′ 0″ × 12″)/1440 = 2″·5 per minute.
Leeds (100′ 0″ × 12″)/1440 = 0″·84 per minute.
Which means that in the Manchester experiments the velocity would have
been three times as large as in Leeds; and it is clear that if the
sewage of both towns was identical the results, so far as the
retention of the suspended matters in the tanks are concerned, could
not have been identical. As a matter of fact, considerably greater
velocities have been used in the Manchester experiments, as will be
shown later on.
[Sidenote: Rate of flow through septic tanks should not be expressed
by the length of sojourn in tank but by some linear measurement in a
stated time.]
It will be clear from this, that it is most misleading and erroneous
to express the rate of flow by the length of the sojourn of the sewage
in the tank, and that the velocity should in each case be expressed by
some linear measurement in a stated time—probably inches per minute.
The next point to consider is the velocity to be employed in septic
tanks; and here it is not without interest to refer to the various
experiments enumerated with their results in the next table.
The difference in the results obtained, so far as the suspended
matters are concerned, is probably due to the different character of
the various sewages experimented with; but so low a velocity as 0·52
inch, as used in the Exeter experiments, does not appear to be
necessary.
In the Leeds experiments, it was found that the filling of the tank
once in twenty-four hours gave the best results; and as the velocity
then was 0·84 inch per second it will be somewhat near the mark to
recommend generally a velocity of 1 inch per minute. On the assumption
that the sewage shall remain twenty-four hours in the tank, this gives
a length of tank of 120 feet, which is a very suitable one.
SEPTIC TANK EXPERIMENTS.
_Rate of Flow and Deposition of Suspended Matters._
-----+------------+-----------------------+
| | Rate of Flow. |
| +-----------------------+
| | | |
No. | Name of | Length of | Velocity |
| Town. | Sojourn | of Flow |
| | of Sewage | per |
| | in Tank. | minute. |
| | | |
-----+------------+-----------+-----------+
| | | |
| | days. | inches. |
1 | Exeter | 1·0 | 0·52 |
| | | |
2 | Manchester | 0·44 | 5·58 |
| | | |
3 | “ | 0·56 | 4·44 |
| | | |
4 | Leeds | 0·5 | 1·68 } |
| | | |
5 | “ | 1·0 | 0·84 } |
| | | |
6 | “ | 2·0 | 0·42 } |
-----+------------+-----------+-----------+
-----+-----------------------------------------------
| Suspended Matters in Sewage.
|-----------+-----------+------------+----------
| | | |
No. | Remaining | Destroyed | Leaving |
| in | and | Tank | Total.
| Tank. | Liquefied | in |
| | in Tank. | Effluents. |
| | | |
-----+-----------+-----------+------------+----------
| | | |
| per cent. | per cent. | per cent. |
1 | 17 | 39 | 44 | 100
| | | |
2 | 41 | 22 | 37 | 100
| | | |
3 | 23 | 33 | 44 | 100
| | | |
4 | .. | .. | .. | ..
| | | |
5 | average | 28 | .. | ..
| say | | |
6 | | .. | .. | ..
-----+-----------+-----------+------------+----------
[Sidenote: Septic tanks reduce the sludge difficulty to some extent,
but do not altogether remove it.]
(_e_) _Destruction and Liquefaction of Sludge in Septic Tanks._—It was
formerly maintained that the employment of a septic tank did away with
all sludge difficulties, and one sees even now advertisements to that
effect, that there is “no sludge” with a septic tank; but experience
everywhere does not bear out this contention. On the contrary, there
must be sludge with a septic tank, and the only question one has to
consider is, to what extent does a septic tank reduce the quantity of
sludge?
The table above contains the results obtained in the various
experiments, and from these it would appear as if on an average, with
a velocity of 1 inch per minute, 25 per cent. of the total sludge
would be destroyed or liquefied in a septic tank. Generally speaking,
therefore, the following figures will be somewhat near the mark, where
the plant is worked systematically and carefully supervised.
Per cent.
Suspended matters remaining in tank 35
“ “ destroyed or liquefied in tank 25
“ “ escaping in effluent 40
---
Total 100
These figures mean that 35 per cent. of the total suspended matters
will have to be dealt with as sludge, 25 per cent. will be destroyed
or liquefied in the septic tank, and the remaining 40 per cent. will
be deposited on and in the contact beds.
It has already been pointed out that it is claimed that the septic
tank sludge is denser and contains less moisture than ordinary sludge,
and that about half of it is mineral matter.
As previously stated, at Manchester a reduction of about 16 per cent.
in the dissolved matter has been observed in the open septic tank.
(_f_) _Formation of Gas in Septic Tank._—It was at one time suggested
that the gases formed in septic tanks during anaerobic fermentation
might be utilised for lighting or heating purposes, but anyone well
acquainted with the subject will admit that such a use is outside the
range of practical possibilities.
At Manchester, 100 gallons of sewage evolved in twenty-four hours
about a cubic foot of gas, which on an average contained:
Per cent.
Marsh gas, CH{4} 73
Carbon dioxide, CO{2} 6
Hydrogen, H 5
Nitrogen, N (by difference) 16
---
Total 100
At this rate 1 million gallons of sewage will evolve 10,000 cubic feet
of gas, or 0·2 tons of gas, in twenty-four hours.
[Sidenote: Septic tank effluent more suitable for nitrification.]
(_g_) _Mixing Action of Septic Tank._—There is one advantage possessed
by a septic tank which cannot be disputed, and that is the mixing
action going on within it. The fresh sewage on its arrival becomes
mixed with stale sewage, and, owing to the rising of lumps of sludge
from the bottom, and other causes, the contents of the tank become of
a more uniform composition, which must entail a corresponding
advantage for the subsequent contact bed treatment.
[Sidenote: The septic tank effluent is so far as bacterial purity is
concerned practically raw sewage.]
(_h_) _Micro-organisms in Effluent from Septic Tank._—Although the
available number of experiments on the micro-organisms contained in
the effluent from a septic tank is not large, yet they support the
conclusion which one would form by analogous reasoning, that so far as
the bacterial flora is concerned the effluent is practically raw
sewage.
4. CONTINUOUS CONTACT BEDS.
[Sidenote: Continuous contact beds still in an experimental stage.]
It is necessary to make at this point a few short observations on the
artificial self-purification in continuous contact beds.
This method of artificial purification has frequently been called
“continuous filtration,” but it will be much better to reserve the
term “filtration” for the percolation of water through fine material,
such as sand, and to call the continuous flow of sewage through
coarser material continuous contact bed treatment, as the processes
going on during the same are more analogous to those going on in an
intermittent contact bed than to those taking place in a waterworks
filter.
Formerly it was attempted to use the same kind of contact bed for
continuous treatment as is used for intermittent treatment, but, as
was to be expected, the results obtained were so unsatisfactory that
the experiments had to be discontinued. Now somewhat different forms
are utilised, which are mostly protected by patent rights, and the
mode of distribution has also been altered by the introduction of
patent distributors or sprinklers, which cause the sewage to fall in
very thin streams upon the filling material.
In the Manchester experiments, the proprietary continuous contact bed
does not appear to have given satisfactory results. Better effluents
were obtained at Leeds, and at York the results obtained are said to
have been very good.
On the whole, however, it is but right to say that the experience
gained so far is not sufficient to entitle us to form definite
opinions, and for this reason it will be better to await further
results.
VI. MANAGEMENT OF PLANTS FOR THE ARTIFICIAL SELF-PURIFICATION OF
SEWAGE.
[Sidenote: Plants for the artificial self-purification of sewage
require very careful handling.]
It was formerly frequently concluded that neither septic tank nor
contact beds required careful superintendence, but that they could be
worked by automatic machinery and left to themselves. It was therefore
maintained that the working expenses of plants of this nature would be
next to nil. This was, however, not Mr. Dibdin’s view, who, after
years of careful study, came to the conclusion that they were delicate
pieces of mechanism which required careful watching.
Since, Mr. Dibdin’s conclusions have been amply confirmed by all
careful experimenters.
For instance, Mr. Fowler, the chemist in charge of the Manchester
experiments, observed before the Royal Commission on Sewage Disposal
as follows: “It is a delicate operation (the management of septic tank
and contact beds), which requires careful watching! There is no doubt
whatever about that!” (Question 5651.)
Again, the conditions of successful working of contact beds, laid down
by the same gentleman on page 64 of the Manchester report for the year
ending March 27, 1901, are ample proof of this, and they show very
clearly how extremely careful the supervision of such a plant ought to
be, and that in the hands of inexperienced men it will soon come to
grief.
Professor Percy Frankland stated in his evidence before the Royal
Commission, that in his opinion land required less skilled supervision
than contact beds. (See Questions 9937, 10071-74.)
A similar view was expressed by Mr. H. M. Wilson, the chief inspector
of the West Riding of Yorkshire Rivers Board. (Question 6380.)
VII. SOME OBSERVATIONS ON THE DEPOSITION OF SUSPENDED MATTERS IN
TANKS.
[Sidenote: Definition of the term “deposition.”]
The term “deposition” shall here be held to mean the precipitation of
the suspended matters without chemicals or other artificial means,
i.e. the unaided subsidence of these matters at such a rate of flow
that septic action is not set up within the tanks.
The question that is of interest here, is: Which is the most
favourable rate of flow of the sewage through the tank, so far as
the deposition of the suspended matters is concerned? To some extent
the answer to this question will depend on the special characteristics
of the particular sewage under consideration, but for general purposes
the following observations will not be without interest.
Although of very great importance, this question does not appear to
have received very general consideration, as the available number of
careful experiments is but small.
[Sidenote: Tank velocity at Barking.]
It appears that the calculated velocity in the channels of the
precipitation tanks at Barking is about 4 feet per minute, and that
with this velocity about 77 per cent. of the suspended matters were
deposited in the year 1894.
[Sidenote: Tank velocity at Manchester.]
At the Manchester tanks it is stated that a velocity of 3 feet 4
inches per minute is employed.
[Sidenote: Velocity frequently adopted.]
A rate of velocity now frequently adopted in this country for new
works is 6 inches per minute.
[Sidenote: Frankfort experiments.]
In the settling tanks at Frankfort on the Main there are deposited
about 84 per cent. of the suspended matters, with velocities ranging
from 9½ inches to 16½ inches per minute.
[Sidenote: Cassel experiments.]
With a velocity of 7 inches per minute, it is stated that at the
Cassel sewage works 97 per cent. of the suspended matters are retained
in the tanks.
[Sidenote: Hanover experiments.]
At Hanover a set of interesting observations has lately been made, on
tanks 246 feet long, with a view to ascertaining the most advantageous
rate of flow.
With a velocity of 9·44 inches per minute, 62·7 per cent. of the
suspended organic matters were precipitated, with a velocity of 14·17
inches per minute 61·7 per cent. were deposited, and with a velocity
of 35·43 inches per minute 57·3 per cent.; from which figures it will
be clear that there is not much difference in the result on the
suspended matters between these velocities.
Against these results must be placed the results obtained with septic
tanks, where, as has frequently been stated, a velocity of 1 inch per
minute and a sojourn of twenty-four hours in the tank may be expected
to lead to a deposition of about 60 per cent. of the suspended
matters.
[Sidenote: Reduction of cost.]
Where, therefore, a previous septic treatment of the sewage by
anaerobes is not necessary, it is clear that the substitution of
ordinary settling tanks for septic tanks will be accompanied by a very
considerable reduction of cost.
VIII. CONCLUDING REMARKS.
Since the foregoing observations were penned, the Chairman of the
Royal Commission on Sewage Disposal has delivered a very interesting
inaugural address, in August last, at the Congress in Exeter of the
Institute of Public Health, to which attention ought to be drawn at
this point.
According to _The Times_ he is reported to have stated as follows: “He
regretted that he could not give some idea of the probable date at
which the Commission would issue its final report and recommendations.
They would soon, he hoped, be able to publish the results of a
prolonged investigation into the treatment of sewage on land; and
their experts were now making elaborate parallel examinations of some
of the processes of filtration by artificial means. But he feared that
they would ultimately be obliged to bring their proceedings to an
arbitrary close; for, however much they could learn, he was quite
certain they could never come to a point at which they could say there
was nothing more to be learned. The subject was inexhaustible.”
These very guarded observations are almost in direct contrast with the
very positive assurance of some enthusiastic supporters of artificial
treatments, who a year or two ago did not hesitate in proclaiming
throughout this country that the panacea for all sewage difficulties
had been discovered, and that the investigations of the Royal
Commission were a mere matter of form and a foregone conclusion.
To all those who did not share these very sanguine expressions of
faith, and who were painfully aware of the great gaps in our knowledge
of the processes taking place in sewage purification, these words of
Lord Iddesleigh will prove an assurance that the commissioners are not
swayed by popular likes and dislikes, however fascinating they may be,
but that they are earnestly endeavouring, in an impartial manner, to
throw such light upon this abstruse question as will enable them to
arrive at correct conclusions.
For a like purpose the foregoing remarks have been written; and if the
facts recorded in the previous pages, and the opinions expressed
therein, should prove of assistance to anyone in forming correct
views, the labour spent on them will be amply repaid.
POSTSCRIPT.
Since the foregoing remarks were written, I have been somewhat struck
with the views expressed at one or two meetings by some of those who
ought to have the full facts of the case at their fingers’ ends. There
seems to be a considerable vagueness as to the sanitary results to be
obtained by either the natural or one of the artificial methods of
sewage treatment, and with a view to making this point quite clear I
have prepared the following comparative statement (see next page),
which, I trust, will show at a glance what one may expect from either
system.
This statement has not been prepared from experimental installations,
where, as a rule, better results are obtained than in actual every-day
work; but it refers to fairly large works dealing from day to day with
the whole town’s sewage. It has further been assumed, that the plant
both for the natural as well as for the artificial treatment is
suitable, and managed carefully and on intelligent lines.
As in all sewage treatments the sanitary results have first to be
considered, I have only dealt with them in the statement, but even if
I had extended it to economic considerations the result would have
been practically the same.
From a careful examination of the facts recorded in the statement it
follows that with natural treatment we get five distinct advantageous
results, against which we have to place only two on the side of the
artificial treatments; but this means, that if we wish to bring up the
results of these treatments to those obtained by the natural
treatment, we have to supplement them by three further treatments for
the extraction of pathogenic germs, and of the manurial elements, and
for the reduction of the liquid.
It is these facts which ought to be carefully considered by all those
who wish to study the comparative advantages of these two systems, or
who have to decide on a definite method to be employed in a particular
case, and no step ought to be taken before every one of these five
points has been very carefully weighed. Generally speaking, that
system will be preferred which confers the greatest number of
advantages.
STATEMENT.
_Results to be obtained from_
(A) _Natural Treatment._ (B) _Artificial Treatment._
1. Removal of suspended 1. Removal of suspended
matters. matters.
2. Removal of from 75 to 95 2. Removal of from 50 to 75
per cent. of the dissolved per cent. of the dissolved
organic matters. organic matters.
3. Removal of pathogenic 3. Nil. Effluent bacterially
germs. practically raw sewage.
4. Utilisation of large portion 4. Nil. All manurial elements
of manurial elements. escape into the rivers.
5. Great reduction of quantity 5. No appreciable reduction
of liquid. of quantity of liquid.
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E.
Transcriber’s Note:
Words and phrases in italics are surrounded by underscores, _like
this_. Chemicals are displayed with the numbers surrounded by braces,
e.g. CO{2}.
Footnotes were numbered sequentially and moved to the end of the
section in which the related anchors appear.
Wide tables were split for easier viewing on small screens.
An unprinted letter “S” was added to sidenote, “Springs.”
Obsolete and alternate spellings were not changed.
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