[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.