SEWAGE AS A MANURE.
The value of sewage as a manure has been in the past enormously
overrated, and much misunderstanding has existed on the part of the
public on the question of the profitableness of the disposal of town
sewage as an agricultural manure. Not a few of the erroneous opinions
prevalent in the past regarding sewage have been due to statements made
by scientific and other writers as to the enormous wealth lost to the
world by many of the present methods of sewage disposal. Fortunately,
however, the sewage question is now increasingly regarded as a question,
in the first instance, of sanitary interest. As much has been written on
the subject, and many schemes have been devised, at the expense of much
ingenuity, for utilising its manurial properties, it may be desirable
here to say a few words on the purely agricultural side of the question.
The two most important points about sewage are its enormous abundance
and its extremely poor quality. If the most important consideration
were not the sanitary one, but its manurial value, then indeed our water
system, so universally used in towns, must be regarded as a most
wasteful one; for by its means the value of the excrementitious matter
from which it derives its manurial ingredients is tremendously lessened.
When we reflect that a ton of sewage, such as is produced in many
European cities, contains only 2 or 3 lb. of dry matter, and that the
total amount of nitrogen in this is only an ounce or two, while the
phosphoric acid is considerably less, and that it is on those two
ingredients that its value as a manure entirely depends, we see very
strikingly how poor a manurial substance sewage is. Various methods have
been devised and experimented with for extracting these manurial
ingredients, and many methods are in operation in different parts of the
world. The methods of utilising sewage for agricultural purposes may be
broadly divided into two classes.
_Irrigation._
One of these, which may be classed under the heading of irrigation,
consists in pouring the sewage on to certain kinds of coarse green
crops. Sometimes the land is made to filter large quantities of sewage
by special arrangements of drains and ditches. The land is first
carefully and evenly graded down a gentle incline. At the top of the
field the sewage is conducted along an open ditch from which it is
permitted to escape, by the force of gravity, by several smaller
ditches running at right angles from the main ditch. By means of stops
which may be shifted at will, the sewage can be directed to flow over
different parts of the field. Modifications in this plan may be made so
as to suit the nature of the ground. In the case, for example, of a
steep incline, the field may be sewaged by means of what are known as
"catch-work" trenches running horizontally along the hill. In this way
the sewage is allowed to pass over the whole of the field, and is caught
at the bottom in a deep ditch, whence it is allowed to flow into the
nearest river or stream. This is the system which has been employed at
the famous Beddington Meadows, near Croydon.
Another method of distributing the sewage is by means of underground
pipes, which are laid in a sort of network over the ground to be
manured. At certain intervals pipes with couplings for hose are fitted
on, and by keeping a certain amount of pressure on the main pipes the
sewage may be distributed over the different parts of the field as it is
required.
A third modification is subsoil irrigation. This resembles the
last-named system, with this difference, that the pipes used are either
porous or perforated with small holes.
Total submersion can only be applied in the case of absolutely level
lands, and is practised to an enormous extent in Piedmont and Lombardy.
There has been little dispute as to the thorough efficiency of
irrigation--when conducted under favourable conditions--as a method of
purifying sewage and utilising to the full its constituents of manurial
value. It is the only method which has been conclusively shown to
extract from sewage that to which it owes most largely its value as a
manure--viz., ammonia; and from this fact it deserves a first place in
the consideration of agriculturists. For however admirable other methods
may be from a sanitary point of view, it is obvious that a method which
would allow the ammonia in sewage wholly, or at least to over 90 per
cent, to be lost, cannot claim the same place in the judgment of
agriculturists as a method which can extract for the soil not only the
whole of this valuable constituent, but all else in the sewage which in
any way is of value to plant-life.
_Effects of continued Application of Sewage._
When sewage is continuously applied to the same land, what generally
takes place is this: At first the sewage is purified, and the soil
derives corresponding benefit from the valuable fertilising ingredients
it thus extracts. After a time, however, the land becomes what has been
termed "sewage-sick." The pores in the soil become choked up by the
slimy matter the sewage contains in suspension; the aeration of the
soil, which, as we have already mentioned, is so necessary, is
consequently to a large extent stopped; and the result is, that the land
rapidly deteriorates, and the sewage is no longer purified.
_Intermittent Irrigation._
This is obviated to some extent by intermittent irrigation. The land,
instead of receiving sewage continuously, only receives it at intervals,
and is allowed some time to recover between each dose. It is, however,
the opinion of those who have given the subject much attention, that
land, even although intermittently sewaged, never recovers its original
efficacy.
Irrigation, therefore, under favourable conditions, is a most successful
method of utilising the manurial value of sewage; but the great
difficulty in practice is to obtain those favourable conditions. It has
long been known that if soil is properly to discharge its function as a
purifier of sewage water, it must be properly aerated; and we now know
that in every fertile soil the process of nitrification must be
permitted free development. Now the application of large quantities of
sewage to a soil is apt to prevent this free development. As we have
already seen, absence of air and the lowering of the temperature of the
soil distinctly tend to retard nitrification; and these two conditions
accompany the application of large quantities of sewage.
_Crops suited for Sewage._
Another objection to irrigation has been found in the alleged limited
number of crops sewaged land is suited to yield. It has been repeatedly
stated that rye-grass is about the only crop it is profitable to grow
on it. In opposition to this statement, however, is the opinion
expressed in the conclusions arrived at by the committee appointed by
the British Association for the consideration of the sewage question. A
vast number of experiments were carried out by them between the years
1868-72, and the result they arrived at was as follows: "It is certain
that all kinds of crops may be grown with sewage, so that the farmer can
grow such as he can best sell; nevertheless, the staple crops must be
cattle food, such as grass, roots, &c., with occasional crops of kitchen
vegetables and of corn." While, therefore, it is probably a mistake to
say that rye-grass is the only crop sewaged land is capable of growing
profitably, the bulk of experience goes to show that such a crop is best
suited for such land. This being so, the question naturally arises, What
is the farmer who uses sewage as a manure to do with the large green
crops he obtains from his land? He is, in most cases, unable to use them
himself or dispose of them at the time. And while this has hitherto
proved to be a most important drawback, now that we have in ensilage a
means of preserving our green crops in a condition suitable as fodder
for as long a time as is necessary, the grounds on which this objection
rests are almost entirely removed.
It will be obvious, of course, that some soils are naturally much better
fitted to perform purification of sewage than others; but it must be
frankly admitted that even the best of soils can only deal with a
certain quantity of sewage. Various calculations have been indulged in
as to the amount of sewage an acre of land can successfully deal with.
According to one of these, an acre can purify some 2000 gallons per day,
or that produced by 100 persons; while other calculations estimate it at
60 persons; and others, again, at 150. The capacity of a sandy soil in
this respect will be much greater than that of a heavier soil; and at
Dantzic an acre of the sand-dunes is regarded as being capable of
purifying the sewage of 600 persons. The late Dr Wallace has calculated
that, in order to treat the sewage of Glasgow, over twelve square miles
of land would be required. Of course, if the sewage is subjected to
previous treatment, which is often the case, by the method immediately
about to be described--namely, precipitation--the amount of sewage the
soil is capable of purifying will be correspondingly increased. A
difficulty which may also be pointed out in connection with irrigation
as a means of disposing of sewage, is the impossibility of carrying it
on during frosty weather, when the land is frost-bound. In warm climates
irrigation has much to recommend it as a means of sewage disposal. In
damp and cold climates, on the other hand, there are many objections.
_Treatment of Sewage by Precipitation, &c._
We now come to consider the methods grouped under this second heading.
Mechanical filtration, of course, only aims at purifying sewage to the
extent of removing all insoluble suspended matter which it contains.
Different substances have been used as filters, the most generally used
being charcoal. Charcoal mixed with burnt clay, gravel, sand, &c., has
also been used.
In chemical precipitation, however, we have a method which claims to do
more. Beyond the extracting of all solid matters in suspension, it
removes (at any rate most chemical precipitants do) nearly all the
phosphoric acid, which, next to the ammonia, is the most valuable
constituent the sewage contains. Of all precipitants, lime has been the
most universally used; and on the whole, it is perhaps the best, for it
is both cheap and obtainable almost anywhere. According to an analysis
by the late Professor Way, the difference in the percentages of
phosphoric acid, potash, and ammonia, before and after treatment with
lime, in a sample of sewage, was as follows:--
_Grains per Gallon._
Before. After.
Phosphoric acid 2.63 .45
Potash 3.66 3.80
Ammonia 7.48 7.50
From the above we see that while sludge caused by lime as a precipitant
contains nearly all the phosphoric acid, there is not a trace of the
potash or ammonia removed. Sulphate of alumina has also been used, both
alone and in conjunction with lime. The advantage claimed by it over
lime is, that the resulting precipitate is much less bulky. In other
respects, however, it does not seem to be any more efficient as a
precipitant. In the well-known A, B, C process, a mixture of alum, clay,
lime, charcoal, blood, and alkaline salts, in different proportions, has
been used. This mixture is said to extract, in addition to the
phosphoric acid, a certain proportion of the ammonia; but the amount is
so small as scarcely to be worth considering.
Numerous other chemical substances have been used, alone and also in
conjunction with one another, such as perchloride of iron, copperas,
manganese, &c. All alike, however, have failed to do more than effect
partial purification,--the best results, it may be added, being obtained
when the sewage thus treated was fresh. With regard to the manurial
value of the resulting sludges, much difference of opinion has existed.
The small percentage of phosphoric acid and nitrogen they contain has
prevented them from being used to any extent as a manure, as their value
did not admit of carriage beyond the distance of a few miles. By the
introduction a few years ago of the filter-press, their value has been
considerably enhanced. The old method of dealing with the sludge at
precipitation-works was to allow it to dry gradually by exposure to the
atmosphere. The result, however, of leaving sewage-sludge with over 90
per cent of water in it to dry in the air, was to encourage the rapid
decomposition and putrefaction of its organic matter, so that in many
cases the decomposing sludge proved to be as great a nuisance as the
unpurified sewage itself would have been. By the use of Johnson's
filter-press, however, a sludge containing 90 per cent of water was at
once reduced to 50 per cent or even less. By this means the percentage
of its valuable constituents was very much increased, and the
sludge-cake, besides being much more portable, was neither so
objectionable nor so liable to decomposition as before.
_Value of Sewage-sludge._
As to the value of this sludge-cake as a manure, we are happily in
possession of some very interesting and valuable experiments by
Professor Munro of Downton Agricultural College. The sludge experimented
upon was that produced by sulphate of alumina, lime, and sulphate of
iron, and contained, after being subjected to Johnson's filter-press,
from .6 to .9 per cent of nitrogen, and over 1 per cent of phosphoric
acid. It was found that the benefit resulting from the application of
the sludge was far from what in theory might have been expected. The
experiments were made with turnips; and the results obtained with
superphosphate and farmyard manure respectively, in the same field and
under exactly the same conditions, were contrasted with those obtained
with sludge. Thus it was found that 53 lb. of phosphoric acid as
superphosphate, or 60 lb. as farmyard manure, produced a considerably
larger crop than 240 lb. of phosphoric acid in the sludge. That is to
say, that the phosphoric acid in the sludge did not exert more than
one-fifth of its theoretical effect. The explanation of this somewhat
strange result Dr Munro finds in the unsuitable physical character of
the sludge-cakes. In farmyard manure we have a loose texture and a large
amount of soluble constituents when well rotted. It thus quickly
distributes its fertilising elements throughout the soil. In the case of
the sludge, on the other hand, its composing particles are closely
compacted together, and thus offer the greatest resistance to mechanical
and chemical disintegration. "As a matter of fact," says Dr Munro, "the
sludge-plots in my experimental series were all readily identified, when
the roots were pulled, by the presence of unbroken and undecomposed
clods of cake, which had evidently given up, at most, a small portion of
their valuable ingredients to the soil."
Briefly stated, therefore, the objections to chemical precipitation as a
means of dealing with sewage are these--viz., that while it relieves
sewage of all its organic matter, and to a large extent of its
phosphoric acid, it fails to extract any ammonia, which is thus lost;
that the resulting sludge is consequently so poor in fertilising matters
as scarcely to make it worth while to remove it any distance for
manuring purposes; and that, further, owing to its unfavourable
physical character, as at present made, even the small percentage of
plant-food it contains is not realisable, within, at any rate, anything
like a reasonable time, to its full theoretical extent.
The most profitable method of treating sewage must be determined by
various local conditions; and it must be clearly understood that the
question of sewage disposal is primarily a sanitary one, and that it
must be dealt with from the sanitary aspect. The most profitable way of
applying sewage as a manure, however, will doubtless be found by
combining chemical precipitation and land irrigation.