The object of dealing with sewage on land may be taken as twofold,
namely, to purify it (which is the sanitary object), and to utilize its
manurial products (which is the agricultural object). Where want of
skill or where prejudice has existed, these two have not been properly
separated, and the results have been in many cases unfavorable to
sewage disposal on land from either of the before mentioned points
of view. It has been regarded as an axiom that clay land cannot be
employed to clarify sewage. This is true when it is proposed to pour
the sewage on it as if the land were porous. Very recent experience,
however, has led to clay land being converted from an impervious to a
pervious condition, by which it has been successfully utilized. This is
effected by digging out the clay to a depth of about 6 ft., burning it
into ballast and replacing it in layers interposed with an occasional
layer of open alluvial soil, the whole area being well drained with a
free outlet for the effluent. We have successfully carried this plan
out, and with this result, that whereas it was not possible previously
to clarify the sewage of 100 people to an acre of clay land, the
prepared filtration area has been able to continuously clarify the
sewage of about 1,500 people to the acre. The cost of converting clay
land into this form of filter may be taken as varying from £750 per
acre to £1,000 per acre, according to local circumstances. One area
which we have just completed has cost £1,000 per acre. Before sewage is
passed on to these filters (or on to land) it should be strained so as
to remove the larger particles. The best arrangement for this purpose
is to pass the sewage upward through a straining medium (not downward),
and to run the solids from the bottom of the straining tank on to a
low lying piece of land for digging in as they are run out. Where such
a filtration area is made to form part of a sewage farm it acts as a
safety valve, and enables the land and crops to have a rest when they
do not require further irrigation; at the same time the process of
purification is not interrupted. If open, porous land is available for
sewage purification, and if it can be drained 6 ft. deep to a good free
subsoil, so that the effluent can get readily away, we find that the
sewage of from 600 to 700 people can be dealt with on each acre per
annum with both good agricultural and sanitary results.
In our address as President of the Engineering and Architectural
Section of the Congress of this Institute at Newcastle upon Tyne, in
1882, we directed attention to the important investigation which had
been conducted by Mr. R. Warrington, of Rothamsted, the result of which
was to show the action which goes on in the soil when sewage is passed
through it. Further information which the same observer has published
since that date is of equal value, and deserves to be read by all who
have to advise in regard to sewage disposal on land. The process of
"nitrification" (as it is termed), which he has so fully investigated,
consists in the conversion into nitrates (which serve to nourish plant
life) of the organic matter in sewage. This takes place by the action
of a living ferment of the bacteria family, which is created by and
feeds on the impurities in sewage, and these organisms both consume the
impurities and convert them into nitrates. The action of living agents
thus brings about the oxidation of the organic matter in sewage, just
as worms, larvæ, fungi, and insects feed on the vegetable matter in the
soil, increasing the amount of nitrogenous material in it. Experience
during the past year or two has proved the feasibility of preserving
green crops in a succulent state by compressing them in silos, so that
they can be utilized for cattle fodder in the winter. This system
deserves notice in connection with sewage farming, as we are of opinion
that it will prove a valuable means of getting over the well known
practical difficulty which is experienced of finding a market for the
large amount of green crop which is produced by sewage irrigation. In
speaking of this system the term "silo" is applied to the artificial
chamber or receptacle for green crops (such as grass, vetches, clover,
etc.).
The term "silage" is applied to the crop thus treated, and the term
"ensilage" is applied to the process of making "silage." The details of
the construction of silos cannot be referred to here, beyond stating
that what is required is to construct a pit or chamber either in the
form of an excavation in the ground, with a brick or other lining, or
by building it above the ground. The object is to enable the green crop
to be deposited in an air and water tight chamber, in which pressure
can be applied to the crop to compress it. This is effected in some
cases by well treading the crop after it is laid in the silo, and then
spreading layers of earth to about a couple of feet, and pressing
the covering well down. Another way is to construct the silo with a
movable covering of the exact size and shape of its interior. This
cover is raised and lowered by suitable chains and rollers. After the
crop is placed in the silo, the cover is lowered and weighted so that
a thorough compressing is effected; the weight applied giving about
200 lb. or so per square foot of surface. Salt is sometimes added as
the crop is placed in the silo. A crop thus dealt with is stored for
months; when the silo is opened the fodder is found preserved, and in
a state readily taken to by cattle. It is desirable to choose the site
for the silos so that the fodder is preserved somewhere near the place
of consumption; also to lay out the works so that as little handling
as possible is required. For instance, the silo should be on sidelong
ground, so that the crop can be carted and tipped at a high level, and
the silage taken out for use at a lower level.