THE DISPOSAL OF SEWAGE.
The water-carriage system of sewage is, as the late Dr. Parkes put it,
“the cleanest, the readiest, the quickest, and in many cases the most
inexpensive method.” But when the sewage is conveyed to the outfall of
the sewer, its ultimate disposal is still one of the most difficult
problems of the present day. Various plans have been adopted, of which
the following are the chief:—
1. Discharge into running water.
2. Discharge into the sea.
3. Separation of solid and liquid parts {By settlement.
{By precipitation.
4. Filtration through various artificial media or through land.
5. Irrigation.
6. Bacterial methods.
=Discharge at once into running water= was formerly the favourite plan,
as it was certainly the most convenient. The sewage was turned into the
nearest water-course, regardless of the facts that this might have to
supply the drinking water of people at a lower point, that the mouth
of the river tended to become obstructed by sewage mud, that valuable
stocks of fish were destroyed, and that the river which had practically
become a sewer was a source of annoyance and danger to all on it or
near it. The enforcement of the Rivers Pollution Prevention Act of 1876
has not been followed by as great improvement as is desirable.
The sewage entering rivers undergoes considerable purification by
subsidence, by oxidation, by the influence of water plants, and
still more by the active work of microbes, causing nitrification of
nitrogenous matter. The vitality of the typhoid bacillus and of the
cholera vibrio when discharged by sewage into a large river is probably
not very protracted; but water from such a river would form a very
dangerous source of domestic supply.
=Discharge into the Sea= is resorted to in seaboard towns. The outfall
must be carried well below the lowest low-water mark, and to such a
point that the incoming tide or wind will not bring the sewage back
upon the shore, or on the shore of neighbouring places.
=Discharge into an Estuary= is only justifiable when the flow of the
river is rapid, when the volume of water passing out to sea is very
greatly in excess of the volume of sewage, and when there is no
possibility of contaminating oyster-layings or beds of mussels or other
molluscs.
Objection has been taken to the above method on the ground of waste of
manure; but modern sewage is so dilute that its profitable utilization
on land still remains a dream.
For a single house or small village, the sewage may be stored in =a
tank, with an overflow-pipe=, out of which the liquid parts escape, and
are systematically used to irrigate land, while the solid parts are
removed at intervals.
A similar subsidence system has been employed on a larger scale, the
liquid parts being irrigated over land, while the solid parts are mixed
with street sweepings, and sold as manure.
If the liquid parts in any such system as this are turned into a
stream, they are as dangerous as the entire sewage, and the legal
prohibition to discharging sewage into streams applies equally to them.
The =precipitation of the solid parts= of the sewage is rendered much
more perfect by the use of =chemical agents=, and at the same time the
dissolved matters are to some extent removed.
Milk of lime has been employed, 6 to 12 grains of quicklime being used
for each gallon of sewage. Secondary decomposition is apt to occur in
the effluent, causing an offensive smell. Salts of alumina, iron salts,
and various combinations of these have also been employed, but with
imperfect results.
The London sewage for some years has been treated by adding 2·5 grains
of sulphate of iron and 3·7 grains of lime to every gallon of sewage;
a reduction of 15 to 20 per cent. in the amount of dissolved organic
matter being secured. Polarite or magnetic spongy carbon is used as a
filter in certain places, the solid and some of the dissolved sewage
being first precipitated by magnetic ferrous carbon (ferrozone). The
Amines process consists in applying a mixture of herring brine and lime
to the sewage. The sewage is stated to be sterilized by this means.
Electrolysis has also been applied to the purification of sewage, as
in the Hermite process. In this process sea-water is electrolysed,
oxygen-yielding compounds and chlorine being produced.
With regard to all the chemical processes hitherto introduced, the
following general statement appears to hold good: they are expensive
and not thoroughly efficient.
=Sewage sludge= is deposited in the tanks in chemical processes
and needs separate disposal. At Birmingham the amount of sludge
produced daily from the sewage of a thousand persons is nearly a
ton. This sludge has been run into rough filter beds and left to
dry or carted away for manure, but in its crude state its manurial
value is very slight. At Ealing it is mixed with house-refuse and
burnt in a destructor. The more modern method is to pass it through a
filter-press, thus compressing it into solid cakes which can be sold
for manure.
=Filtration= of the sewage matter has been accomplished in various
ways.
_Intermittent downward filtration_ through a considerable depth of soil
was stated by the Rivers Pollution Commission to be attended with good
results. A porous soil is chosen, and the purified water is received
in drains under it. A large part of the organic matter is removed by
bacterial agency. Frankland’s experiments shewed that upward filtration
through the same media did not purify.
_Filtration through artificial media_ has not been successful with
crude sewage. Precipitation by ferrozone followed by filtration through
polarite is said to be satisfactory.
=Broad Irrigation= purifies the sewage efficiently under favourable
conditions, the possible exceptions being during rainstorms and during
frosty weather. The effluent into the river cannot, however, be
regarded as certainly innocuous, though it is better than the effluent
from most other processes. Sewage farms are not a commercial success.
In such a farm liquid sewage is allowed to flow at intervals over the
land, different fields being irrigated in rotation. Immense crops of
grass are obtained, but the grass is coarse and rank.
The soil to be irrigated should have a gentle slope, and the effluent
be conveyed by subsoil drains about 5 or 6 feet deep into the nearest
water-course. The sewage should be delivered in as fresh condition as
possible, and should be freed from its coarser portions by settlement
or precipitation. The amount of land required is about 1 acre for the
sewage of 100 persons. The irrigation must be on the intermittent plan,
in order that the soil may undergo aeration; as it is only in this
way that the best purifying results can be obtained. The sewage farm
should be well drained by deep-laid agricultural drains. The chief
purification of the sewage occurs in the superficial layers of the
soil. Nitrification ceases at a depth of about 18 inches. The great
point, therefore, is to keep the superficial soil in good condition. A
similar nitrification occurs in earth-closets (page 195). No nuisance
need arise in connection with a sewage-farm, and the supposition that
milk and other products from such a farm are less wholesome than the
same products from other farms has proved to be unbased.
=Bacterial Methods of Treating Sewage.=—Chemical precipitation of
sewage is likely to be completely superseded by _biological or
bacterial_ methods of sewage disposal. When sewage is treated by
filtering through land or by broad irrigation the process is bacterial,
bacteria or microbes in the soil converting injurious organic matter
into innocuous mineral products. The typical process is one of
_nitrification_. The novelty of recent methods is in utilising bacteria
for the whole process of purification, and not only for its final
stages. The object is, in fact, not as in chemical processes to arrest,
but by confining the sewage in tanks to aid and hasten decomposition or
putrefaction. Two kinds of microbes serve in this process; those living
in air, known as _aerobic_, and those living in other gases than air,
called _anaerobic_.
Three biological methods of preliminary treatment of sewage are
employed. (1) Mr. Scott Moncrieff passes the sewage slowly upwards
through a filter 14 inches thick, consisting of successive layers of
flint, coke, and gravel. This is called a “_cultivation tank_.” The
solid sewage becomes liquefied in passing through this medium, the
microbes in the filter dissolving the sewage. (2) In the “_septic tank
method_,” introduced by Mr. Cameron at Exeter, a tank is employed which
is covered in to exclude light, and to a large extent air. The tank
is large enough to hold 24 hours’ flow of sewage. The microbes in the
sewage under these conditions multiply rapidly, attack, and liquefy
the sewage. As in the first process little or no sludge is left. The
ultimate products of the decomposition are water, ammonia, and carbonic
acid, and other gases. The effluent from the tank is comparatively
clear and inoffensive. (3) _Aerobic biological filters_ are employed,
as in Mr. Dibdin’s installation at Sutton, where the filtering material
is coke. The sewage slowly passing through the filtering medium becomes
liquefied, the solid matter being peptonised. This action is in part
at least due to anaerobic microbes. The filtering beds are used
intermittently to allow of aeration, and the liquefaction of solid
organic particles entangled in the filter probably chiefly occurs at
this stage. It is desirable to have small subsidence tanks, for the
removal of large suspended matters and of road debris, etc., before
the sewage is spread over the filtering beds. The material used in the
filter varies. Most commonly coke-breeze has been employed, but coal
slack and other material have also been utilised.
After the preliminary treatment above described, the sewage requires to
be passed over finer filtering beds, in which aerobic microbes complete
the purification by changing the dissolved organic matter into inert
inorganic compounds, by the process known as nitrification. The two
processes run into one another, to some extent going on together.
Hitherto the Local Government Board have required filtration of sewage
through land before any sewage effluent is allowed to pass into a
stream. In view of the successful results now obtainable by bacterial
processes this requirement will be occasionally waived. It is unsafe
to assume, however, that the clear effluent obtained is free from all
disease-producing microbes; and drinking water should not be obtained
from even a very large river below the point of discharge of such an
effluent, without the most efficient sand filtration.