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.