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.
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.
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