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.