1. Wet methods.
2. Dry methods.
1. Wet methods. These comprise the removal of excreta—(1) By discharging it into running water. (2) By storage in tank with overflow. (3) By carrying it into the sea. (4) By precipitation. (5) By irrigation and filtration.
(1) By discharging it into running water. Our previous remarks have already shown in what respect this proposal is fallacious, and why it has, therefore, been discontinued.
(2) By storage in tank with overflow. In this process the sewage runs into a well-cemented tank fitted with an overflow pipe, which sometimes leads into a second tank arranged in the same manner; the solids subside, and are removed from time to time, whilst the liquid is allowed to run away. Instead of permitting the liquid to escape into a ditch or stream, it has been proposed to carry it into drain pipes, which are buried from half a foot to a foot in the subsoil, where it will be readily sucked up by the roots of grasses. This plan is only suited for small villages, or for a single house or mansion.
(3) By carrying it into the sea. The precautions to be observed in the working of this system are, wherever possible, to let the outlet or discharge pipe, which conveys the sewage to the sea, be always under water even at ebb tide, and to take special care that the wind does not blow up the sewers. A tide-flap, opening outwards, which is usually fixed by a hinge on the sewer at its outlet, will obviate this last contingency. At high water the tide will fill the outfall sewers to its own level, and to that extent will check the discharge of sewage, and thus cause a deposit in the sewers filled with mixed sea water and sewage. It is most important that this should be removed.
“If the sewage cannot be got well out to sea, and if it issues in narrow channels, it may cause a nuisance, and may require to be purified before discharge.”[152]
[152] Parkes.
(4) By precipitation. The simplest of the plans proposed for this method of removal is by subsidence only, and would afterwards
permit the discharge of the supernatant sewage water into running water or over the land. The removal of the solid material is effected in a manner somewhat similar to that followed in plan No. 2, but as the thin water which runs off must, when poured into rivers or streams, be almost as dangerous as the sewage itself, the process of precipitation by settlement alone has little to commend it over the old rude and objectionable practice, a circumstance that in these days will doubtless lead to its entire prohibition.