When cesspools are used, they should be situated as far as possible from the building and clear of any storage tanks or source of domestic water supply, their walls and bottoms being perfectly water-tight, and the tank large enough to obviate frequent clearing out; though the other extreme is just as bad. They should be well ventilated by pipes terminating at the most favourable position that the surrounding circumstances will allow; and the overflow should be so constructed that as little harm as possible is likely to occur from it, though cesspools are bad and injurious, even when necessary, constructions; and in all cases the house sewer should be disconnected from them, as hereinbefore referred to.
There are two methods of disposal by irrigation - namely,
1. Surface irrigation, which consists in distributing the sewage matter over the surface of properly prepared land, the liquid being drained off at a depth which will ensure its being deodorised before being allowed to flow into a stream or river; and,
2. Sub-irrigation, by which solid matter is collected in tanks, constructed and subdivided, so that the liquid part is forced through beds of sand and charcoal into other compartments (as Fig. 1001), from which it should be automatically flushed out and distributed below the ground, at a depth of, say, 2 feet, by means of ordinary field drains laid a certain distance apart in dry rubble.
It is necessary that the distribution should be large and intermittent to prevent the trickling of small discharges, which make the soil round the tanks boggy and unpleasant, because the liquid never gets beyond the first two or three pipes; and it is desirable to have all irrigation tanks constructed in duplicate, to allow of one set being cleaned out while the other is in operation, without interfering with the regular working of the scheme.
It is needless to add that sewage irrigation depends very much on the nature of the ground through which the pipes are run - a gravelly soil, or any other through which the liquid will percolate quickly, being the desideratum of a sanitary engineer.
Before leaving house sewers, and passing on to the consideration of soil pipes and water closets in connection therewith, the writer would impress upon the student the necessity of careful thought in dealing with drainage schemes, as errors have, frequently, most serious results.