EFFLUENT SETTLING TANKS OR HUMUS PITS.
Reference has already been made, under the heading “Grading of Filtering Material,” to the advisability of using coarse material, for the reason that the converted organic matter will in that case readily pass away in the effluent, and thus prevent the choking of the filter. Even with finer material, a certain amount of solids in suspension will be found in the effluent, and, in order to produce a final effluent suitable for discharge into any stream or watercourse, it is necessary to arrest and remove these suspended solids. This fact has been recognised by the Royal Commission on Sewage Disposal, who recommend the adoption of effluent settling tanks with a capacity equal to two hours’ flow of the sewage, and provided with means for removing the deposit.
The solids in effluents from percolating filters are rather difficult to arrest, as they are in the form of very finely divided matters in suspension. Many methods have been tried in various places, but in the author’s experience he has found that the chief factor in securing a satisfactory settlement of these solids, is the reduction to the minimum of the velocity of the effluent in its passage through the settling tank. If this principle is adopted, the simplest form of tank is similar in construction to that suggested for detritus tanks, so long as the outlet end is constructed in the form of a weir of the greatest possible length under the circumstances. Such a tank is illustrated in [Fig. 134]. By this means the rate of flow over the weir may be reduced to the minimum; and if, in addition to this, the outlet at the bottom of the tank is arranged in the form of a plug valve fixed in a pocket below the lowest point of the floor proper, near to the inlet end, and the floor is laid with a sharp slope towards this outlet, it will be found possible, as a rule, to draw off the deposit without discharging the entire contents of the tank, as long as it is done at frequent intervals. As there is a tendency for a scum to form on the surface of the liquid in these tanks, it is desirable to provide a scum-plate of wood, slate, or other material, as shown in the illustration, and in all cases, except the smallest schemes, these tanks should be constructed in duplicate. It may be mentioned here that the hydrolytic tank, [Fig. 34, page 52], and the separator of the Septic Tank Co., Ltd., [Fig. 47, page 65], have both been adopted for use as effluent settling tanks.
Fig. 134.—Effluent Settling Tank.
The deposit from effluent settling tanks, as a rule, rapidly dries without creating a nuisance when it is spread out in a thin layer upon a suitable draining bed similar to that suggested for dealing with sludge from settling tanks ([Fig. 58, page 83]). Under favourable conditions as to fall, the draining bed can be constructed below the level of the sludge outlet from the effluent settling tank, and the deposit can then be drawn off by gravitation. There is, however, still the problem of disposing of the liquid flowing from the bed, and as this should have a free outlet, it usually happens that the levels do not permit of the discharge of the deposit by gravitation. Under these circumstances the outlet should still be arranged as shown, [Fig. 58], but it should be connected to a sludge well fitted with a chain-pump, or other means of raising the deposit to the draining bed, which may thus possibly be situated at the same level as the similar beds for the sludge from the settling tanks. When dry, this deposit may be spread out on the land, or used in gardens and on farms as a manure.