Fig. 31.
The Dortmund type of tank, described under the heading of detritus tanks, may also be adapted for sedimentation tanks, but the outlet should be arranged in such a manner as to reduce the velocity of the flow at this point to the minimum. This is usually secured in by causing the liquid to flow over a weir formed by the circular wall of the tank, or by a number of weirs consisting of cast-iron channels laid transversely across the top of the tank. In either case it becomes necessary to form a circular effluent channel round the top of the tank, to receive the effluent after it has passed over the weirs. These two arrangements are illustrated in [Figs. 32] and [33], the former showing the circular weir wall, and the latter the transverse cast-iron channels. Both edges of each of these channels act as weirs, so that the total effective length of weir is thus greatly increased. The inlets, conical bottoms, and sludge outlets for these two tanks, would be similar to those shown in connection with this form of detritus tank ([Fig. 14]). Mr. S. R. Lowcock, M.Inst. C.E., has stated that in his experience an excellent effluent can be obtained by drawing off the liquid at one point, and at about two feet below the top water level. A method of accomplishing this is shown in dotted lines on [Fig. 32].
Fig. 32.
Special Types of Tanks.—One of the troubles which frequently arises in the operation of all types of natural sedimentation or septic tanks is a nuisance from smell, due to offensive gases given off by the effluent. These are the result of the decomposition under anaerobic conditions of the organic matter deposited in the tanks. It is possible to arrange them in such a way, that the conditions which cause the trouble may to a great extent be avoided even in the ordinary types of tanks.
Fig. 33.