Sewage Disposal Works: Their Design and Construction

by William Charles Easdale

2. Continuous-flow settlement without chemicals: 6 tanks, with 2 additional spare tanks.

3. Continuous-flow settlement with chemicals: 6 tanks, with 2 additional spare tanks.

4. Quiescent-settlement without chemicals: 8 tanks, with 2 additional spare tanks.

5. Quiescent settlement with chemicals : 8 tanks, with 2 additional spare tanks.

The general features of construction are:—

Substantial walls in brickwork, concrete, plain or reinforced, and of a suitable thickness to withstand with safety the pressures they are required to resist; sloping floors provided with suitable outlets for both liquid and solid contents at the bottom, and specially arranged inlets and outlets at the top. In connection with the floors, sufficient care has not always been devoted in the past to the consideration of the most convenient method to adopt, in view of the necessity of removing the sludge. In some cases, the floors have been laid with a slope towards the outlet end, and, as the greatest accumulation of deposit takes place at the inlet end, great difficulties have been experienced in removing the sludge. There is very little doubt that if suitable arrangements are made, by means of which the accumulation of solids deposited at the inlet ends of tanks can be removed without drawing off the total contents of the tank, much labour will be saved. With this end in view, the design illustrated in [Fig. 16] is suggested as a model which may be adopted exactly as shown, or, with some modifications, adapted to meet the special requirements of particular cases. It will be noticed that a submerged weir wall is introduced at some distance (which will vary with the method upon which the tank is operated and with the character of the sewage) from the inlet end of the tank, so as to retain the larger portion of the solids in this separate compartment. The floor of this section is laid with a comparatively steep gradient leading to the sludge outlet. A separate outlet, fitted with a floating arm, may be provided for drawing off the top water down to the level of the top of the weir wall. Below this level, in ordinary circumstances, only the contents of the separate compartment at the inlet end of the tank will be drawn off in removing the sludge. A valve is provided at the bottom of the weir wall, so that the entire contents of the tank may be drawn off should it be found necessary at long intervals. An alternative to the submerged weir wall is shown in [Fig. 17], in the form of a division wall carried up to the top of the tank, with orifices below the top water level through which the sewage passes when the tank is in use. These apertures are provided with valves, so that they may be closed when the solids in the compartment at the inlet end of the tank are drawn off, and thus obviate the necessity for emptying the whole of the tank.

Fig. 17.

From observations which have been made in various places, it has been found that although the actual capacity of the tanks corresponded to anything from 12 up to 24 hours of the daily dry weather flow, the period during which the sewage remained in the tank, or rather the time taken for the sewage to pass through the tank, was much less than it was anticipated would be the case. In one instance, it was noticed that the sewage passed through a tank of a capacity equal to 15 hours’ dry weather flow in 4 hours, and, although it is obvious that the same efficiency of sedimentation could not be secured by passing the sewage at the same rate through a tank of a capacity of 4 hours’ flow, it would seem that the full effect of the larger tank was not brought into play. A possible explanation is that the form of the tank and the arrangement of the inlet and outlet were such that the flow of sewage through the tank was more or less in a direct line from the inlet to the outlet, and this, if correct, would lead to the conclusion that there is room for improvement in the design of the tank, in order to cause the sewage in its passage to be spread out over the whole area of the tank. With this end in view the author has specially designed the arrangement illustrated, [Fig. 18], as a suitable method of preventing the sewage passing direct from the inlet to the outlet. It will be noticed that the sewage enters the first compartment about 3 feet below the top water level, and by means of three cross walls is made to flow down to within a short distance of the floor in one compartment, and up to within a short distance of the top water level in the next, and that this occurs twice in the total length of the tank. By sloping the floor from the centre both ways, i.e. to the inlet and outlet ends, and providing sludge outlets at the lowest points in each case, every facility is made for removing the deposit and for emptying each half of the tank whenever it may be found necessary. Further, by arranging the sludge outlets in pockets or sumps, situated below the level of the lowest point of the floor itself, it is possible to draw off the sludge in small quantities at frequent intervals without emptying the tank itself. The chief factors in causing the sewage to be uniformly spread out over the whole area of the tank are, however, the valves or penstocks on the inlet and outlet pipes, and on the pipes in the central cross wall. By suitable adjustment of these penstocks, partially closing those through which the sewage has a tendency to flow most freely and opening the others, there should be no difficulty in securing a uniform distribution of the sewage. In any case the actual direction of the flow of the sewage is, by means of these penstocks, entirely under control. The inlets to the tank being submerged below the water level in the supply channel, will secure a more uniform rate of flow through all the inlet pipes than if they were placed at the top water level, and the valves on these pipes provide facilities for any further regulation that may be required. The most important point to be observed, however, is that the rate of flow from the outlets of the tank should be uniform. In order that this may be secured, these pipes are submerged on the inside of the tank, but have their outlets set at the top water level, so that the actual discharge may be visible, and thus render it possible to regulate the rate of flow from each pipe by means of the penstocks provided for the purpose. Further, the openings in the middle cross wall may be adjusted to control the direction of the flow through the tank by means of the penstocks, which also serve to shut off either half of the tank when the other is emptied.