Fig. 29.—Type of Floating Arm.
The various types of inlets and outlets described above are more particularly suitable for tanks which come under the terms “septic” and “continuous-flow sedimentation without chemicals.” It is not necessary that the inlets and outlets should both be of the same type. Various combinations may be adopted, according to the requirements of each case and the judgment of the engineer. Similar methods may be utilised for “continuous-flow sedimentation tanks with chemicals,” but they need the addition of floating arms for the purpose of drawing off the top water before the sludge is removed. The type of inlet and outlet more generally in use for chemical precipitation processes is shown in [Fig. 28], as in these cases there is no need to preserve a scum on the surface. The connection between the sewage carrier and the tank is usually in the form of a sluice gate, and simple wooden boxes are provided round the inlet and outlet in order to divert the flow towards the bottom of the tank. It is also found desirable in some cases to provide scum-boards for the purpose of arresting the grease, which naturally rises to the surface, and must not be allowed to pass away with the effluent. The floating arm outlet is essential, particularly for tanks which are designed for “quiescent sedimentation with or without chemicals,” and the usual form of outlet into a channel a few inches only below the inlet level is not needed, as tanks of this type are filled and allowed to stand full for a certain period, and the contents are then drawn off through the floating arm. The function of this appliance is to draw off the whole of the clear liquid contents, from a point a few inches below the surface, at a slow rate, and without disturbing the sludge at the bottom.
Fig. 30.—
Decanting
Valve.
A type of floating arm is shown in detail in [Fig. 29]. In order to prevent any possibility of these arms drawing off sludge by an oversight, when approaching the floor of the tank, the chain attached to the float should be arranged to check the fall of the arm at a point which will be above the level of the sludge, or, if there is any possibility of the chain being tampered with by unauthorised persons, the fall of the arm may be arrested with certainty by means of a bracket, built into and projecting from the wall of the tank, or by means of a short pier of brickwork and concrete, built up on the floor of the tank under the arm to the required level. Another method of drawing off the top water from tanks has been introduced by Messrs. Willcox and Raikes, Civil Engineers, and is manufactured by Messrs. Adams Hydraulics, Ltd. As will be seen from the illustration, [Fig. 30], it consists of a cast-iron stand-pipe, in sections, each of which makes a tight joint with the one below it. A spindle, working in a screwed nut in a bracket or pillar at the top, passes through crossbar guides inside the stand-pipe sections. This spindle has projections at irregular intervals, arranged in such a manner that as the spindle is screwed up it lifts the top section first, then the second, and lastly the third, and thus makes it possible to draw off the supernatant water in three layers, each of which may if desired be discharged in different directions. Finally, the sludge may be drawn off through the same outlet to the sludge-disposal area.
As the distance which the sewage travels in “continuous flow settlement tanks with chemicals” is frequently an important factor in securing the maximum efficiency, it may be found economical to arrange the tanks in the form shown in [Fig. 31], where each tank has a division wall, carried through from the inlet end to within a few feet of the opposite end, so that the sewage travels a distance equal to twice the length of the tank before passing to the outlet. This arrangement requires only one carrier, but this must be provided with suitable sluice-gates opposite to each tank, in addition to similar gates on the inlet and outlet from each tank.