In this article we will discuss about the design of flushing tanks with the help of a suitable diagram.
When the gradients of the sewers are flat and the velocity of sewage is very low, the suspended matter of sewage starts settling in the bed of the sewer and causes clogging of sewer lines. At such places where self- cleaning velocity is not available, flushing tanks are provided to flush the sewers. These tanks are usually provided at the beginning point of the sewers and may be either automatic or worked by hand. Automatic flushing tanks are most commonly used.
In manually operated flushing arrangement water or sewage is held up in the sewer by plugging the inlet and outlet ends of the sewer. When the flushing tank is filled up with water by tap the outlet plug is removed, all the water rushes towards outlets and flushes the sewer.
In automatic flushing tank, the water is automatically released from the tank at required interval, which can be adjusted by the supply pipe tap, and flushes the sewer.
ADVERTISEMENTS:
An automatic flushing tank is shown in Fig. 7.8. It consists of a masonry or concrete chamber fitted with a tap for filling the tank with water. A U-tube with a bell cap at its one end connects the chamber with sewer. When the water level increases in the chamber, it also increases in the bell cap.
As soon as it reaches a certain level, siphonic action takes place and the whole water of the chamber rushes to the sewer pipe and flushes it. The capacity of these tanks is usually 900-1400 litres and it is adjusted in such a way as to work twice or thrice a day depending on the quantity of deposits in the sewer and size of sewer.
Table 7.1 gives the capacity of the flushing tanks for flushing 65 m long sewer of different diameter:
Table 7.2 gives the approximate quantities of water required for flushing the sewer lines:
