In this article we will discuss about the wells provided to pumping stations.
The sewage from the city is received at pumping station in a tank known as sump or wet well. The capacity of the wet well is such that it can store D.W.F. of at least 2 hours, which is the maximum period during which pumps can be repaired or replaced.
Modern practice is to pump the sewage as nearly as possible after it has been received; therefore a tank having capacity of 20-30 minute average flow is sufficient.
The sump well is an underground masonry or R.C.C. chamber, placed at such a level that sewage from the trunk sewer can flow into it by gravity only. The tank may be rectangular, ring shaped or circular in shape in the plan. Mostly circular wet wells are used, the bottom of which is given a 1:1 slope towards a central pit where the end of suction pipe of the pump is placed.
The depth of the well mainly depends on the depth of the incoming sewage inlet. The main storage inside the well should be free fall inside the wet well. This will also avoid the surcharging and back flow inside the incoming sewer. Guide pipes filled with flats, switches of motors, cables of electric motors, sewage level indicators etc. should be installed in the wet well at suitable places.
Gate-valves should be fitted on the incoming sewer lines to stop the sewage flow during inspection, repair and cleaning of the wet well etc.
In the top of wet well manholes with ladders are provided for its cleaning, inspection and maintenance.
The overflow weirs and by-pass arrangements are also provided in the wet wells for diverting the sewage during emergency floods.
Design of Wet-Well:
Storage capacity is generally required in all the sewage and storm water pumping stations, where automatic controls and variable speed drives are not provided.
The capacity of the wet well depends on the following factors:
(i) The frequency of operation of pumping equipment, and
(ii) The time for which the sewage will be retained in the pumping station.
The detention time and shape of the wet well should be such that sewage may not deposit nor become septic. The capacity of the well is reckoned between the level at which air affects the suction line of the pump of minimum duty installed in the pump house and the designed sewage level of the incoming sewage i.e. the portion of the well below the uppermost starting point and the lowest most stopping point.
The principle followed is that any pump should work for at least 5 minutes before it is stopped. The size of the wet well is kept such that any combination of inflow and pumping, the cycle of operation for each pump will not be less than 5 minutes. The maximum detention period should not exceed 30 minutes of average flow.
Buffle walls should be provided in the well at suitable places to ensure uniform flow at each suction. To avoid deposition the flow should be provided 1: 1 slope. For uniform distribution of sewage flow, dividing walls should be provided between the pump sections. Adequate provision for the removal of sludge should be provided.
If possible suitable overflow arrangements should be provided against flooding due to breakdown of plant or power failure. Coarse screens having 40-50 mm clear opening for manual type and 25 mm for mechanical type cleaning, should be provided before the wet well.
The screening units conforming to IS: 6280 should be provided in duplicate. For increasing the life of pumps grit removing chambers ahead of wet wells should be provided.
Pump Room or Dry Well:
It is placed in a convenient place and pumps are installed inside it. Its location should be such, so that pumps can easily function.
This is an underground masonry or R C.C. room having circular or rectangular shape in the plan. The sewage pumps, their driving units. Control valves and necessary pipes with the fittings are installed in it.
The size of the dry well should be sufficient for the movement of the operator during operation, maintenance and repairs as well as installation of the pump etc. It is better to take deep these wells, so that pumps may not require any priming during starting. In some cases a small pump is also provided to pump the leakage from the wet well in this well.
This size of the dry well should be sufficient to accommodate installation of more pumps for the further expansion with the design period. Sometimes in small sewage pumping station no separate dry well is provided. The pumps may be submerged in the wet well itself or may be provided on the roof of the wet well.
But the pumps submerged in the wet well give the following troubles:
(i) Difficulty in the inspection and repairing, because for this purpose, they are to be taken out.
(ii) Acids, alkalies and other harmful substances, reduces the life of the pumps.
(iii) It is costly in maintenance, as the worker will have to enter the wet well having foul explosive and poisonous gases. The explosion may damage the pumping station due to fire in explosive gases by the sparks of the circuits etc.