Equalization Tank

Flow equalization is the process of mitigating changes in flow rate through a portion of a system by providing storage to hold water when it is arriving too rapidly, and to supply additional water when it is arriving less rapidly than desired. In treating wastewater, the rate at which the waste arrives at the treatment process might vary dramatically during the day, so it is convenient to equalize the flow before feeding it to the various treatment steps. In either case, the engineering issue is deciding how large an equalization basin is required to allow a steady, the treatment processes to operate with a steady, average flow.

The sewage from the bar screen chamber and oil, grease and grit trap comes to the equalization tank. The equalization tank is the first collection tank in an STP. Its main function is to act as buffer: To collect the incoming raw sewage that comes at widely fluctuating rates, and pass it on to the rest of the STP at a steady (average) flow rate. During the peak hours, sewage comes at a high rate. The equalization tank stores this sewage, and lets it out during the non-peak time when there is no/little incoming sewage.

Design consideration

The incoming sewer line is usually gravity-fed, and is likely to be at considerable depth below the ground level. Therefore it is prudent not to make the tanks of STP too deep, otherwise it requires very deep excavations and expensive construction. It also makes the maintenance and cleaning processes very hazardous. In it necessary to force compressed air in the sewage held in the tank. This is mandatory for two reasons:

» It keeps the raw sewage aerated, thereby avoiding septicity and suppressing odor generation
» It keeps solids in suspension and prevents settling of solids in the tank, thereby reducing frequency of manual cleaning of the tank

The tank may be of any shape, provided it permits placement of air diffusers for full floor coverage and uniform mixing over the entire floor area. The diffusers should be retrievable: Individual diffusers (or sets of diffusers) may be lifted out and cleaned for routine maintenance. This will reduce frequency of shut down of the Equalization tank for manual cleaning purposes.

If membrane diffusers are used, they will fail frequently, due to the repeated cycles of expansion and contraction caused by fluctuating water levels in the equalization tank. Therefore, only coarse bubble diffusers must be used in the equalization tank.