The wetwell shall be designed to accommodate two identical pumps, one of which is a standby, each capable of handling the peak flow. The pumps are not to be automatically alternated at every start. The volumetric capacity of the wetwell, to prevent the overly frequent starting and stopping of pumps, may be determined using the following formula:

where T = Length of time between pump starts in minutes.

Q = Pump capacity in gallons per minute.

V = Wetwell volume in gallons between lead pump start and lead pump stop.

Q_in = In-flow rate in gallons per minute.

The minimum pump cycle time occurs when the pump capacity (Q) equals two times the inflow (Q_in). The volume required for a single constant speed pump to maintain a given minimum cycle time is:

For T, use the minimum restart time for the motor or seven minutes, whichever is greater.

The physical size of the wetwell structure may need to be larger because of other considerations like physical size and spacing of pumps, bar rack, sluice gate, access and working space requirements, etc. The wetwell structure shall be sized taking all these factors into consideration.

The length of time between pumping cycles is the retention time. Retention time must be limited to acceptable limits so that bacterial action taking place in the wetwell does not cause an odor problem in the vicinity. When sizing the pumps, velocity in the forcemain is also to be considered to prevent sedimentation. As many different and often conflicting aspects of the overall system have to be considered in selecting the pumps, wetwell volume, etc., this process shall be done in close consultation with the WSSC on a case-by-case basis. (Design Guideline DG-08 § 5.2, dated August 2006)