Sump Volume Calculation

The volume of sumps in sewage pump stations is often determined by the allowable number of starts per hour for the electrical motor. The sump volume for a normal small sewage pump station equipped with two pumps can be calculated based on the allowable start frequency, the maximum inflow and the pump capacity. Generally, the dimensioning of the sump volume is based on the assumption that the constant inflow is at the critical value, i.e. half of the pump capacity.

In reality the inflow is rarely constant and will differ according to the time of day, the weather etc. If the critical inflow is used as a constant inflow value, the calculated sump volume is usually over-specified leading to longer periods where the pumps are inactive, i.e. during dry-weather and night flow conditions. This could result in sedimentation build up on the sump floor and allow floating waste material to accumulate on the water surface. Left for longer periods of time this could result in odours and poisonous gases being generated and also increase the threat of the pumps being blocked during start-up.

One way of solving this problem is to reduce the sump volume, which will result in the pumps starting more often. In order to calculate the active sump volume Xylem Water Solutions has developed a software program called SUVO. In this software the user is able to specify the geometry of the sump, the inflow to the station and the pump capacity. This enables the program to calculate active sump volume and stop levels for the alternative ways of cycling the pump units within the station.

Xylem Water Solutions has also developed a method of optimising the sump volume using the "real" inflow variation. More information about this method and the problem in general can be found in the brochure.