Two types of pump might for found in a sewage system. One is a fairly high powered (HP or so), sewage pump that pushes treated sewage out of the tank where the tank is situated in lower ground than the surrounding area. This type of pump should only run infrequently, and for a short time.
The other type of pump that might be found in a sewage system, mine included, is an air compressor, which is used to agitate and aid with the digestion of the sewage. In the majority of systems, this pump runs continuously (mine seems to run about half of the time, under the control of some expensive electronics). These typically are rated at 100W or less.