The Tamale Metropolitan Assembly (TAMA) sewage treatment plant is located at Gbalahi, a village about 5km away from Tamale and serves a total population of about 240,000 people (GSS 1993). The sewage treatment plant consists of two units of three ponds each: a 2,432 m2 anaerobicpond (4m deep), a pair of 1, 216 m2 of both primary andsecondary facultativeponds (3m deep each) in series, which are connected to a common 4, 464 m2aerobic pond (2 m deep) (Fig 1). The system design allows for both units to operate in rotation such that one side can be closed while the other is in operation.