Fundamentals of Microbiology

4.4.3 Primary Treatment

It is the process of removal of solid matter called sludge from sewage. About 40 ~ 60% of suspended solids are removed by passing the sewage through sedimentation tanks. Sometimes flocculating chemicals are added to remove more solids.

Primary treatment removes approximately 25% to 35% of the biochemical oxygen demand (BOD) of the sewage. BOD is a measure of the biologically degradable organic matter in water. BOD is determined by the amount of oxygen required by bacteria to metabolize the organic matter.