ABSTRACT
Th e main purpose of secondary treatment (sometimes referred to as biological treatment) is to provide biochemical oxygen demand (BOD) removal beyond what is achievable by primary treatment. There are three commonly used approaches, all of which take advantage of the ability of microorganisms to convert organic wastes (via biological treatment) into stabilized, low-energy compounds. Two of these approaches, the trickling filter [and/or its variation, the rotating biological contactor (RBC)] and the activated sludge process, sequentially follow normal primary treatment. The third, ponds (oxidation ponds or lagoons), however, can provide equivalent results without preliminary treatment.
