chapter  14
22 Pages


The main purpose of secondary treatment (sometimes referred to as biological treatment) is to remove suspended solids and provide biochemical oxygen demand (BOD) removal of 90% or more, beyond what is achievable by primary treatment. The three commonly used approaches all 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-or its variation, the rotating biological contactor (RBC)—and the activated sludge process, sequentially follow normal primary treatment. The third approach, ponds (oxidation ponds or lagoons), can provide equivalent results without preliminary treatment.