ABSTRACT

National Pollutant Discharge Elimination System (NPDES) permits limit the final effluent’s total suspended solids (SS) and 5-day biochemical oxygen demand (BOD

). Other quality parameters and sampling requirements are included as needed. However, most biological processes produce effluents that contain only a few mg/L of soluble BOD

, or less, and the BOD

of the final effluent is mostly biomass that has escaped capture by the final clarifier. Thus, the NPDES permits control final clarifier design and operation, not the biological process itself. Some biological processes, like completely mixed activated sludge, produce flocs that settle slowly, and this should be a consideration in their selection and operation. If effluents containing much less than 10 mg/L of SS or BOD

are required, then additional effluent treatment like coagulation, settling, and filtration should be considered.