text: Inert solids in wastewater will accumulate on the bottom of the lagoon and gradually reduce depth until there is inadequate room for the facultative zone.  Lagoon depths between 2 and 5 feet (60 to 150 cm) are preferred for effective treatment.  Parallel facultative lagoons with common polishing ponds allow wastewater treatment to continue while one lagoon is out of service for sludge removal.
  Precipitation falling on the surface of the lagoons and polishing ponds will increase the volume of wastewater requiring disposal.  Conversely, the volume of wastewater may be reduced by evaporation from the water surface in arid climates.
  Wastewater nutrients may cause continuing growth of algae in the polishing ponds after the original wastes have been catabolized.  Algae may cause measurable contribution to biochemical oxygen demand (BOD) and total suspended solids (TSS) concentrations where discharge regulations include limitations on those concentrations.  The TSS contribution of algae tends to peak in the summer months, but the long-term BOD of decomposing algae may not be evident within the typical 5-day test.  United States Environmental Protection Agency regulations describe facultative lagoons as providing "treatment equivalent to secondary treatment" when 65 percent of influent BOD and TSS are removed and effluent BOD and TSS concentrations do not exceed a 7-day average of 65 mg/L and a 30-day average of 45 mg/L.  Individual States may establish alternative effluent limitations.