periodic sloughing seen in the standard filter. Since solids are not retained in the high-
capacity filter for long periods as they are in the standard filter, they are less stable and
continue to exert a considerable oxygen demand in settling tanks receiving filter
effluent. Likewise, the solids continue wastewater clarification and BOD removal in the
settling tanks as long as aerobic conditions are maintained. The settling tanks,
including primary tanks if filter effluent is returned through them, are integral parts of
high-capacity filter treatment.
Wastewater flow diagram at plants using high-capacity filters.
(A) single stage. (B) two-stage filters.