Dissolved Air Flotation as Secondary Clarification

Dissolved Air Flotation (DAF) units have long been used in the food industry to remove fats, oils and grease (FOG), total suspended solids (TSS) and biochemical oxygen demand (BOD). The effluent from the DAF units has often discharged to a downstream activated sludge treatment facility complete with conventional gravity secondary clarification. The activated sludge from these facilities frequently exhibits filamentous sludge bulking requiring large secondary clarifiers, high return activated sludge pumping rates, and supplemental waste activated sludge (WAS) thickening to reduce sludge disposal volumes.This treatment system can be greatly simplified by simply eliminating the primary DAF and conventional gravity clarification and WAS thickening and replacing it with a more robust activated sludge treatment system with DAF as the secondary clarifier. This system is capable of accommodating slug loading and filamentous sludge bulking while discharging effluent BOD, TSS, FOG, and total phosphorus (TP) within pretreatment compliance. Secondly, these systems are capable of discharging WAS concentrations equivalent to or greater than that achieved with traditional WAS thickening. At the same time, these simplified systems occupy a much smaller footprint. A review of the performance and relative costs for two such facilities operated by the Dean Foods Dairy group are described below.