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A Discussion of The Advantages of Co-Thickening of Primary and Secondary Sludges in Dissolved Air Flotation Thickeners
Author: Richard Butler, Richard E. Finger, Jim Pitts, Barbara Strutynski

Dissolved air flotation thickeners (DAFTS) are commonly used to thicken waste activated sludge; however, their use for co-thickening primary and waste activated sludges is not commonly practiced. This paper describes the results of a full-scale DAFT co-thickening operation. Soluble BOD reduction across the DAFTS is approximately 80% while soluble COD reduction is approximately 60%. This represents a reduction in BOD loading to the activated sludge facilities of approximately 4%. In addition, substantial amounts of fine grit is removed form the primary sludge during the flotation process. Thus, DAFT co-thickening of primary and secondary sludges can significantly reduce secondary organic loading while concurrently reducing the amount of grit being discharged to the digesters. It also eliminates the need to independently thicken primary sludge. These impacts lead to reductions in secondary operating costs as well as reduced operating costs associated with digester cleaning and thickening of primary sludge.