ADVANTAGES OF COTHICKENING PRIMARY AND SECONDARY SLUDGES IN DISSOLVEDAIR FLOTATION THICKENERS

Dissolved air flotation thickeners (DAFTs) are commonly used to thicke n waste activated sludge (WAS). However, their use for cothickening pr imary sludge and WAS is not commonly practiced. A full-scale DAFT coth ickening process has been operating at the East Division Reclamation P lant at Renton, Wash. (EDRP), since 1988. The development of the cothi ckening operation at EDRP from predesign to present is summarized. Rec ent cothickening DAFT performance is also presented. The paper focuses on the DAFT's ability to remove soluble biochemical oxygen demand/che mical oxygen demand (BOD/COD) and fine grit in the mixed sludge feed. DAFT cothickening of primary and secondary sludge can significantly re duce secondary organic loading while concurrently reducing the amount of grit being transferred to sludge digesters. Soluble BOD is reduced by similar to 80% across the DAFTs; soluble COD is reduced similar to 60%. This represents about a 4% BOD loading reduction to the activated -sludge facilities. By contrast, previous studies using primary sedime ntation tanks to thicken primary sludge increased primary effluent COD by 10-15%. Substantial amounts of fine grit are removed during the fl otation process as evidenced by solids buildup in the digester and the volatile solids content of the float and bottom DAFT sludge. In 1995, EDRP DAFTs averaged 6.2% thickened total solids with 81% capture effi ciency (disregarding bottom sludge) al solids loading rates of 98-122 kg/m(2)/d (20-25 Ib/d/sq ft). These rates are nearly twice the rates t hat would be required for thickening WAS separately. Thus, the DAFT si ze required for cothickening was not substantially greater than that n eeded for WAS thickening.