Advantages of cothickening primary and secondary sludges in dissolved air flotation thickeners: Fid-Bau Portal

Advantages of cothickening primary and secondary sludges in dissolved air flotation thickeners

Butler, Richard C. / Finger, Richard E. / Pitts, James F. / Strutynski, Barbara

Dissolved air flotation thickeners (DAFTs) are commonly used to thicken waste activated sludge (WAS). However, their use for cothickening primary sludge and WAS is not commonly practiced. A full‐scale DAFT cothickening process has been operating at the East Division Reclamation Plant at Renton, Wash. (EDRP), since 1988. The development of the cothickening operation at EDRP from predesign to present is summarized. Recent cothickening DAFT performance is also presented. The paper focuses on the DAFT's ability to remove soluble biochemical oxygen demand/chemical oxygen demand (BOD/COD) and fine grit in the mixed sludge feed. DAFT cothickening of primary and secondary sludge can significantly reduce secondary organic loading while concurrently reducing the amount of grit being transferred to sludge digesters. Soluble BOD is reduced by ∼80% across the DAFTs; soluble COD is reduced ∼60%. This represents about a 4% BOD loading reduction to the activated‐sludge facilities. By contrast, previous studies using primary sedimentation tanks to thicken primary sludge increased primary effluent COD by 10–15%. Substantial amounts of fine grit are removed during the flotation 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 efficiency (disregarding bottom sludge) at solids loading rates of 98–122 kg/m²/d (20–25 Ib/d/sq ft). These rates are nearly twice the rates that would be required for thickening WAS separately. Thus, the DAFT size required for cothickening was not substantially greater than that needed for WAS thickening.