Treatment of Spent Filter Backwash Wastewater using Dissolved Air Flotation Process

Spent Filter Backwash Water (SWW) is a major recycle stream that accounts for 10% (about 30 mgd) of the average flow at Blue Plains Advanced Wastewater Treatment Plant (AWTP) and a similar proportion of nitrification and denitrification treatment process capacity (DCWASA Focus, November 2004). The peak SWW flow and load result from increased filter washing during high flow events. The combined flows of SWW recycle plus raw influent especially during high flow (washout) events ultimately cause disruptions in process performance throughout the plant. Treatment of SWW by folded flow dissolved air flotation was explored to reduce the hydraulic load at the head of the plant and to ultimately eliminate this recycle flow stream from the treatment process. A bench-scale Dissolved Air Flotation (DAF) tester followed by a pilot-scale DAF unit was used to conduct the study. The bench scale unit was used to determine the reduction of suspended solids from SWW and the pilot scale unit was used to estimate the maximum hydraulic loading rate at which total suspended solids of 10 mg/l or less can be achieved.One-liter samples were individually batch-tested using 10% recycle ratio, 60 psi pressure and 0.7 mg/l emulsion polymer (Ciba, Zetag 8816, 40% active, 1% solution) for 10 min detention time. The folded flow pilot unit was set up in-line at Blue Plains AWTP using 44–45 psi pressure, 13% recycle and 0.3 mg/l emulsion polymer. The raw water and subnatant were analyzed for Total Suspended Solids (TSS) before and after DAF treatment. The impact of recycle ratio and polymer dose was investigated at varying hydraulic loading rates.The impact of recycle on effluent TSS concentration was investigated at 7%, 10% and 13% recycle, using 44–45 psi pressure, 1 mg/l emulsion polymer (Ciba, Zetag 8818, 40% active, 1.8% dilution) at an influent flow rate of 150 gpm. The lowest average effluent TSS concentration was at 13% recycle.The impact of polymer on TSS effluent concentration and % SCOD removal was investigated by varying the polymer dose at 0, 0.3. 0.6, and 1 mg/l, using 17% recycle, 44–45 psi pressure and an influent flow rate of 120 gpm. The optimal polymer dose was achieved at 0.6 mg/l when the polymer-particle uptake was no longer evident.The impact of hydraulic loading rates on effluent TSS concentration was investigated by varying the loading rates at 10, 12.5, 13 and 15 gpm/sf, using 13% recycle, 44–45psi pressure and 1 mg/l emulsion polymer (1.8% dilution). An average effluent TSS of 10 mg/l was achieved before failure occurred at 13 gpm/sf.The maximum hydraulic loading rate was estimated by examining the loading rates results at varying operating parameters throughout the study. The maximum hydraulic loading rate at which the TSS effluent concentration of 10 mg/l or less can be achieved was 12.5 gpm/sf.