Water Environment Federation (WEF)

Operation of a solids contact tank at low dissolved oxygen and low total suspended solids concentrations

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Courtesy of Courtesy of Water Environment Federation (WEF)

Solids retention time (SRT) and dissolved oxygen concentration (DO) set points were optimized at a 32 MGD trickling filter-solids contact tank system. OPTImasterTM on-line optimization software was used for set points selection. Set points were maintained automatically using SRTmaster and DOmaster - sludge age and dissolved oxygen automatic control systems. Referenced above biological system was successfully operated at DO=0.5 mg/l and MLSS less than 900 mg/l. As a result of set points optimization and automatic control, airflow demand was decreased by 25% while water quality and sludge settleability continued to be excellent.

Tests performed at the first phase of optimization work showed an increase of total and nonsettleable effluent TSS when DO at the end of the solids contact tank was reduced to 0.3 mg/l or less. SVI has decreased to 60 ml/g-70 ml/g during the same time. Reduction of DO at the beginning and in the middle of the solids contact tank did not produce similar results. It can be preliminary concluded that DO concentration at the end of the tank is more important for flocculation than in the first part of the tank.

Despite absence of any flocculation baffles, rectangular clarifiers showed excellent flocculation ability reducing non-settleable solids concentration from 20 mg/l to 6 mg/l. Total effluent suspended solids concentration was only 1 mg/l higher than non-settleable fraction.

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