In December 1985, the City of Columbus, OH initiated a $220 million program for upgrading capacity and treatment from secondary to advanced wastewater treatment (AWT). The two plants (4990 L/s and 2630 L/s) were required to be fully operational by July 1988. The existing plants had a history of bulking sludges and it was necessary that design/construction proceed concurrently with a laboratory 18-month sequencing batch reactor (SBR) study to evaluate bulking sludge control while producing the required effluent quality of CBOD5, TSS and NH4N of 8,16 and 1 mg/L, respectively. Completion was on schedule and full-scale results paralleled those produced by the SBR sludge. Average effluent concentrations from the larger plant for the 2.5 years of operation were 2 mg/L CBOD5, 5.5 mg/L TSS, 1.5 mg/L TKN, 0.13 mg/L NH4N and 1.1 mg/L TP. The problems resulting from unexpectedly high sludge yields and inadequate sludge transport capacity in the 61 mø clarifiers are discussed and resolutions presented.

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