A pilot plant with a capacity of 20∼30m3/day was operated for 10 months for simultaneous removal of nitrogen and phosphorus by a modified Phostrip process. In the process, a denitrification tank was installed before the aeration tank in the Phostrip process, which was developed mainly for phosphorus removal. To maintain the effluent TKN concentration in the range of 1∼3 mg/L, the allowable maximum TKN loading rate in the aeration tank was found to be 0.065 kg TKN/kg MLSS·d. The nitrification rate in the aeration tank increased as the influent COD/TKN ratio declined. The optimal internal recycle rate from the aeration tank to the denitrifier was found to be 2Q when the yearly average COD and TKN concentration of the influent was 197 mg/L and 37.1 mg/L, respectively. To keep the effluent T-P concentration below 1 mg/L, T-P loading to the aeration tank should be less than 0.008 kg T-P/kg MLSS·d at the sludge recycle rate of 0.3Q from the 2nd clarifier to P-stripper. However, the loading should be kept at less than 0.0065 kg T-P/kg MLSS·d at the sludge recycle rate of 0.15Q.

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