Most Korean community represents the primary effluent of 180 mg/L COD, 80 mg/L BOD, 25 mg/L TKN and 4 mg/L TP. A/O, A₂/O and MUCT (Modified University of Cape Town) systems were applied to laboratory scale reactor with a temperature of 10 to 20℃. A total of 6 hour hydraulic retention time including anaerobic, anoxic and aerobic zones was used with a maximum 3,000 mg/L MLss to simulate the existing municipal plants. All BNR systems represented effluent BOD less than 10 mg/L. MUCT produced better quality; 0.5 mg/L SP (soluble phosphorus) with 10 mg/L TN vs 1.8 mg/L SP with 12 mg/L TN for A²/O with the same internal recycle ratio. Performance of BNR systems would suggest the primary effluent used for this study represents a nature of slowly biodegradable COD. As that result, anaerobic fraction must be increased to attain lower effluent P concentrations. However, prefermentation to increase P removal was not necessary since P was also limited. Microbial mass fractions computed from COD and nitrogen mass balances suggested that poly-P microbes were about 33% in A/O and MUCT, denitrfier fractions were about 30% in A²/O and MUCT. Nitrifier fractions were about 2%.