Preanoxic(Continued Aer。b Retum activated suoge Sludge Preanoxic zones can olso be used in a step-feed BNR process. Because step-feed BNR processes are usually adapted to existing mulfiple-pass full scole tanks, symmetrical anoxic/ aerobic stages are generally used. How- ever, nonsymmetrical designs with smaller initial anoxic/aerobic stages can take better a vontoge of the higher MLSS concentration in the early due bo less RAS dilution, resulting in greater treatment copacity. A possible aunt Aow spliting percent distribution for a 4- pass sy 15: 35: 30: 20. The final ow portion to the last anoxic/aerobic zone is criti reduced and w f thus determine the final eluent NO N concentrotion EfRuent NOx-N concentrations of less than 8 mg/L are possib14