External nitrification in biological nutrient removal activated sludge systems

Abstract

A biological nutrient removal (BNR) activated sludge (AS) scheme incorporating external nitrification in a fixed media system is proposed. A laboratory-scale evaluation of the scheme indicates that it holds considerable potential for BNRAS system intensification through major reduction in sludge age and oxygen demand and significant improvement in sludge settleability. Because the BNRAS system is not required to nitrify, its anoxic mass fraction can be considerably enlarged at the expense of the aerobic mass fraction creating conditions that allow it to achieve high N removals with domestic wastewaters with high TKN/COD ratios; and promote anoxic P uptake polyphosphate accumulating organisms (PAO) to develop in the system. From this, and earlier investigations with conventional nitrification denitrification biological excess P removal (NDBEPR) systems, it appears that anoxic P uptake BEPR is only about 2/3 of aerobic P uptake BEPR. Hence, inclusion of aerobic P uptake PAOs in the BNRAS system is desirable for the proposed scheme.

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