External nitrification in biological nutrient removal activated sludge systems

A biological nutrient removal (BNR) activated sludge (AS) scheme incorporat ing external nitrification in a fixed media system is evaluated. A laborato ry scale investigation of the scheme indicates that it holds considerable p otential 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 fract ion can be considerably enlarged at the expense of the aerobic mass fractio n creating conditions that (i) allow it to achieve high N removals with dom estic wastewaters with high TKN/COD ratios and (ii) promote anoxic P uptake polyphosphate accumulating organisms (PAO) to develop in the system. From this, and earlier investigations with conventional BNR systems, it appears that anoxic P uptake biological excess P removal (BEPR) is only about two t hirds of aerobic P uptake BEPR. Inclusion of anoxic P uptake PAOs in, and e xclusion of nitrifiers from, the BNRAS system are not essential for the sch eme. However, conditions that promote aerobic P uptake to maximize BEPR, ar e also conducive to nitrifier growth, which, if supported in the BNRAS syst em, would require virtual complete nitrification in the fixed media system to avoid nitrate interference with BEPR. Before the scheme can be implement ed at targe scale, an engineering and economic evaluation is required to qu antify its potential benefits and savings.