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Considerable research has been performed on biological nutrient removal (BNR) systems which remove the problematic nutrients, nitrogen and phosphorus, that cause eutrophication. This research focussed on setting up two laboratory-scale anaerobic/anoxic/oxic (A/A/O) systems and investigating their reliability while undergoing various parameter changes. Pump failure, in the ®rst trial, R1, led to a decrease in pH, exposure of the sludge to relatively low nitrate concentrations and reduction of the suspended solids concentration within the system. This adversely affected the phosphorus removal ef®ciency. Shock loading the system with increased in¯uent phosphate concentrations for 56 days was shown to aid remediation of the phosphorus removal ef®ciency to values between 65 and 70% (w/w). The second trial, R2, highlighted the presence of bacteria capable of P-uptake under anoxic conditions (in the presence of nitrate). The characteristic anaerobic P-release was also evident. The bacteria responsible for phosphate uptake under anoxic conditions are thought to be the denitrifying phosphate removing bacteria (DPB). However, the presence of higher nitrate concentrations retarded the P-removal ef®ciency to some extent. Secondary release of P was evident in the clari®er of the A/A/O system during the R2 trial and especially during times of increased nitrate concentrations in the system. Between 20 and 40% (w/w) of the P taken up in the oxic stage of the system was released in the clari®er at various stages throughout the trial.

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