Low Dissolved Oxygen Operation Enhances Biological Phosphorus Removal in Wastewater Treatment

Biological phosphorus (bio-P) removal occurs when bacteria are exposed to anaerobic/aerobic conditions that stimulate polyphosphate-accumulating organisms (PAOs) to uptake phosphorus. In most treatment plants, dissolved oxygen (DO) concentrations in the aeration tanks are commonly at or above 2.0 mg O2/L, which is often referred to as the most optimal aeration condition for bio-P. If bio-P can happen efficiently at lower DO (< 0.3 mg O2/L), then the energy demand of aeration can be decreased appreciably. This research was conducted in a biological nutrient removal (BNR) pilot plant to assess the performance of bio-P at progressively lower DO operation. This pilot employed an anaerobic-oxic (AO) process and was fed real sewage at a constant temperature. Throughout this study, the pilot operating DO setpoints were steadily decreased from 2.0 mg O2/L to 0.2 mg O2/L and then raised back up to 2.0 mg O2/L to monitor the adaptation of PAOs to low DO and to characterize bio-P performance. Measurements were conducted to assess carbon uptake and phosphorus release ratios (C/P), the PAO oxygen half saturation coefficient (Ko), effluent phosphorus concentrations, maximum phosphorus uptake rates, as well as PAOs and glycogen-accumulating organisms (GAOs) prevalence. The measured Ko for P-uptake was consistently very low (< 0.1 mg O2/L) at both high and low operating DO setpoints, demonstrating that PAOs were not inhibited by low DO operation and did not need to adapt to low DO. Additionally, in-situ OP removal remained stable and high (≥90%) even at the lowest operating DO setpoints with effluent OP concentrations averaging the lowest at an operating DO setpoint of 0.2 mg O2/L (0.18 mg P/L). The C/P ratios and PAO/GAO measurements showed that PAOs outperformed GAOs at lower DO setpoints. Unexpectedly, PAOs were also found to exhibit Haldane kinetics when OP uptake rates were measured across various batch DO setpoints, highlighting oxygen inhibition at high DO concentrations. These results provide strong evidence that low DO operation is not only compatible with bio-P, but also incredibly beneficial – exactly the opposite of most design and operational guidance.