Optimization of operation conditions for the mitigation of nitrous oxide (N2O) emissions from aerobic nitrifying granular sludge system.

The optimization of operation parameters is a key consideration to minimize nitrous oxide (N2O) emissions in biological nitrogen removal processes. So far, different parameters have only been investigated individually, making it difficult to compare their specific effects and combined influences. In this study, we applied the Plackett-Burman (PB) multifactorial experimental design and response surface methodology (RSM) analysis to find the optimized condition for the mitigation of N2O release in a nitrifying granular sludge system. Seven parameters (temperature, pH, feeding strategy, C/N ratio, aeration rate, Cu(2+) concentration, and aeration mode) were tested in parallel. Five of them (other than chemical oxygen demand/nitrogen (C/N) ratio and Cu(2+) concentration) were selected as influential factors. Since the type of feeding strategies and aeration modes cannot be quantified, continuous feed strategy and anoxic/oxic aeration mode were applied for the following study. Influences of temperature, pH, and aeration rate on N2O emissions were tested with RSM analysis to further investigate the mutual interactions among the parameters and to identify the optimal values that would minimize N2O release. Results showed the minimum emission value could be obtained under the temperature of 22.3 °C, pH of 7.1 and aeration rate of 0.20 m(3)/h. Predicted results were then verified by subsequent validation experiments. The estimated N2O emission value of each design by RSM was also observed in good relationships with experimental result.