Evaluation of Alternative DO Control Strategies for High Rate BNR Processes

Assessment of the level of DO control necessary to meet nutrient removal performance targets and contain energy costs is critical in large wastewater treatment plants. An additional challenge for high rate, step feed BNR processes are the changing DO demands, due to both the diurnal variation and the process' step feed pattern. A comprehensive evaluation using dynamic process simulation showed that a high degree of DO control automation would not materially enhance process performance. By balancing control system complexity with process performance, it was found that utilizing a simplified control strategy with DO control provided to each pass would provide an optimal balance between O&M requirements and process performance. In case of DO control system failure, a preset dry weather diurnal air flow pattern would allow for adequate control with minimal impacts on performance. The process analysis showed that a greater reduction in effluent nitrogen levels, and reduced reliance on the DO control system would be obtained by modifying the process operating mode to allow for separate nitrification of high ammonia in-plant recycles. A probabilistic analysis of the effluent nitrogen levels under both operating modes is ongoing.