A Modeling Approach to Analyze Performance of a Minimal Aeration Biological Nutrient Removal (BNR) Pilot-Scale Plant
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ABSTRACT The aeration of wastewater in the activated sludge process for biological nutrient removal is energy intensive. For the average wastewater treatment plant (WWTP), aeration contributes 60% of the electrical energy costs required for plant operation. Conventional wastewater treatment operates the aeration basins at dissolved oxygen (DO) concentrations greater than 2 mg-DO/L, where stable nutrient removal is achieved. However, a 5-month study conducted at the Nine Springs WWTP in Madison, WI, demonstrated that operating a large portion of the aeration zone at less than 0.5 mg-DO/L achieved reliable phosphorus and nitrogen removal. Research conducted by the University of Wisconsin-Madison, in collaboration with the Madison Metropolitan Sewerage District, demonstrated that sequentially reducing the DO concentration within the aeration zone from conventional high aeration to low-DO operation of a University of Cape Town (UCT) pilot-scale plant throughout one year was effective at providing stable nitrogen and phosphorus removal; this research is an extension of that project. This pilot plant, continuously fed settled wastewater, was operated for an additional 19 month period at an average DO concentration throughout the aeration zone of 0.39 mg-DO/L. This long-term operation revealed challenges in performance that need to be understood before full-scale implementation of low-DO operation can be recommended. With seasonal water temperatures reaching below 14°C, the pilot experienced reduced nitrogen removal performance. A BioWin™ 5.2 dynamic model, utilizing experimentally determined nitrogen removal kinetic parameters, was used to simulate the performance of the pilot plant, elucidate the causes of poor performance, and propose corrective actions. Low DO operation affords a significant operational energy cost reduction; by understanding the factors that drive performance and simulating conditions that lead to effective low-DO operation, this research contributes to proving the efficacy of a stable and sustainable approach to wastewater treatment.