Pilot plant demonstration of stable and efficient high rate biological nutrient removal with low dissolved oxygen conditions.

Aeration in biological nutrient removal (BNR) processes accounts for nearly half of the total electricity costs at many wastewater treatment plants. Even though conventional BNR processes are usually operated to have aerated zones with high dissolved oxygen (DO) concentrations, recent research has shown that nitrification can be maintained using very low-DO concentrations (e.g., below 0.2 mg O2/L), and therefore, it may be possible to reduce energy use and costs in BNR facilities by decreasing aeration. However, the effect of reduced aeration on enhanced biological phosphorus removal (EBPR) is not understood. In this study, we investigated, at the pilot-scale level, the effect of using minimal aeration on the performance of an EBPR process. Over a 16-month operational period, we performed stepwise decreases in aeration, reaching an average DO concentration of 0.33 mg O2/L with stable operation and nearly 90% phosphorus removal. Under these low-DO conditions, nitrification efficiency was maintained, and nearly 70% of the nitrogen was denitrified, without the need for internal recycling of high nitrate aeration basin effluent to the anoxic zone. At the lowest DO conditions used, we estimate a 25% reduction in energy use for aeration compared to conventional BNR operation. Our improved understanding of the efficiency of low-DO BNR contributes to the global goal of reducing energy consumption during wastewater treatment operations.

Taking advantage of aerated-anoxic operation in a full-scale University of Cape Town process.

To evaluate the potential benefits or limitations of aeratedanoxic operation in high-rate biological nutrient removal processes, we conducted a full-scale experiment in a University of Cape Town (UCT)-type wastewater treatment plant by reducing oxygen supply and increasing flowrates within one treatment train so that aerated-anoxic conditions (i.e., zones that receive oxygen but maintain dissolved oxygen concentrations below 0.5 mg/L) could be implemented in a section of the aerated zone. With this retrofitted configuration, total nitrogen removal increased from 54 to 65%, but was limited by the organic carbon available for denitrification. Furthermore, the significant reduction in dissolved oxygen concentrations in the aerated zone did not negatively affect enhanced biological phosphorus removal, demonstrating that the implementation of an aerated-anoxic zone within a UCT-type reactor can contribute to a reduction in operational costs and a slight improvement in total nitrogen removal, without compromising the extent of phosphorus removal.