Operational implications of low dissolved oxygen in water resource recovery facilities: A simulation study on greenhouse gas emissions and carbon footprint reduction

Water resource recovery facilities (WRRFs) play a crucial role in mitigating urbanization's environmental impact by reducing nitrogen and phosphorus discharges through biological and chemical treatments to prevent eutrophication. Energy-intensive biological processes like aeration represent a significant portion of the facilities' total energy consumption. Additionally, greenhouse gas (GHG) emissions, chemical usage, and solids disposal can play a role in the overall footprint of a WRRF. This study investigated the implications of implementing low dissolved oxygen (DO) operation in WRRFs through model simulations. A SUMO model was calibrated using 2 years of field collected data on biochemical oxygen demand (BOD), total suspended solids (TSS), total ammonia (NH x ), and total phosphorus (TP) from a biological nutrient removal (BNR) WRRF in Texas. Four scenarios were simulated, comparing low DO (0.5 mg L −1 ) and conventional DO (2 mg L −1 ) levels treating carbon-rich and carbon-deficient influent conditions to better understand the impact of DO setpoints on nutrient removal performance, GHG emissions, chemical usage, and solids disposal impacts. The results indicated that total indirect GHG emissions were lower for low DO operations compared to high DO operations, with reductions of 48 % and 63 % for high carbon (HC) and low carbon (LC) wastewater, respectively. In scenarios with LC wastewater, low DO operations resulted in lower GHG emissions (800–1330 kg CO 2 -eq) compared to high DO operations (1170 to 1890 kg CO 2 -eq). Similarly, in HC wastewater scenarios, blower-related emissions were higher under high DO conditions (1400 to 2095 kg CO 2 -eq) than under low DO conditions (1005 to 1540 kg CO 2 -eq). The combination of low DO and HC conditions led to reduced effluent nitrogen levels, reducing the need for external carbon dosing, and subsequently lowering associated emissions. Direct emissions from nitrous oxide (N 2 O) were not estimated in the model, but rather a N 2 O production budget was estimated. The GHG reductions from indirect emissions, including blower savings, associated with low DO operation were added to a potential N 2 O production rate where low DO would still result in reduced emissions. Based on this N 2 O budget, an additional 0.1–0.5 % of influent nitrogen could be converted to N 2 O before low DO operation reaches high DO operation in terms of carbon footprint (CF). The findings suggest that operating the biological component of a WRRF at low DO setpoints can significantly reduce the CF. Further research is necessary to quantify GHG emissions, especially N 2 O, and determine if low-DO operations can achieve a net positive reduction in CF.