Using a Process Modeling and LCA Approach for Balancing Energy, Operating Costs, Greenhouse Gases, and Water Environment

Process models can capture the dynamic effects of activated sludge processes that impact greenhouse gas (GHG) emissions, particularly nitrous oxide (N2O) emissions and indirect carbon dioxide (CO2) emissions from energy consumption. Coupled with instrumentation, control and automation (ICA), process models can be used to develop biological nutrient removal (BNR) control strategies that can minimize the GHG footprint of wastewater utilities, as well as to help balance operating costs and treatment goals. A life cycle assessment (LCA) model can help quantify the overall GHG footprint and environmental impact of a particular control strategy due to electricity, chemical addition, N2O emissions, and effluent water quality. Several control strategies for a four-stage BNR plant were modeled and compared in terms of Scope 2 (electric) GHG emissions, aeration costs, and effluent water quality. The N2O emissions potential of each control strategy was also investigated based upon recent research findings. A three loop control strategy to control: 1) dissolved oxygen (DO) in the primary oxic zone per ammonia in the effluent, 2) the internal recycle rate based on nitrate in the primary anoxic zone, and 3) the wasting rate based upon total solids, proved to reduce aeration electricity costs and electric GHG emissions by 27 percent, and improve total nitrogen (TN) in the effluent by 2 mg/L. However, it did not improve N2O emissions potential since it was not originally designed to mitigate N2O emissions. Another control strategy, increasing the internal recycle rate, showed promise in reducing the N2O emissions potential and effluent TN.