LAUSR

Abstract Nitrate contamination of groundwater has been recognized as a significant environmental problem world widely. Sulfur-driven mixotrophic denitrification has been demonstrated as a promising groundwater treatment process, which though plays an important role in nitrous oxide (N 2 O) emissions, significantly contributing to the overall carbon footprint of the system. However, the current process optimizations only focus on nitrate removal and excess sulfate control, with the N 2 O emission being ignored. In this work, an integrated mathematical model was proposed to evaluate the N 2 O emission as well as the excess sulfate production and carbon source utilization in sulfur-driven mixotrophic denitrification process. In this model, autotrophic and heterotrophic denitrifiers use their corresponding electron donors (sulfur and organic matter, respectively) to reduce nitrate to nitrogen gas, with each modeled as three-step denitrification (NO 3 − to N 2 via NO 2 − and N 2 O) driven by sulfur or organic matter to describe all potential N 2 O accumulation steps. The developed model, employing model parameters previously reported in literature, was successfully validated using N 2 O and sulfate data from two mixotrophic denitrification systems with different initial conditions. Modeling results revealed substantial N 2 O accumulation due to the relatively low autotrophic N 2 O reduction activity as compared to heterotrophic N 2 O reduction activity, explaining the observation that higher carbon source addition resulted in lower N 2 O accumulation in sulfur-driven mixotrophic denitrifying system. Based on the validated model, optimizations of the overall system performance were carried out. Application of the model to simulate long-term operations of sulfur-driven mixotrophic denitrification process indicates that longer sludge retention time reduces N 2 O emission due to better retention of active biomass. High-level total nitrogen removal with significant N 2 O emission mitigation, appropriate excess sulfate control and maximized COD utilization can be achieved simultaneously through controlling the influent nitrate and COD concentrations.