Benchmarking of Control Strategies for the Prevention of Nitrous Oxide Accumulation in Wastewater Treatment Plants

Nitrous oxide (N 2 O) is an ozone-depleting substance and a harmful greenhouse gas with a global warming potential three hundred times larger than that of carbon dioxide. Measurement campaigns at domestic wastewater treatment plants (WWTPs) have found significant N 2 O emissions [1–5]. Specifically, N 2 O is found to be produced during the typical WWT processes involved in the nitrogen removal [6]. Suboptimal operating conditions can trigger the biological processes producing this gas. For the purpose of reducing the carbon footprint of WWTPs, control strategies accomplishing those environmental conditions minimizing the production of N 2 O have to be developed. Several investigations have been performed to find out how to reduce N 2 O emissions [7,8]. There is a considerable amount of evidence showing that denitrification by ammonia-oxidizing bacteria (AOB) is the main producer of the total N 2 O emitted [9]. For this reason, a control strategy regulating the aeration regime of the aerated tanks in the mainstream of WWTPs was developed by Boiocchi et al. [10]. The control strategy showed its effectiveness in drastically reducing the N 2 O emissions. However, denitrification by heterotrophic bacteria (HB) can contribute to the total N 2 O emitted as well. For this reason, control strategies minimizing N 2 O produced during HB denitrification should be developed. N 2 O is produced by HB as an intermediate compound during the reduction of nitrogen oxides, such as nitrate (NO 3-) and/or nitrite (NO 2-), to dinitrogen (N 2). If the rate of N 2 O production is higher than the rate of N 2 O reduction to N 2 , N 2 O accumulates in the liquid phase. Several factors are known to trigger the accumulation of N 2 O during HB denitrification: high oxygen concentrations, low organic biodegradable carbon availability, low anoxic hydraulic retention time, and accumulation of nitrite and/or nitric oxide are the main factors. In full-scale WWTPs, N 2 O production by heterotrophs can occur in the aerobic zone as a consequence of scarce oxygenation regime where nitrite accumulates. This contribution is already drastically reduced through the same controller used to minimize the N 2 O production by AOB [10]. However, in pre-denitrification systems, the amount of N 2 O accumulated in the anoxic zone ends up in the subsequent aerobic zone where it can be removed by stripping. Therefore, it is of interest to reduce the amount …