Model-based identification and testing of appropriate strategies to minimize N2O emissions from biofilm deammonification

Abstract

Based on a one-year pilot plant operation of a two-step biofilm nitritation-anammox pilot plant, N2O mitigation strategies were identified by applying a newly developed biofilm modeling approach. Due to adapted plant operation, the N2O emission could be diminished by 75% (8.8% → 2.3% of NH4-Noxidized_AOB). The results (measurement and simulation) confirm the huge importance of denitrification as an N2O source or N2O sink, depending on the boundary conditions. A significant reduction of N2O emissions could only be achieved with a one-step deammonification system, which is related to low nitrite and HNO2 concentrations. Increased oxygen concentrations in the bulk phase are not related to decreased emissions. N2O formation by ammonium-oxidizing bacteria (AOB) just shifts deeper into the biofilm; zones with low oxygen concentrations are not avoidable in biofilm systems. Low oxygen concentrations in the bulk phase, however, result in a reduction of the total net N2O formation due to increased activity of heterotrophic bacteria directly at the source of N2O formation (outer biofilm layer). For the model-based identification of mitigation strategies, the standard modeling approaches for biofilms were expanded by including the factor-based N2O formation and emission approach. The new model ‘Biofilm/N2OISAH’ was successfully validated using data from pilot-scale measurement campaigns. Altogether, the investigation confirms that the employed digital model can strongly support the development of N2O mitigation strategies without the need for specialized measurement inside the biofilm. HIGHLIGHTS Modeling provides deeper insights into the dynamics of N2O formation, conversion, and emission in biofilms.; N2O emission rates of biofilm systems are limited by diffusion rates.; N2O degradation mainly depends on the concentrations of oxygen and HNO2.; Ambivalent role of oxygen concentration: Lower concentrations promote heterotrophic N2O denitrification, but increase N2O formation by AOB.;

Keywords

• aeration strategies
• control algorithms
• denitrification
• ghg emissions