Including Nitrite as an Intermediate in Simultaneous Nitrification/Denitrification Membrane-Aerated Biofilm Reactor Models

Abstract

Abstract

Our understanding of biofilm function has been greatly increased with the aid of one-dimensional biofilm models. However, to date, there has been no evaluation of how the inclusion or exclusion of nitrite within nitrification/denitrification biofilm models may impact simulated prediction of the biofilm community composition and effluent water quality. As such, several variations of commonly assumed nitrification and denitrification models with a common set of kinetic parameters were simulated in a one-dimensional membrane-aerated biofilm model to identify the differences in simulation output with respect to simultaneous nitrification and denitrification. Our results indicated that the inclusion of nitrite as an intermediate affected the biofilm community composition, microbial activity, and effluent water quality. Even under circumstances when the predicted water quality results were similar, significant differences persisted in the microbial activity. For example, chemical oxygen demand (COD) removal was independent of the including nitrite as an intermediate; however, the processes that contributed to COD removal (i.e., aerobic oxidation vs. anoxic denitrification) varied considerably between model assumptions, and as a result, affected differences between the predicted oxygen demand. Simulations also helped to identify inefficiencies caused by nitrite looping, a phenomenon in which NO₂⁻ cycles between oxidation within aerobic regions and reduction within anoxic regions. Overall, results indicated that if one-dimensional biofilm models are used to inform our understanding of fundamental biofilm processes, then the manner in which nitrite is modeled must be carefully considered to avoid introducing modeling artifacts into our interpretation of simulation results.