Enhancing ammonium oxidation fluxes and nitritation efficiencies in MABRs: a modeling study

Abstract

The membrane aerated biofilm reactor (MABR) is a novel technology based on gas-supplying membranes that supply dissolved O₂ (DO) to biofilms growing on the membrane surface. The counter-diffusion of DO, supplied from base of the biofilm, and the electron donor, supplied from the bulk liquid, make membrane-aerated biofilms (MABs) behave differently from conventional, co-diffusional biofilms (CABs). We used 1-D biofilm modeling to explore ammonium oxidation fluxes, J_NH4, and nitritation efficiencies, η_NO2, for MABs and CABs under nitrifying conditions. The J_NH4 for MABs has three regions of behavior: biomass limitation at low biofilm thicknesses, single substrate limitation for moderate thicknesses, and dual substrate limitation at higher thicknesses. For both MABs and CABs, for a given bulk DO and NH₄⁺ concentration, the J_NH4 increases with increasing biofilm thicknesses up to a critical value. For further increases in thickness, CAB fluxes remain the same, while MAB fluxes decrease. CABs typically have lower maximum J_NH4 values than MABs, but achieve high η_NO2 values more easily. For MABs, η_NO2 is high as long as NH₄⁺ fully penetrates the biofilm. A low membrane mass transfer coefficient, K_m, results in lower DO concentrations at the membrane surface, decreasing J_NH4 values but increasing the η_NO2. The effects of a low K_m can be offset by increasing the O₂ supply pressure. Increasing the bulk DO can significantly increase J_NH4 values, especially for thick biofilms. Bulk BOD has a detrimental effect on J_NH4 values, but MABs are less susceptible to BOD inhibition than conventional biofilms. Biofilm roughness significantly impacts both J_NH4 and η_NO2 values, depending on the biofilm thicknesses and substrate concentrations, so average biofilm thicknesses should be used with caution. Results from this study are consistent with experimental results from the literature. This work provides a mechanistic understanding of MABR behavior. This can allow researchers to better design experiments and better interpret results, and for practitioners to develop more effective applications.