Modeling nitrogen removal in membrane aerated biofilm reactors: the role of nitritation, denitritation, and anammox nitrogen removal

This study develops a two-dimensional, multi-species biofilm model to investigate the influence of environmental factors, specifically temperature and concentrations of oxygen, acetate, and ammonium on nitrogen removal in membrane aerated biofilm reactors (MABRs). The resulting model is a highly nonlinear reaction-diffusion system, explored through computer simulations, and captures microbial interactions, substrate transport, and nitrogen transformations within a biofilm, incorporating the counter-diffusion mechanism. Three nitrogen removal pathways have been examined in this study: nitritation-denitritation (ND), partial nitrification-anammox (PN/A), and conventional nitrification-denitrification (CND). The simulation results show that temperature and concentrations of oxygen and acetate significantly affect nitrogen removal rates and contributions of each pathway. ND dominates under most conditions, while PN/A prevails in oxygen-limited scenarios ($ O_{\infty} = 0.25-0.5\; gm^{-3} $) and co-dominates with ND at moderate oxygen levels ($ O_{\infty} = 0.5-1\; gm^{-3} $). CND is significant only at higher oxygen concentrations ($ O_{\infty} = 5\; gm^{-3} $) with low ammonium ($ N_{1\infty} = 5-15\; gm^{-3} $) and acetate levels ($ A_{\infty} = 6\; gm^{-3} $). Moreover, it has been shown that temperature enhances nitrogen removal primarily by increasing the contribution of anammox. Effective removal rates ($ > 0.1\; g/m^2/d $) occur at $ O_{\infty}\geq 1\; gm^{-3} $ with low to moderate acetate levels ($ A_{\infty} = 6\; gm^{-3} $ to $ < 100\; gm^{-3} $). The simulations further indicate that MABRs can achieve a stable ND nitrogen removal efficiency with biofilm thickness exceeding approximately $ 0.8\; mm $. In this scenario, ammonium-oxidizing bacteria (AOB) and ND denitrifiers outcompete aerobic heterotrophs and nitrite-oxidizing bacteria, resulting in a biofilm structure predominantly composed of AOB and ND denitrifiers. The findings of this study provide valuable insights for optimizing MABR design and operation to achieve energy efficient nitrogen removal.