Heterotrophic activity compromises autotrophic nitrogen removal in membrane-aerated biofilms: results of a modeling study

A 1-d multi-population biofilm model was constructed to study the effect of heterotrophic activity on completely autotrophic ammonium (NH4+) removal in membrane-aerated (counter-diffusion) versus conventional biofilm systems (co-diffusion). Growth of heterotrophic bacteria (HB) was supported either solely by biomass decay products or by organic carbon (as chemical oxygen demand (COD)) in the influent. Three scenarios were considered: influence of HB growing on biomass decay products on steady-state performance (total nitrogen (TN) removal efficiency); influence of the influent COD/N ratio on steady-state performance (supplying COD in the influent); and impact of dynamic changes in the influent COD/N ratio on TN removal efficiency. The results revealed that the TN removal efficiency in the counter-diffusion biofilm was significantly different when HB were included in the simulations at NH4+ surface loads of LNH4>2.7 g - N m(-2) d(-1). Influent COD significantly altered the microbial community composition in the counter-diffusion biofilm and anaerobic NH4+ oxidation could not be sustained at COD/N>2. The co-diffusion system, however, was less affected and more than 50% of the TN removal originated from anaerobic NH4+ oxidation at those ratios. Perturbation experiments showed that step increases to influent COD/N ratios of 2 or higher over a period of 50 d or longer caused a loss of anaerobic NH4+ oxidation capacity which could not be regained within a reasonable time frame (>1000 d) in the counter-diffusion system. In contrast, simulating a 1-d sloughing event only caused a disturbance of 200 d although a maximum biofilm loss of 90-95% occurred. These results clearly indicate the importance of heterotrophic activity in autotrophic N removal biofilms, especially in counter-diffusion systems where they may compromise N removal capacity.