| In a membrane-aerated biofilm (MAB), a gas-permeable membrane supplies oxygen to a biofilm growing on its surface. Stratification of microbial populations within the biofilm is important for achieving simultaneous nitrification and denitrification. The goal of this research was to characterize changes in MAB stratification and determine the effects of reactor conditions on nitrification and denitrification. Biofilms were grown in a flow-cell reactor under different conditions of oxygen partial pressure, fluid velocity, chemical oxygen demand (COD) concentration, and COD to ammonia-nitrogen (COD:N) ratio. Profiles of dissolved oxygen (DO) with depth were obtained with a microelectrode. Biofilm slices (100--300 mum) were analyzed for protein concentration by the Lowry method, respiratory activity by iodonitrotetrazolium chloride reduction, and populations of nitrifying and denitrifying bacteria by competitive polymerase chain reaction (cPCR).; Increasing fluid velocity from 2 to 14 cm/s resulted in increased oxygen consumption, biomass density, and respiratory activity. Populations of nitrifying and denitrifying bacteria increased with increasing fluid velocity as a result of increased mass transfer of substrate into the biofilm. Increasing the COD concentration from 40 to 200 mg/L resulted in an increase in thickness, density, and respiratory activity and a decrease in oxygen penetration depth. In an air-fed MAB, the increase in COD concentration resulted in a decrease in nitrifying bacteria due to increased competition for oxygen with aerobic heterotrophic bacteria. Increasing the available DO by feeding the membrane oxygen resulted in greater concentrations of nitrifying bacteria at the higher COD concentration. As COD:N ratio increased from 4 to 10, respiratory activity and microbial populations decreased. In particular, a high COD:N ratio and high COD concentration, produced conditions unfavorable for simultaneous nitrification and denitrification.; Model predictions were similar to the experimental results at low COD concentration (40 mg/L) and COD:N ratios (4), but tended to break down under conditions of high COD concentration (200 mg/L) or COD:N ratio (10). Suggested areas for improvement in the model include incorporating (1) biofilm structure heterogeneity, (2) assimilation of ammonia by heterotrophic bacteria, and (3) competition for nutrients between aerobic heterotrophic and nitrifying bacteria. |
