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The purpose of this study was to examine the effects of different nutrient (carbon, nitrogen, oxygen) concentrations on the microbial activity and community structure in membrane-aerated biofilms (MABs). MABs were grown under well-defined conditions of fluid flow, substrate concentration, and membrane oxygen partial pressure. Biofilms were then removed and thin-sliced using a cryostat/microtome parallel to the membrane. Individual slices were analyzed for changes with depth in biomass density, respiratory activity, and the population densities of ammonia-oxidizing and denitrifying bacteria populations. Oxygen-sensing microelectrodes were used to determine the depth of oxygen penetration into each biofilm. Our results demonstrated that ammonia-oxidizing bacteria grow near the membrane, while denitrifying bacteria grow a substantial distance from the membrane. However, nitrifying and denitrifying bacteria did not grow simultaneously when organic concentrations became too high or ammonia concentrations became too low. In conclusion, membrane-aerated biofilms exhibit substantial stratification with respect to community structure and activity. A fundamental understanding of the factors that control this stratification will help optimize the performance of full-scale membrane-aerated biofilm reactors for wastewater treatment.

Biofilms cultivated on oxygen-filled gas-permeable membranes grow differently than conventional biofilms, as the chemical species required for growth diffuse from different sides of the biofilm. Oxygen is delivered directly to the base of the biofilm by the membrane, while organic substrates and other soluble nutrients are provided to the upper surface of the biofilm via the water in which the membranes are immersed. This counterdiffusion of nutrients results in a growth environment very different from that of conventional biofilms that receive both oxygen and other nutrients from the water. In recent years, membrane-supported biofilms have been shown to simultaneously remove chemical oxygen demand (COD) and inorganic nitrogen from wastewater in laboratory studies. Several investigators have developed computer models of these biofilms, but they have all focused on a single population of aerobic bacteria. While these models are useful in characterizing the behavior of these biofilms in pure cultures, they are not useful in modeling the behavior of the biofilms in mixed cultures such as those found in wastewater treatment. In this study, a multipopulation biofilm model was developed that includes aerobic heterotrophs, nitrifiers, denitrifiers, and acetoclastic methanogens. The model was constructed with Aquasim software and can predict the COD and inorganic nitrogen removal behavior observed previously in experimental studies. In this paper we present examples of predicted biofilm behavior and compare the results of this multiple-population model with the single-population models published previously. In addition, the behavior of the biofilm is discussed in terms of application to wastewater treatment.

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