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Recent studiesin aerobic-nitrifying biofilms demonstrated the heterogeneity of biofilms used in wastewater treatment and led to modifications of the homogeneous assumptions in the conventional biofilm kinetic models. However, the stratification in aerobic-anaerobic biofilms has not been well investigated because of a lack of effective experimental tools. In this study a suite of microelectrodes, recently developed in the authors' laboratory, was used to examine the stratification of microbial processes and the change of oxidation-reduction potential (ORP) within an aerobic, sulfate-reducing biofilm. The microelectrodes have tip diameters of 3 to 20 m and a high spatial resolution. They were used to measure the profiles of oxygen, total dissolved sulfide, ORP, and pH as a function of depth in the biofilm. The biofilm reactor was used to treat an azo-dye-containing wastewater with a chemical oxygen demand of 160 mg/L. The reactor bulk-phase dissolved oxygen concentration of the biofilm was 1.7 mg/L. The experimental results demonstrated that the microbial processes in the biofilm were stratified. In this biofilm, aerobic oxidation took place only in a shallow layer of 0.55 mm near the surface and sulfate reduction occurred in the deeper anoxic zone. The ORP changed with the shift of primary microbial process. The ORP was ϩ362 mV at the biofilm surface and Ϫ166 mV near the substratum. Near the interface between the aerobic zone and the sulfate reduction zone, a surprisingly sharp decrease of ORP from a positive potential of ϩ194 mV to a negative potential of Ϫ77 mV was observed. This occurred within a narrow band of 50 m in depth. These new experimental findings support the concept of stratification of microbial processes and the associated ORP change in biofilms. Water Environ. Res., 73, 368 (2001).

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