Stratification and oxidation-reduction potential change in an aerobic and sulfate-reducing biofilm studied using microelectrodes

Recent studies in aerobic-nitrifying biofilms demonstrated the heterogeneit y 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 invest igated because of a lack of effective experimental tools. In this study a s uite of microelectrodes, recently developed in the authors' laboratory, was used to examine the stratification of microbial processes and the change o f oxidation-reduction potential (ORP) within an aerobic, sulfate-reducing b iofilm. The microelectrodes have tip diameters of 3 to 20 mum and a high sp atial resolution. They were used to measure the profiles of oxygen, total d issolved sulfide, ORP, and pH as a function of depth in the biofilm. The bi ofilm reactor was used to treat an ate-dye-containing wastewater with a che mical oxygen demand of 160 mg/L. The reactor bulk-phase dissolved oxygen co ncentration of the biofilm was 1.7 mg/L. The experimental results demonstra ted that the microbial processes in the biofilm were stratified. In this bi ofilm, 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 +362 m V at the biofilm surface and -166 mV near the substratum. Near the interfac e between the aerobic zone and the sulfate reduction zone, a surprisingly s harp decrease of ORP from a positive potential of +194 mV to a negative pot ential of -77 mV was observed. This occurred within a narrow band of 50 mum in depth. These new experimental findings support the concept of stratific ation of microbial processes and the associated ORP change in biofilms.