title: Stratification and oxidation-reduction potential change in an aerobic and sulfate-reducing biofilm studied using microelectrodes.
abstract: Recent studies in 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 microm 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 +362 mV at the biofilm surface and -166 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 +194 mV to a negative potential of -77 mV was observed. This occurred within a narrow band of 50 microm in depth. These new experimental findings support the concept of stratification of microbial processes and the associated ORP change in biofilms.
authors.affiliation: University of Cincinnati, USA. Tong.Yu@ualberta.ca
authors.forename: T
authors.initials: T
authors.lastname: Yu
authors.forename: P L
authors.initials: PL
authors.lastname: Bishop
mesh_terms: D001391:Azo Compounds / Q000378:metabolism; D001421:Bacteria, Anaerobic / Q000502:physiology*; D018441:Biofilms; D006863:Hydrogen-Ion Concentration; D008839:Microelectrodes; D010084:Oxidation-Reduction; D011157:Population Dynamics; D013431:Sulfates / Q000032:analysis / Q000737:chemistry*; D013440:Sulfides / Q000032:analysis / Q000737:chemistry; D014865:Waste Disposal, Fluid
chemical_list: D001391:Azo Compounds; D013431:Sulfates; D013440:Sulfides