Inner-profiles of a spherical bio-carrier determined by microelectrodes

The microbial activity within bio-carrier is a key factor in the performance of biofilm reactor. In this paper, a spherical bio-carrier comprised of a netlike shell, plastic stuffing, and a perforated hollow duct was introduced. We examined four runs of biofilm processes for simultaneous nitrification and denitrification of the bio-carrier. The heterogeneity of microbial processes and the impact of substrate, oxygen concentrations, as well as the hollow duct on the inner environment of the bio-carrier was determined by microelectrodes for DO, ORP, NH4+ and NO3−. Clear evidence was found that aerobic/anoxic zones could be concurrently in existence at run 2 and run 4. Due to microbial oxygen utilization, the aerobic zone was limited to the surface layer (14–17 mm) of the bio-carrier. Although the anoxic zone disappeared at a lower substrate loading and a higher oxygen concentration in the bulk solution, the bio-carrier had great potential for denitrification as long as the quantity of organic substances was not the limiting factor. The contaminant and oxygen concentrations in the bulk solution were correlated with the oxygen uptake rate, which directly influenced the oxygen penetration in the bio-carrier, thus resulting in redox potential changes correspondingly. Different from the DO profiles, the redox potential reflected a comprehensive result of the mixed microbial and chemical activities in the bio-carrier. Moreover, the measured NH4+ and NO3− profiles showed the obvious nitrification dominated in the surface layer while denitrification occurred in the anoxic zone in the bio-carrier. Based on the kinetics of transport and reaction processes, the calculation of oxygen transport coefficients demonstrated that the intra-carrier convection, which was influenced significantly by the bulk oxygen concentration, prevailed against the molecular diffusion in the bio-carrier.