Abstract
A novel packing medium comprised of a spherical net-like shell, plastic stuffing, and a perforated hollow duct for biological reactors was investigated. In this study, we examined a biofilm process that employed the packing medium for combined aerobic carbonaceous substances removal, nitrification, and denitrification. As a result, we were able to obtain the average of nitrification, denitrification, as well as the chemical oxygen demand (COD) removal capacities, which correspond to 0.2 kg N/m3 · day, 0.3 kg N/m3 · day, and 5.56 kg/m3 · day, respectively. Moreover, the spatial concentration distribution of chemical species inside the packing medium was studied, using microelectrodes for dissolved oxygen, NH4+, NO3−, and redox potential. Clear evidence was found that aerobic/anoxic spatial stratification was formed inside the packing medium, and that due to microbial oxygen utilization, the aerobic zone was limited to a surface layer of 12 mm, which had a corresponding decrease in redox potentials from 157.4 mV to −99.5 mV. It was also found that nitrification dominated in the top 4 mm of the surface layer, whereas denitrification occurred in the deeper part of the packing medium. Furthermore, a gap zone only for aerobic heterotrophic oxidation was found to be present between nitrification and denitrification. This implied that a high C/N ratio could contribute to a decrease in the effective size of the packing medium. Based on the measured DO and NH4+ depth profiles under steady-state conditions, the effective diffusion coefficients of DO and NH4+ in the packing medium were calculated to be 0.0172 m2/day and 9.7 × 10−5m2/day, respectively. The turbulent diffusion of oxygen ensured the sufficient oxygen supply not only for aerobic carbon oxidation but for nitrification inside the packing medium as well.