Nitrification and denitrification were evaluated in multiple CSTR continuous flow fixed film systems at 12°C. Three systems were operated with three types of media installations and a fourth as a control without media. The media configurations evaluated included: (i) frame mounted fine pore sponge media supported on wires; (ii) free floating fine pore sponge media; and (iii) fixed location frame mounted rope media (ringlace). The pore size for sponge media was 14 pores per centimeter. Each system included a two-cell anaerobic zone with 17 percent of total volume, two-cell anoxic zone with 17 percent of total volume, and a three-cell aerobic zone with 64 percent of total volume. The multi-cell configuration was used to maximize kinetics of removal with suspended growth biomass and evaluate improvements in nitrogen removal beyond the levels achieved with suspended growth.
At the optimum location (aerobic cells 1 and 2), the nitrification in cells containing free-floating sponges was 143 percent of the control at aerobic MCRTs of 3.1 and 3.4 days, with 35 percent media volume to cell volume. The nitrification with rope media was 136 percent of the control in middle third of the aerobic zone. Removals with ringlace were poorer in the first aerobic cell operating at higher COD levels, and in the third aerobic cell which did not contain sufficient biofilm growth at low levels of COD. Nitrification was 14 percent higher in Systems 1 and 2 with fixed and free floating sponges, respectively, and 8 percent higher in System 3 (ringlace) when all systems were spiked with 20 mg/L additional ammonium over a 24 hour period. Optimizing location of the media, with higher density of media at locations where adequate COD and ammonium-N are available for biofilm growth increase nitrification on media. Studies in full scale systems show that COD and ammonium-N concentrations at downstream locations in the activated sludge basin increase during peak flows and with step feed of wastewater. This helps increase biofilm growth and improve nitrification on the media downstream locations.
Denitrification observed in the aerobic cells of the fixed film systems was in excess of the control. All aerobic cells were operated at D.O. levels in excess of 6.0 mg/L. The fraction of total denitrification under aerobic conditions was 0.0 in the control as compared to 0.14 to 0.24 with ringlace media and fixed sponge media, respectively, at an aerobic MCRT 3.1 days. Fraction aerobic denitrification in all systems increased with an increase in MCRT of suspended growth - to 0.21 in the control and 0.35 and 0.39 with ringlace and sponges, respectively, at aerobic MCRTs of 3.4 to 7.7 days.