The development of a low-cost and efficient hydrogenotrophic denitrification (HD) system for nitrate removal from groundwater is urgently required in developing countries. In the present study, a sponge-based HD reactor was developed to examine the effects of various carrier filling ratios (0%, 10%, 20%, and 30%) on the HD performance. HD reactors with sponges showed higher nitrogen removal capacities than that without sponges. There was no significant difference in the nitrogen removal efficiency at filling ratios of 10%, 20%, and 30%. The NO₃－N removal rate varied based on the filling ratio, and reached 382, 470, 548, and 530 g－N/(m³・d) at filling ratios of 0%, 10%, 20%, and 30%, respectively. Furthermore, the attached biomass was considered to play an important role in the enhancement of NO₃－N removal. The coexistence of hydrogenotrophic and heterotrophic denitrification activity was observed in the reactors, and there was a strong correlation between total volatile sponge-attached biomass and heterotrophic activity. However, heterotrophic activity accounted for a maximum of only 5.1% of the dissolved inorganic nitrogen removal. The high nitrogen removal rate achieved in this study shows that sponge-based HD reactors can potentially be used for NO₃－N removal from groundwater.

 Both the stepwise advanced treatment method (experimental system) for reducing the biochemical oxygen demand, total nitrogen, and total phosphorus in secondary treated water and standard activated sludge method (control system) were compared for 14 months. Performance was evaluated by Student’s t-test. The results revealed that the water quality items in the experimental system met the target water quality standard for stepwise advanced treatment and were significantly reduced compared to those in the control system.

 Next, the removal efficiency of hygienically relevant microbes (total coliform, Escherichia coli, fecal streptococci, and enterococcus form) was compared. The concentrations of these microbes and removal ratio in the experimental system were significantly reduced and improved compared to in the control system, respectively. Particularly, the concentration of total coliform bacteria met the effluent standards (≤3,000 colony-forming units/mL) before disinfection treatment. The results of Pearson product moment correlation analysis and hierarchical variable cluster analysis indicated that the removal ratio of hygienically relevant microbes in the secondary treatment process were highly correlated with mixed liquor suspended solids, aerobic-solids retention time, dissolved oxygen, sludge age, air-to-flow ratio, and activated sludge biota. These results clarify the operating factors that increase the hygienically relevant microbe removal ratio in the stepwise advanced treatment method.

 The simultaneous anaerobic ammonium oxidation (anammox) and heterotrophic denitrification (SAD) process has been proposed for the nitrogen removal from domestic wastewater. In the SAD process, anammox bacteria are inoculated into the denitrification reactor. Nitrate is reduced to nitrite by heterotrophic denitrifiers. Then, nitrite is partly intercepted by anammox bacteria from heterotrophic denitrifiers and is reduced to N₂ using ammonium. The purpose of this study was to investigate the possibility of nitrogen removal by the SAD process in the 24－h batch mode at 20℃. Both samples of anammox sludge acclimated at 20℃ and activated sludge collected from a wastewater treatment plant were inoculated to 20mL synthetic wastewater at 1500 mg－MLSS/L, respectively. The nitrogen removal of 49－80% in the SAD process was demonstrated in a wide range of the C/N ratio of 0.4－1.5 with synthetic wastewater containing 70 mg－N/L nitrate, 50 mg－N/L ammonium, and acetate as the sole carbon source. The nitrogen removal in the SAD process was higher than the typical denitrification process at lower C/N ratio. However, the nitrogen removal in the SAD process at 20℃ was much slower than that obtained at 33℃ in a previous study (Park et al., J. Biosci. Bioeng., 123, 505－511) and highly depended on the activity of the heterotrophic denitrifiers. The high nitrite-producing activity of heterotrophic denitrifiers will be needed for developing the efficient SAD process at low temperature.