The processes for traditional nitrogen removal from wastewater consist principally of two sub-processes, nitrification and denitrification. In the nitrification process ammonium (NH4+) is oxidized generally to nitrate (NO3-) by autotrophic bacteria; in the subsequent denitrification process, NO3- is reduced to dinitrogen gas (N2) by heterotrophic bacteria. The latter process requires electron donors like organic carbon sources for the heterotrophic microbial reaction. It is reported that CH4 is useable for the denitrification under circumstantially micro-aerobic conditions in the presence of CH4 and O2. Denitrification using CH4 has been experimentally applied to wastewaters that do not contain sufficient organic carbon for denitrification, such as landfill leachate (Werner and Kayser, 1991), nitrified municipal wastewater (Houbron et al., 1999), and ground water (Eisentraeger et al., 2001). Methane is present at high concentration in biogases produced in anaerobic waste treatment reactors and landfills, which would make it substantially less expensive to use than introduced chemicals. While use of CH4 for denitrification would be advantageous at such sites from the point of view both of carbon source reuse and cost reduction, it has not yet been in actual use. Both NO3- and NH4+ are frequently contained in the wastewater targeted for nitrogen removal using CH4 such as effluent from aerobic reactors treating animal wastewater, owing to the imperfect nitrification that occurs due to high and varying N concentrations (Tanaka et al., 1999). Therefore, simultaneous removal of NO3- and NH4+ would be necessary for practical application of nitrogen removal using CH4. However, there are few studies with actual wastewater. In this study, a novel type of reactor, a semi-partitioned reactor (SPR) was operated with a varying gas supply rate and was used to create a biological reaction using methane and oxygen in the water phase. The SPR then discharged the unused gases separately. Successful removal of NO3- and NH4+ was observed when biogas and air of 1 L/min was supplied to an SPR of 9 L water phase with a NO2,3-N and NH4+ –N removal rate of 0.10 g/L/day and 0.060 g/L/day, respectively. The original biogas contained an average of 77.2% methane, and the discharged biogas from the SPR contained an average of 76.9% of unused methane that was useable for energy like heat or electricity production. Methane was contained in the discharged air from the SPR at an average of 2.1%. When gas supply rates were raised to 2 L/min and the nitrogen load was increased, NO3- concentration was decreased, but NO2- accumulated in the reactor and the NO2,3-N and NH4+ –N removal activity declined. To recover the activity, lowering of the nitrogen load and the gas supply rate was needed. This study shows that the SPR enables nitrogen removal from purified swine wastewater using biogas under limited gas supply condition.

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