Stable Partial Nitrification in an Up-Flow Fixed-Bed Bioreactor Under an Oxygen-Limiting Environment

Stable partial nitrification followed by a denitrification or anammox process is a cost-effective alternative to conventional biological nitrogen removal processes, especially for wastewaters having a low carbon-to nitrogen (C/N) ratio. This article addresses partial nitrification in an up-flow fixed-film bioreactor operated for more than 265 days using low C/N synthetic wastewater. The study was carried out in the absence of external aeration, for further cutting down the cost of treatment. By controlling influent dissolved oxygen (DO) at 1 mg/L, stable partial nitrification with more than 85% efficiency was achieved (influent NH3N: 102.6 +/- 4.6mg N/L). Maximum ammonium removal rate of 210.5 +/- 6.7 mg N/L/day was obtained at a hydraulic retention time (HRT) of 18 h. Oxygen uptake at influent DO concentration of 1 mg/L was practically negligible compared to the amount of ammonia oxidized to nitrite. Partial nitrification was taking place devoid of oxygen consumption. A possible mechanism of partial nitrification with negligible DO uptake is proposed in this article. The nitrogen oxide (NOx) cycle may be playing a major role for such low oxygen requirement. This strategy of developing a stable partial nitrifying biosystem under a low oxygen environment without external aeration, can lead to considerable savings in aeration cost in the treatment of ammonia laden wastewaters.