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 NH 3 N: 102.6 AE 4.6 mg N=L). Maximum ammonium removal rate of 210.5 AE 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.

SummaryPartial nitritation/anammox (PANAM) technologies have rapidly developed over the last decade, but still considerable amounts of energy are required for active aeration. In this study, a non‐aerated two‐stage PANAM process was investigated. In the first‐stage upflow fixed‐film bioreactor, nitratation could not be prevented at ammonium loading rates up to 186 mg N l−1 d−1 and low influent dissolved oxygen (0.1 mg O2 l−1). Yet, increasing the loading rate to 416 and 747 mg N l−1 d−1 by decreasing the hydraulic retention time to 8 and 5 h, respectively, resulted in partial nitritation with the desired nitrite to ammonium nitrogen ratio for the subsequent anammox stage (0.71–1.05). The second‐stage anammox reactor was established with a synthetic feeding based on ammonium and nitrite. After establishing anammox at low biomass content (0.5 g VSS l−1), the anammox influent was switched to partial nitritation effluent at a loading rate of 71 mg N l−1 d−1, of which 78% was removed at the stoichiometrically expected nitrite to ammonium consumption ratios (1.19) and nitrate production to ammonium consumption ratio (0.24). The combined PANAM reactors were operated for 3 months at a stable performance. Overall, PANAM appeals economically, saving about 50% of the energy costs, as well as technically, given straightforward operational principles.

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