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Using anammox as a novel biological process for nitrogen removal from wastewater containing high concentrations of ammonium has attracted much attention. This study focuses on the potential for removing nitrogen from reject water in a municipal wastewater treatment plant using a combined nitritation/anammox process with fixed-bed reactors. In a bench-scale plant that was fed reject water, the nitrite conversion efficiency was above 80% for about 210 days during the nitritation operation. Nitrate production was negligible during this period. We found that the NO2 --N/NH4 + -N ratio of the anammox reactor influent could be maintained easily by controlling the flow rate of bypass wastewater according to the nitritation performance. For the anammox reactor, significant nitrogen removal was observed immediately after the start-up and a maximum nitrogen removal rate of 2.9 kgN·m -3 ·d -1 was obtained 80 days after start-up. When the influent ammonium concentration was fluctuated intentionally during the experimental operation, the performance of the combined nitritation/anammox process was almost constant and the average nitrogen removal efficiency was 81%.

A swim-bed reactor using the biofringe acryl-fiber biomass carrier was used for partial nitritation treatment for anaerobic digestion liquor of swine wastewater. The sludge in the reactor demonstrated excellent settling properties, and the sludge volumetric index (SVI) was always about 50 ml g -1 . The mixed liquor suspended solids (MLSS) concentration was maintained above 10,000 mg l -1 with a maximum of 16,800 mg l -1 . Satisfactory and stable partial nitritation was obtained at a nitrogen loading rate (NLR) of 1.9 kg-N m -3 d -1 without any operational control. Only a little nitrate was produced almost during the whole operational period and the nitrite to total oxidized nitrogen ratio (NO 2 -N / (NO 2 -N + NO 3 -N)) was always above 95%. In addition, the influence of temperature on partial nitritation efficiencies was also investigated and noncontrolled efficiencies were maintained stably between 15℃ and 30℃ at a NLR of 1.9 kg-N m -3 d -1 , but suddenly deteriorated when the temperature fell below 15℃. Nitrite oxidizing bacteria were inhibited by free ammonia and free nitric acid, which prevented the conversion of nitrite to nitrate and the inhibition due to free nitric acid weaken with a decrease in temperature. It was apparent that these phenomena were crucial to the control of partial nitritation treatment.