A technology to achieve stable and high rates of nitrification of landfill leachate at low temperatures has been desired. Nitrifying bacteria entrapped in a polyethylene glycol (PEG) gel carrier produced high nitrification rates of 0.71 kg N/m3/day at 10 °C for more than 1 year. As a characteristic of nitrification, ammonium nitrogen at 16-35 mg/L remained in effluent water irrespective of nitrogen load and nitrite accumulation was observed. Batch experiments clearly showed that the relationship between ammonium concentration and ammonium removal rate followed a Monod-type equation. It was also revealed that ammonium-oxidizing bacteria cultivated in a gel carrier had a low affinity for ammonium, leading to incomplete nitrification. Moreover, it was suggested that the remaining ammonium in the reactor produced free ammonium, which inhibited the activities of nitrite-oxidizing bacteria. Thus, only nitritation was observed. Molecular biological methods, such as denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH), revealed that Nitrosomonas sp. was the dominant ammonium-oxidizing bacteria in the gel carrier at low temperature.
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