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In this study, a swim-bed reactor was used for partial nitrification treatment of magnesium ammonium phosphate precipitation process treated effluent of methane fermentation digester liquor. Even without temperature, DO, and pH control, the maximum nitrite production rate of 2.66 kg NO(2)-N/m(3)/d was achieved under a nitrogen loading rate of 5.45 kg N/m(3)/d at 18 degrees C and a nitrite production rate of 0.22 kg NO(2)-N/m(3)/d was still attained at a nitrogen loading rate of 0.48 kg N/m(3)/d even the operational temperature decreased to 8 degrees C. The average effluent NO(2)-N/(NO(2)+NO(3)-N) ratio reached 95.4% during the entire experimental period. The mean particle size of suspended sludge increased from 260 to 450 microm and the microscopic observation confirmed the formation of dense-flocs. The results indicated that there were two regimes for ammonium oxidation, one at 20-30 degrees C and the other at 10-20 degrees C. Scanning electron microscopic observation and FISH analysis demonstrated that ammonium-oxidizing bacteria were the dominant microorganisms and were considered to be responsible for the satisfactory partial nitrification performance.