This study investigated partial nitrification treatment of methane fermentation digester liquor effluent from magnesium ammonium phosphate precipitation process in a swim-bed reactor. The reactor was operated at a temperature of 35 degrees C and pH between 7.5 and 7.8. Partial nitrification was achieved at the onset of the experiments even though conventional activated sludge was used as seed sludge. The maximum nitrite production rate was 1.0 kg NO2-N/m(3)/d at a nitrogen loading rate of 2.0 kg-N/m(3)/d. The average effluent NO2-N/NH4-N ratio and the effluent NO3-N concentration were 1.04 +/- 0.34 and 5.7 mg/I, respectively, during the stable experiment periods. After 150 days of operation, the sludge volume index value decreased to 15 ml/g and the mean particle size of suspended sludge increased by approximately 3 times from 80 to 260 mu m. Comparison of mineral analysis between the seed sludge and the partial nitrification sludge demonstrated that the mineral content of the latter increased approximately three-fold in comparison to that of the former. High Ca concentration was considered to be closely related to dense floc formation and superior settleability of the sludge. Both DGGE and DNA clone analysis verified that there were significant microbiological differences between the samples taken at different time periods. Nitrosomonas was confirmed to be the predominant species after stable partial nitrification performance was obtained. The overall results of this study validated our previous results that swim-bed reactor technology could be successfully used as a pre-treatment technology for anammox treatment. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.