Effects of different static magnetic field (MF) intensities (10-50 mT) on the activity of activated sludge were compared at low temperature (5 degrees C). The TTC dehydrogenase activity (TTC-DHA) achieved the maximum of 8.12 mgTF/gSS at the MF intensity of 40 mT. MF intensity enhanced from 20 mT to 40 mT could promote microorganisms to produce more unsaturated fatty acids (UFAs) to stimulate the TTC-DHA and adapt to low temperature. At 30 mT, the Gram-negative bacteria with better cold adaptability were greatly enriched, which ensured the cold resistance of low-temperature reactor. The PLFA diversity strengthened by MF was higher than that of control when MF intensities changed from 20 mT to 50 mT and reached the maximum of 1.31 at 20 mT. Optimal MF intensity is supposed to be a potential pathway to strengthen the activity and cold resistance of activated sludge microorganisms in biological wastewater treatment under low temperature. (C) 2014 Elsevier Ltd. All rights reserved.