The strengthening effects of static (homopolar and heteropolar) magnetic fields (MF) on microorganisms were compared in activated sludge degrading organic matter at low temperature. The TTC dehydrogenase activity improved substantially through external heteropolar MF intensification, and led to the highest COD removal rate of 94.9% at 5 degrees C. Phospholipid fatty acid analysis showed that C16:1 omega 7c, C18:1 omega 7c and C18:1 omega 9c were the essential unsaturated fatty acids in cell membrane at low temperature (4-15 degrees C), accounting for the majority of the whole unsaturated fatty acids. The ME effect increased the Gram-negative bacteria content to improve the cold adaptability. Shannon-Wiener diversity analysis demonstrated the samples with heteropolar MF had a higher PLFA diversity index (1.17-1.25) than that with homopolar ME (0.89-1.13). AFM observation showed MF smoothed part of the microbial cell surface, with some remaining distinct protuberances. Heteropolar ME enhancement performance is much more effective than that of the homopolar ME with identical plate distance. (C) 2013 Elsevier Ltd. All rights reserved.