The strengthening effect of a static magnetic field on activated sludge activity at low temperature

The figure shows the TTC-DHA, main species of unsaturated fatty acids in microbial cell membrane at 5°C (%), and the AFM image of activated sludge strengthening with magnetic field which provides information about the cell membrane unsaturated fatty acid composition and low temperature response of microorganism enzymatic activity at 5°C. It is a concise and illustrative application of PLFA analysis to reflect the composition of activated sludge in response to a magnetic field. •C16:1ω7c, C18:1ω7c and C18:1ω9c were the essential unsaturated fatty acids at 5°C.•MF increased the Gram-negative bacteria content to improve the cold adaptability.•Heteropolar MF enhancement is much more effective than that of the homopolar MF. 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°C. Phospholipid fatty acid analysis showed that C16:1ω7c, C18:1ω7c and C18:1ω9c were the essential unsaturated fatty acids in cell membrane at low temperature (4–15°C), accounting for the majority of the whole unsaturated fatty acids. The MF 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 MF (0.89–1.13). AFM observation showed MF smoothed part of the microbial cell surface, with some remaining distinct protuberances. Heteropolar MF enhancement performance is much more effective than that of the homopolar MF with identical plate distance.