Static magnetic fields enhance microbial aggregation and adhesion to promote aerobic granulation

Aerobic granulation promoted by static magnetic field had been widely noticed because of its environmental friendliness and economic feasibility. The mechanism of strengthening aerobic granulation by static magnetic field needed to be clarified. Consequently, this study aspired to promote aerobic granulation by loading different intensities of static magnetic fields (R1: 0 mT, R2: 10 mT, R3: 50 mT), and elucidate the potential mechanism of static magnetic fields enhancement in terms of microbial interactions, chemical structure and adhesion performance of extracellular polymeric substances (EPS), metabolic pathways. Compared to R1 (day 45), R3 and R2 completed granulation at day 34 and day 42 respectively. Under the influence of magnetic fields, the interfacial adsorption free energy between microorganisms decreased from −55.88 mJ/m2 (R1) to −64.32 mJ/m2 (R2) and −75.02 mJ/m2 (R3), and the energy barriers preventing microbial aggregation in R2 and R3 were reduced. This was attributed to the fact that magnetic fields induced microorganisms to secrete highly hydrophobic EPS and enhanced adhesion to metal cations (Ca2+, Mg2+, etc.), resulting in an enhanced hydrophobic interaction and a reduced electrostatic repulsion interaction between microorganisms. Furthermore, magnetic fields increased the relative abundance of EPS-producing bacteria (Chryseobacterium, Candidatus_Competibacter, Thauera, Zoogloea) and upregulated the abundance of functional genes associated with the synthesis of hydrophobic amino acids (trpA, trpB, ADT, ilvE, proC, metH) and energy supply (mdh, cs, acn, IDH, sucA, frdA). The above results proved that the static magnetic field promoted aerobic granulation by enhancing microbial aggregation and adhesion, which provided sufficient theoretical support for the construction of a new wastewater treatment plant.