Impact of High-Intensity Static Magnetic Field on Chemical Properties and Anaerobic Digestion of Sewage Sludge

The increasing production of sewage sludge at global level has addressed the search for technical solutions to take advantage from it, reducing the environmental burden originating from its disposal. Anaerobic digestion is a suitable option to handle sewage sludge in accordance with circular economy principles, as it generates a methane-rich biogas and a digestate with potential fertilizing properties. Several techniques have been proposed to enhance anaerobic digestion performances and, among these, the application of static magnetic field (SMF) has recently gained attention. Nonetheless, the effects of high-intensity SMF on the sewage sludge destined to anaerobic digestion and its impact on the anaerobic digestion process have not been evaluated yet. This study aims to determine the effects of a 1.5 T SMF on the chemical composition of sewage sludge as well as on methane generation during anaerobic digestion. The main parameters influencing the SMF (i.e., flow rate, mixing ratio of magnetized to non-magnetized sludge, number of pumping cycles, and total solid content) were varied to evaluate the impact of different exposure conditions on the chemical characteristics and methane potential of sewage sludge. An extensive exposure to the high-intensity SMF applied resulted in a 24% decrease of methane production, reduced the concentration of the monitored ionic species (i.e., NH4+, NO3−, PO43−, SO42− and Mg2+) in the liquid phase of sewage sludge, and promoted the precipitation of compounds with valuable fertilizing properties, e.g., struvite. These outcomes suggest that high-intensity SMF, although negatively influencing methane generation, can promote the precipitation, and possibly the recovery/recycle of valuable compounds from sewage sludge, enhancing its proper management in a circular economy perspective.