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Magnetic materials were recently developed for sludge disposal because of their advantages. However, the role of magnetic materials in the process of anaerobic digestion of municipal dewatered sludge remains unclear. The aim of this present study is to investigate the magnetic effect factors such as static magnetic field, magnetic nanoparticles, and ordinary magnetic particles on the anaerobic digestion of municipal dewatered sludge. In addition, the biogas production variations and relevant physicochemical properties of municipal dewatered sludge under anaerobic digestion are also evaluated by adding anaerobic bacteria. Results indicated that the magnetic nanoparticles are superior to both ordinary magnetic particles and static magnetic field at total biogas production; moreover, the aerobic bacteria could shorten the duration of anaerobic digestion. For the sludge dewatering, the organic matter degradation rate, chemical oxygen demand degradation rate, and dewatering rate under the treatment condition with magnetic nanoparticles are 32.1%, 87.1%, and 70.9%, respectively. It is suggested that the magnetic nanoparticles are much suitable for sludge dewatering than ordinary magnetic particles. As combined with bacteria, magnetic nanoparticles can effectively increase gas output, methane content, and shorten the anaerobic digestion period. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 374–379, 2019