<p>One of the inherent problems of activated sludge processes (ASPs) is the production of large volumes of sludge, which creates issues in secondary clarifiers regarding sludge settling. The application of physical methods such as static magnetic fields (SMFs) has attracted considerable attention from researchers in recent years regarding the sedimentation of wastewater sludge in ASPs. One basic parameter for evaluating sludge settling measurement is the rate of settling sludge in the graduated cylinder for 30&#xa0;min (SV₃₀). During the exposure of the SMFs (15 mT), sludge sedimentation with and without the use of metal coagulants was uneven in the case samples. The sludge settled in the graduated cylinder reactor had a smooth surface. The reason for uneven sedimentation of SV₃₀ is related to the formation of Ramin’s zone between the negatively charged particles of the separated sludge blanket layers. Ramin’s zone is temporary and by increase electrostatic forces between negative ions in this area and positive ions above the upper zone, will be stronger. This interaction allows the sludge to float back up and re-sedimentation. The results of present of sludge in SV₃₀ test and with exposure of SMFs (15 mT) by the application metallic coagulants or without them, is the uneven surface of sedimentation of the sludge. This re-sedimentation of sludge happened by use of metal coagulants, too. settling occurs because of the electrostatic attraction between negatively and positively charged particles.</p> Graphical Abstract <p></p>