In this study, the effects of magnetic field on activity of activated sludge in wastewater treatment were investigated in a batch reactor system containing synthetic wastewater. The medium pH and the direct current (DC) magnetic field strength were changed in the ranges of 6.0 to 8.5 and 8.9 to 46.6 mT, respectively. Experiments were also conducted operating at pulsed DC and alternating current. The changes in the substrate (i.e. glucose) and microorganism concentrations were followed. The highest substrate removal and microorganisms growth rates were observed at pH 7.5. The difference between the substrate removal rates of experiments performed with magnetic field and without it was the maximum at this pH, which was about 44%. The substrate removal rate was first increased by increasing the magnetic field strength, and reached the maximum at 17.8 mT, and decreased with further increase. Application of pulsed DC magnetic field did not change significantly the activity of system, hereas using alternating current magnetic field instead of DC form even caused slight decrease in the activity. (C) 2000 Elsevier Science Inc. All rights reserved.