This study investigated the effect of magnetic field on the biological treatment of wastewater at varied liquid volumetric flow rates. Wastewater quality is measured by Chemical Oxygen Demand (COD) which quantifies the amount of oxygen required to chemically oxidize organic compounds present in the water. The results obtained from the present study show that at the flow rate of 6.7 x 10⁻⁵ m³s⁻¹ there was a significant effect on the COD removal. At lower flow rates the magnetic field had more time to act on the microorganisms which in-turn increased the COD removal rate. However at flow rates 3.3 x 10⁻⁴ to 1.2 x 10⁻⁴ m³s⁻¹ the effect of the applied magnetic field on the COD removal decreased slightly.