In this study, a water cooling tower was used as a low-rate biofilm reactor for treating municipal wastewater. The performance of the system was evaluated at three different flow rates. The biofilm reactor gave the best results at a flow rate of 1.6l/s, namely 43.3% nitrogen removal, 42.3% chemical oxygen demand (COD) removal, 1.7% phosphorus removal and 39.8% suspended solids (SS) removal. These results were achieved with a once-through flow and low organic and hydraulic loads. This type of biofilter system is being used in West Malaysia to treat municipal effluents. Our system performance was capable of treating municipal wastewater to meet general effluent standards in South Africa (Government Gazette, 1984). This is sufficient for treating effluent for a population equivalent of 2 800 people.