The virus removal process from the liquid phase in an activated sludge basin possibly consists of physicochemical processes, such as adsorption onto sludge flocs, biological processes such as microbial predation and inactivation by virucidal components excreted by microbes. To describe properly the virus behavior in an activated sludge basin, a simple model is proposed based on the experimental data obtained using a poliovirus type I. A three-compartments model, which includes the virus in the liquid phase and in the peripheral and inner regions of sludge flora Is employed. By using the model, the virus removal process was successfully simulated to highlight the implication of its distribution in the activated sludge basin.