One drawback associated with the activated sludge process is the production of 'sludge'. The expense for treating excess sludge can account for 50-60% of the running costs of a plant. Traditional methods for disposing of excess sludge, which include incineration, the use of landfill sites and dumping at sea are becoming increasingly regulated worldwide due to concerns about the presence of potentially toxic elements in it. Furthermore, a combination of the limited amount of land available for landfill, particularly in urban areas, with stringent legislation has seen the economic costs of using landfill sites increasing sharply. Thus there is a growing interest in methods that reduce the volume and mass of excess sludge produced as part of biological wastewater treatment processes. We investigate a simple model for the activated sludge process in which the influent contains a mixture of soluble mand biodegradable particulate substrate. Within the bioreactor the biodegradable particulate substrate is hydrolyzed to form soluble substrate. The soluble organics are used for energy and growth by the biomass. We investigate how the amount of sludge formed depends upon both the residence time and how the use of a settling unit.