The activated sludge process depends on a dense microbial population being mixed in suspension with the wastewater under aerobic conditions. Activated sludge is the most widely used biological wastewater treatment process in the developed world. It is used to treat both domestic and industrial wastewaters. The process is very flexible in operation, but to obtain maximum efficiency from an activated sludge plant the operator must have a full understanding of the complex process. This book covers all the major theoretical concepts and practical operational factors. Hence it acts as a useful introduction to theoretical principals as well as a practical operational manual and trouble-shooting guide.