In Ontario, the attainment of nitrification (oxidation of ammonia) in activated sludge plants is receiving increased attention. Nitrification of waste water is a necessary requirement because it reduces plant discharge of nitrogenous oxygen demand and/or toxic ammonia. However, this new requirement will result in added responsibility for operations personnel and possibly in the expansion and/or upgrading of facilities. This manual discusses different dimensions of nitrification and relates them to daily plant operations. An overview of the carbon and nitrogen cycles and their relationship to activated sludge systems provides a foundation for understanding the processes and problems discussed. Summary points of major sections include: (1) the two major nitrification bacteria are susceptible to factors such as temperature, carbon-to-nitrogen ratio, and toxic substances; (2) various system configurations are possible for nitrification with particular advantages and disadvantages specific to each; (3) operation of an activated sludge nitrification plant requires control of parameters such as solids retention time, aeration, dissolved oxygen and clarifier solids loading; (4) upgrading of sludge wasting facilities providing low rate continuous flow is highly recommended for solids control; and (5) frequent sampling and analyses will be required to monitor a nitrification plant. (DC)