The Sanitary District of Decatur, Illinois operates a wastewater treatment plant that utilizes an anaerobic sludge digestion process. The gas produced by the digestion process is collected and used to generate electricity at the plant in a cogeneration system. The cogeneration system's two engines have recently required an increased amount of maintenance and are approaching the end of their useful life. In considering replacement options, the District and Malcolm Pirnie, Inc. analyzed the quality and quantity of anaerobic digester gas, identified electricity cogeneration technology alternatives, and further evaluated two gas use technologies and their respective gas treatment requirements.