The research project was undertaken with the overall objective of better defining the oxygen transfer performance, operation and maintenance (O and M) requirements, and proper design approaches for ceramic fine bubble dome and disc diffuser aeration systems used in activated sludge wastewater treatment. A study of dome and disc diffuser aeration systems in 19 North American activated sludge plants was carried out during the project. All were municipal wastewater treatment plants treating predominantly domestic wastes, though some had significant industrial flows as well. Data on design bases, influent and effluent wastewater characteristics, aeration power and air flow, and O and M experience were requested from each plant. These were supplemented as needed by on-site investigations and interviews that focused on the practical operating experience of plant personnel.