This research project was initiated with the overall objective of better defining the oxygen transfer performance, operation and maintenance (O&M) requirements, and proper design approaches for fine bubble dome diffuser aeration systems used in activated sludge wastewater treatment. Working in conjunction with the Water Research Centre of Stevenage, England, a survey of 19 wastewater treatment plants with dome diffuser aeration equipment was carried out. Thirteen of the plants were located in the United Kingdom, two in The Netherlands, and four in the United States. As nearly as possible, data on influent and effluent wastewater characteristics, power demand, air supply, and process parameters were compiled for a 5-yr period. Maintenance personnel were interviewed to develop a summary of long-term operation and maintenance experience. Results of this work indicate that, relative to other devices, fine bubble dome diffuser systems can perform at high energy efficiency when proper design, operation, and maintenance practices are followed and when strong alpha-depressing industrial wastes are absent. O&M experience with this equipment has generally been very good, with 6-10 yrs or longer, between cleanings. Oxygen transfer performance was found to be highly variable between the plants. Many plants carried excessive mixed liquor D.O. in the aeration basins, thereby lowering oxygen transfer efficiency. Plants with multiple-pass plug flow basins generally exhibited poorer power economy than those with single-pass basins.