The activated sludge process was discovered quite by accident in 1914 by Arden and Lockett in England (1). Following that discovery, the first aeration devices were generally simple non-sophisticated diffusers usually consisting of open tubes or pipes with perforations. After World War II the emphasis on water quality improvement led to increased interest in the activated sludge process because of its many advantages. These include economics, flexibility, lower land area requirements than other types of biological treatment and the ability to produce a higher quality effluent. The latter is perhaps the most significant reason for the current upsurge in popularity of the activated sludge process. As would be expected, the intense activity in construction of activated sludge wastewater treatment plants has been accompanied by much research and by the development of new equipment to improve process efficiency and economy.