In the early experiments, air was introduced through open tubes or perforations. As the activated sludge process investigation progressed, greater economy was sought with smaller bubbles produced by porous media. In 1914, experiments were made with natural limestone, fire brick, sand and glass mixtures, pumice and other materials. They were all found to be to dense. In 1915 a porous plate set in a cast iron box was developed. In the following years, experiments with porous media and other media continued.

Diffuser clogging was a problem in numerous cases from the earliest aeration experiments. Around 1915, perforated pipes were abandoned in favor of diffusers because 90% of the jets had plugged. With the advent of porous diffusers, clogging became an even greater problem. Early work between 1922 to 1924 prompted coarser media use to avoid severe clogging problems.

Mechanical aeration was one answer to the clogging problem. Down draft and up draft type mechanical aerators were developed to diffuse air efficiently and to solve clogging problems.

Another approach to the clogging problem was the development of the large orifice-type diffuser. First developed in the 1950's, these units improved on earlier perforated pipes and were designer for easy maintenance and accessibility. These devices generated larger bubbles and sacrificed substantial transfer efficiency for maintenance ease.

With the emphasis on more energy efficient aeration systems in the 1970's, the technology turned to fine bubble systems. Emphasis on fine pore diffusion has lead to many extensive research programs aimed at the evaluation of design, operation, and control processes relative to improving overall system performance.
 

Lagoon Aeration Fundamentals and Techniques