This document is an overview of approaches to and observations of wastewater treatment plants using fine-bubble diffusion. It deals with variations in the effectiveness of these systems installed in activated sludge wastewater treatment plants in small communities. It provides the bases for the design of aeration systems capable of achieving transfer performances greater than those of other aeration plants. This document is earmarked for wastewater treatment plant operators, water treatment specialists and engineering and design departments, as well as all those involved in the sector who hope to improve the performances of their plants.