Suspended growth lagoons are shallow earthen basins varying in depth from 1 to 6m. The aerated lagoons depth ranges usually between 1.8 and 6m, mixing and aeration is provided through the use of slow-speed surface aerators mounted on floats. Non aerated lagoons can be classified in aerobic, facultative and anaerobic lagoons, depending on the main environmental conditions: biological conversion is carried out in aerobic and/or anaerobic conditions.

The aerobic lagoons depth usually ranges between 1 and 1.5m in order to guarantee sufficient oxygen concentration in the water. In facultative lagoons three different zones can be observed: superficial aerobic zone, anaerobic bottom zone (where settleable solids accumulate) and a facultative zone where biological processes are carried out by facultative bacteria. The anaerobic lagoon are deeper than the others and the main biological conversion is essentially anaerobic.