A biotechnological process used to treat and purify domestic sewage and industrial biodegradable effluents. It consists of a tall vertical loop-type bioreactor with a height of between 30 m and 150 m that is usually installed below ground level. Compressed air is introduced into the downflow leg to drive the circulation of the liquid medium containing microorganisms. As the liquid rises up the upflow section, the hydrostatic pressure decreases, and the bubbles add to the mixing and aeration. This type of bioreactor is used where there is a high oxygen demand such as in the processing of activated sludge. The sludge can therefore have a higher density than conventional activated sludge systems. It is also known as a deep shaft airlift fermenter.