Index >> Waste Water Microbiology >>Anaerobic digestion and aerobic oxidation

The sludge is much reduced in volume and is rendered stable and inoffensive. A portion of the solids is transferred into liquid and gases (mostly methane) which rises to the surface in the form of bubbles. The liquid which passes out of the outlet pipe from time to time is called effluent. It contains numerous bacteria, cysts, helminthic ova and organic matter in solution or fine suspension. The effluent is allowed to percolate into the subsoil. There are millions of aerobic bacteria in the upper layers of the soil which attack the organic matter present in the effluent. Thus the organic matter is oxidised into stable end products like nitrates, carbon dioxide and water. This stag of purification is the aerobic oxidation. Thus two stages are involved in the purification of sewage: First stage (anaerobic digestion) takes place in the septic tank and the second stage (aerobic oxidation) takes place outside the septic tank in the subsoil. Sedimented sludge accumulated at the bottom of septic tank is removed and may be dried, ground and spread on soil as a fertiIiser.