In this article we will discuss about the principle of biological treatment of sewage.
Sewage can remain in aerobic or anaerobic condition depending on the availability or non-availability of oxygen. In aerobic condition the bacteria consume oxygen and remain active, without causing any foul smell.
On the other hand in anaerobic condition foul smell is created, due to which only aerobic conditions are preferred. Now for the continuity of this condition, it is necessary that suitable circumstances should be developed.
The first requirement is the availability of large porous surface containing large amount of air in which the aerobic bacteria can grow and perform their functions. When sewage is passed through such contact beds, the coarser particles of suspended matter are removed by mechanical straining and the aerobic bacteria act on the sewage and stabilize it.
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Practically, it has been observed that the major action takes place only at the surface, where a film of aerobic bacteria is formed. All the organic matters are first absorbed by this film, which is then acted upon by the bacteria. This bacterial film also contains algae, protozoa and other organisms in addition to the aerobic bacteria.
When the sewage is passed through the beds, where aerobic action takes place, following actions are done by the aerobic bacteria:
(a) The colloidal and dissolved putrescible organic matters present in the sewage are absorbed within few minutes in the trickling filters and within about one hour in the activated sludge process.
(b) The bacteria feed the organic matters so absorbed which are necessary for their life.
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(c) The bacteria convert the organic matters into stable inorganic forms by oxidising them.
(d) Finally, nitrates, CO2 and sulphates etc. remain in the sewage. The gases escape in the atmosphere.
All the materials separated or precipitated in the biological treatment units are separated in the secondary settling tanks. The sludge of these settling tanks contains colloidal, organic material and the B.O.D. This sludge is highly putrescible and is treated before final disposal. The effluence from the secondary settling tank is directly disposed of on land or in water courses because it is free from all polluting elements.