The two different categories of microbes naturally occuring in sewage water are bacteria and fungi.
The following three treatments are carried out for purifying sewage water.
a. Primary treatment : 1. separation of large debries, 2. sedimentation in tanks.
b. Secondary treatment :
1. Waste water is pumped in shallow stabilization or oxidation ponds, where microbes oxidize its organic matter.
2. The process results in release of carbon dioxide and formation of sludge.
3. Sludge is continuously aerated further for its oxidation.
c. Teritary treatment :
1. Removal of turbidity in waste water caused by the presence of nutrients in dissolved organic matter, metals or pathogens.
2. It involves chemical oxidation by strong oxidising agents such as chlorine gas, perchlorate salts, ozone gas, UV radiations.