Name the two different categories of aerobic microbes naturally occurring in sewage.

The two different categories of aerobic microbes naturally occurring in sewage water are bacteria and fungi. The two together forms flocs a mesh like structure. 

The following three treatments are carried out for purifying sewage water. 

(a) Primary treatment : 

(i) Separation of large debris. 

(ii) sedimentation in tanks. 

(b) Secondary treatment : 

(i) Waste water is pumped in shallow stabilization or oxidation ponds, where microbes oxidize its organic matter. 

(ii) The process results in release of CO₂ and formation of sludge. 

(iii) Sludge is continuously aerated  further for its oxidation.

(c) Tertiary treatment : 

(i) Removal of turbidity in waste water caused by the presence of nutrients (N₂, P, etc.) in dissolved organic matter, metals or pathogens. 

(ii) It involves chemical oxidation by strong oxidising agents such as chlorine gas,perchlorate salts, ozone gas, UV radiation.