Could you please be a little more specific with your question.
Purification of water is done according to requirement of use.
For drinking water the purification process includes- Ion exchange method / RO (Reverse Osmosis) + UV for disinfecting.
In earlier days Chlorination is mostly used for disinfection.
For purification of sewage water, primary treatment, secondary treatment, tertiary treatment etc are used.
In primary treatment process, the insoluble materials of sewage water / Wast water (i.e. paper, plastic, leave etc.) is filtered using coarse screen filed by fine screen. The screens are either manually operated or automatic with mechanical systems.
In secondary treatment sewage water is treated by micro bacterias which purify the water by controlling BOD, COD, PH, etc. of water.
In tertiary treatment the treated water from secondary treatment are disinfected and control eColli to use this water for gardening, to flow in surface water.
The sewage water after tertiary treatment can be used for making it drinkable by applying Ion exchange/ RO with UV treatment.