Water purification have different methodologies and it depends upon how much purified water is required. I will briefly explain these in the descending order of purification, i.e., most purified water first and then lesser ones.
1) Demineralization: In this cation and anion exchange takes place with cation and anion resin. Thus the cations and the anions of water are removed. After some time the resin is exhausted and cation resin is regenerated using Acids such as HCl, HNO3 etc while anion resin is regenerated using Caustic Soda Naoh.
2) Reverse Osmosis: In this technique the impurities are removed due to reverse osmosis through a membrane. The membrane exhausts after some time and is regenerated using certain chemicals.
3) Drinking Water: In this, the water is first boiled and then condensed which forms pure water with no impurities. The desired minerals are then added to it.
4) UV treatment: Ultraviolet rays are passed through water to kill germs. This is normally done after filtration.
5) Chlorine or Sodium Hypochlorite: These are added to water to kill bacteria. It is also usually done after filtration.
6) Filtration: Sand and Multimedia filters are used to remove suspended solids from the water. They are backwashed after exhausting.
7) Clarifier: It uses the process of agglomeration and sedimentation to remove suspended particles. Alum Sulfate and polymers are added to water to make the tiny suspended solid particles come closer and clot to form a bigger particle which then settles due to gravity and scrapped off.
Hope to answer you well. Bye.
I am a chemical (process) engineer and this is my work related area.