How do waste water plants use chemistry to purify water?

the ways of purifing are as follows.

1. water is coming from river or any source.

we will attach a big filter which helps us to out the big things like wood , plastic e.t.c..

2. we will allow to seetle down the water in large tannnk for along time.so allthe impurties get settle down .

3. then we wiill purifies the water by passing though sand (raiti).

4.after that we will add chlorine in it . so it get well purfied.

and suplly to the homes