Water gets recycled naturally. It rains down, is used by us, gets evaporated again and so on. But still there is a shortage of it. That's because water gets polluted and does not remain fit for use.
However, with the help of a Sewage Treatment Plant this can be significantly reduced. Such a plant takes the polluted water from homes / industries and treats it in several different ways in order to make it fit for use again.
Sewage Treatment Plants vary in size and number of stages. If a lot of sewage water needs to be treated, the plant should be big and vice versa. Similarly, if the water is too polluted / contaminated, several stages are needed to make it clean. Different physical, chemical and biological processes are used to do so.