For urban and industrial uses, water is collected in the natural environment and after being used is returned to the natural environment.
The process of utilization is simultaneously a process of pollution that is transferred to the wastewater and from it to the water-course. More than 50 years were necessary after the introduction of the techniques of house plumbing to the appearance of artificial wastewater treatments.
Since the beginning of this process, it has been necessary to count on the natural assimilative capacity of the waters to deal with the pollution. But artificial treatment and artificially enhanced natural treatment have considerably augmented the possibility that the return of used waters to the environment became a true integration in it, its quality not being changed, or being changed in a way that does not significantly affect water uses.
Three main technical means are available to do this, only one being artificial treatment.