This animation illustrates the main stages of the wastewater treatment process.
The treatment of wastewater is essential before the water is returned to nature. With more than 16,500  treatment plants, the United States  collects and treats 90% of its wastewater.

The treatment cycle combines mechanical procedures (screening, clarification…) and physiochemical ones (biological treatment).  The net output of unpolluted water from these plants is estimated to be, on average, 75%.