Treated wastewater is injected into underground aquifers , or in some cases spread over a surface of land .
Treated wastewater is allowed to percolate down into aquifers below , where it can later be extracted to be added to the urban water supply .
Treated wastewater is injected into underground aquifers , or in some cases spread over a surface of land and allowed to percolate down into aquifers below , where it can later be extracted to be added to the urban water supply .