wastewater instead of being discharged into a river or stream is utilized through an independent piping network. it is used for industrial processes such as industrial evaporative colling and irrigation. It is more predominantly in place out west where water scarcity is a concern. In public water and sewer utility operations the term grey water system is a specialized sewer collection whereby each customer has a septic tank which collects solids and the leachate or wastewater is discharged into small diameter gravity lines that ultimately work their way to larger diameter gravity sewers or pumping stations and to the WWTP. Each customer or utility has to pump out the septic tanks when needed. I worked on a very large non-potable system back during the drought of 2007. Bought property for tank had the entire backbone designed and was ready to go. Then it started raining and the project died.