It is a simple answer. The people residing in the basin / watershed area should be considered as the owners of the waste water. i.e If the person is considered the owner of a new vehicle as well as his scrap (Waste) vehicle, then why is that person who is using fresh water and creating waste water not considered responsible and the owner of his own waste water?
It is true that Governance in the WATSAN sector is needed to manage the water supply and sanitation facilities. Ownership of waste water has to be defined by considering the local situation as well. Because waste water can be treated and sold in the Market at the same time, one must identify the persons responsible for wasting that water.