A wastewater treatment plant.
In this article from The San Bernardino County Sun, market forces will soon be stronger than the "ick" factor when it comes to using treated sewer water in potable applications.
Due to the harsh drought in the western US, treated sewage water is already being used to recharge aquifers, and research into direct reuse is moving apace.
According to the article:
As they progress to large-scale plants, water agencies have used demonstration projects to help sway public opinion on the practice of injecting treated sewer water into underground drinking supplies or blending it with surface water... "Part of the message that needs to be conveyed to the public is that these systems have redundant treatments,” said Mark Millan, president of Windsor-based Data Instincts, a public outreach consulting firm.