A Water Treatment Facility is a facility that processes and treats water to make it suitable for a variety of uses, including drinking, industrial applications, or returning it safely to the environment.
Water treatment facilities were a key part of starships as they would recycle waste water - including that found in sewage - generated by the crew back into a potable form of water for crews to reuse. (TNG reference: Star Trek: The Next Generation Technical Manual)
Such facilities could be ground based as well. In the wake of the sabotage of the New Anchorage electrical distribution network by people loyal to former Governor Kodos, relief teams verified that the treatment plant was still functional. (DSC novel: Drastic Measures)