The Coachella Valley’s biggest water district recycles wastewater at three of its six sewage treatment plants, churning out water to irrigate golf courses, parks and lawns at housing developments. Now it’s proposing to reuse more water by converting a sewage plant in Thermal to a water-recycling plant. Environmentalists are objecting to the Coachella Valley Water District’s proposal, saying it would reduce the flow of water into the Salton Sea and eliminate an important source for wetlands that are planned to be built on the north shore of the shrinking lake.