The largest water recycling plant in the world can be found in Fountain Valley, California, and work is underway to make it even bigger. According to the Orange County Water District, its Groundwater Replenishment System takes about 110 million gallons of wastewater from the county’s sanitation district every day that would normally be dumped in the Pacific Ocean. It then puts it through a three-step treatment process to produce purified water. … Patel said that water is then used to replenish a local groundwater aquifer, which is the main source of drinking water for 2.5 million people in the county.