“Potable water reuse” means applying very high-level treatment to wastewater, bringing it up to drinking-water standards. With indirect potable reuse, wastewater is first treated biologically to remove carbon and nitrogen, followed by advanced water purification, which typically includes microfiltration, reverse osmosis and an advanced oxidation process. The recycled water is then added to an environmental buffer, such as an aquifer or lake, where it mixes with other sources of drinking water. In direct potable reuse, wastewater is treated and directly redistributed as drinking water without “recharging” in an environmental buffer, according to the EPA.

This isn’t new technology. A drought-induced water crisis in 1957 prompted the development of the world’s first large-scale potable reuse plant in Windhoek, Namibia. It became operational in 1968.

Today, most of the world’s potable reuse plants are in the United States. Globally, the total number of such facilities in operation or under construction is 59. Of these, 40 are in the U.S., including 16 in Southern California; 10 are in Africa; five in Europe; two in Australia; and two in Asia (Singapore and Manila). There are none in South America or the Middle East, according to Water360, an information portal run by the Water Services Association of Australia.

Only three of these plants are direct potable water systems: the original one in Namibia; one in Big Spring, Texas; and a small facility in the Philippines.

Indirect potable systems are more popular with consumers, who see the environmental buffer and the passage of time as extra safeguards. Water scientists, however, point to inefficiencies.

“You’re taking a really high-quality water that you’ve spent significant effort treating, and then you’re putting it into a groundwater aquifer or surface water reservoir where there can be potential for recontamination,” said Adam Smith, who directs USC Viterbi’s Environmental Engineering Program.

Orange County now faces this problem, he added. Certain groundwater wells had to be taken offline due to contamination with harmful “forever chemicals” called PFAS, requiring an added step for remediation: either ion exchange or activated carbon filtration. As the EPA’s recent PFAS maximum contaminant level regulations go into effect, the OC Water District is installing treatment facilities at contaminated wells to remove PFAS from already-treated drinking water stored in the ground.