The processes at Orange County and most other plants that clean the water include microfiltration that strains out anything larger than 0.2 microns, removing almost all suspended solids, bacteria and protozoa.

After that comes reverse osmosis, which involves forcing the water across a membrane, which removes other impurities, including viruses, pharmaceuticals and dissolved minerals. A zap with powerful ultraviolet light and a bit of hydrogen peroxide disinfects further and neutralize other small chemical compounds.

But after all that, 13 percent of adult Americans say they would absolutely refuse to even try recycled water, according to a recent study in the journal Judgment and Decision Making. “A small minority of people are very offended by this, and can slow it down or stop it because of legal and political forces,” said Paul Rozin, a psychology professor at the University of Pennsylvania who studies revulsion and a co-author of the study.

Image A cup of treated drinking water at the Orange County plant. The water is cleaner than water that flows out of a home faucet or comes in a brand-name bottle. Credit... John Schwartz/The New York Times

Opponents of reusing water have long had the upper hand, said Paul Slovic, a professor of psychology at the University of Oregon, because of the “branding problem.”

People tend to judge risk emotionally, he said, and a phrase like “toilet to tap” can undercut earnest explanations. “The water industry has not been good at marketing reuse,” he added. But research has shown that highlighting the benefits of recycled water — and the need — can shift emotions to a more positive reaction and help diminish the sense of risk.

“Under crisis, people accept things that they wouldn’t accept otherwise,” Dr. Rozin noted.

The cities now considering using recycled water for drinking have watched Orange County’s success carefully. San Diego and the Santa Clara Valley Water District have opened demonstration plants, and conducted tours and talks.