We followed the pipes up to a sunlit, metal-clad building where the water, now dosed with an antiscalant and sulfuric acid to lower its pH, was forced at high pressure through hundreds of white tubes filled with tightly spiraled sheets of plastic membranes. Reverse osmosis, Wildermuth says, stops cold almost all nonwater molecules (things like salts, viruses and pharmaceuticals). The stuff that’s removed is washed back to a pipe that discharges into the ocean. The filtered water, now known as permeate, moves one building over, where it’s spiked with hydrogen peroxide, a disinfectant, and then circulated past 144 lamps emitting ultraviolet light. “Destruction of compounds through photolysis,” Wildermuth said, nodding. Anything that’s alive in this water can no longer reproduce.

Strolling back through the campus, Wildermuth took me to a three-part demonstration sink with faucets streaming. The basin on the right contained reverse-osmosis backwash: it was molasses black, topped with a rainbow slick of oil. “Don’t touch,” Wildermuth warned as I leaned in for a better look at the ocean-bound rejectamenta. The middle basin contained the chamomile water from microfiltration. And on the left was the stuff Orange County would eventually drink. It was clear and had no smell.

But even this suctioned, sieved and irradiated water wasn’t quite set for sipping; it still needed to be decarbonized and dosed with lime, to raise its pH. Finally it would enter a massive purple pipe, which dives into the ground inside a nearby pump house and reappears 13 miles to the north, in Anaheim. There, the water would pour into Kraemer Basin, a man-made reservoir, where it would mix with the lake water and filter for six months through layers of sand and gravel hundreds of feet deep before utilities throughout the county pumped it into taps.

The reservoir is a prosaic ending for a substance that’s been through the glitziest of technological wringers, transformed from sewage to drinking water only to be humbly redeposited into the earth. This final filtering step isn’t necessary, strictly speaking, but our psyches seem to demand it.

To understand the basics of contemporary water infrastructure is to acknowledge that most American tap water has had some contact with treated sewage. Our wastewater-treatment plants discharge into streams that feed rivers from which other cities suck water for drinking. By the time New Orleans residents drink the Mississippi, the water has been in and out of more than a dozen cities; more than 200 communities, including Las Vegas, discharge treated wastewater into the Colorado River. That’s the good news. After heavy rains, many cities discharge untreated sewage directly into waterways  more than 860 billion gallons of it a year, according to the Environmental Protection Agency. However  and this is where we can take solace  the sewage is massively diluted, time and sunlight help to break down its components and drinking-water plants filter and disinfect the water before it reaches our taps. The E.P.A. requires utilities to monitor pathogens, and there hasn’t been a major waterborne-disease outbreak in this country since 1993. (Though there have been 85 smaller outbreaks between 2001 and 2006.)

So confident are engineers of so-called advanced treatment technologies that several communities have been discharging highly treated wastewater directly into reservoirs for years. Singapore mixes 1 percent treated wastewater with 99 percent fresh water in its reservoirs. (In Orange County, the final product will contain 17 percent recycled water.) Residents of Windhoek, Namibia, one of the driest places on earth, drink 100 percent treated wastewater. For 30 years, the Upper Occoquan Sewage Authority, in Virginia, has been mixing recycled wastewater with fresh water in a reservoir and serving it to more than a million people. Still, no system produces as much recycled water as Orange County (currently 70 million gallons a day, going up to 85 million by 2011), and none inserts as many physical and chemical barriers between toilet and tap.

Environmentalists, river advocates and California surfers  the sort of people who harbor few illusions about the purity of our rivers and oceans  generally favor water recycling. It beats importing water on both economic and environmental grounds (about a fifth of California’s energy is used to move water from north to south). “The days are over when we can consider wastewater a liability,” says Peter Gleick, president of the Pacific Institute, an environmental research group in Oakland. “It’s an asset. And that means figuring out how best to use it.”