After touring the $481 million plant in Orange County, visitors are offered a glass of the water. Is it safe? The new National Academy analysis suggests that the risk from potable reuse “does not appear to be any higher, and may be orders of magnitude lower” than any risk from conventional treatment. There are currently no national standards for water reuse processes, only for drinking-water quality.

Of course, the treatment process is much more expensive than tapping local groundwater — in Southern California, about 60 percent more, and in El Paso about four times more. But to remain sustainable, groundwater must be used sparingly. Orange County’s reclaimed water costs $1.80 per thousand gallons when regional water subsidies are factored in. This is similar to what it pays to import either Colorado River water or water from Northern California. Without the benefit of subsidies, reclaimed water’s cost was just 14 percent less than desalinated water’s, which experts say requires 3 to 10 times the energy output.

The bigger hurdle to public acceptance may be psychological. Carol Nemeroff, a psychologist at the University of Southern Maine, said the notion of treated sewage “hooks into the intuitive concept of contagion” and contamination. To overcome this, she said, a city must “unhook the current water from its history.” That proved to be the case in 1998 in San Diego when the water department’s initiative was derided as “toilet to tap” during a bruising City Council campaign. Council members refused to allow further discussion of it.

A 2004 poll commissioned by the San Diego County Water Authority found that 63 percent of respondents opposed reuse. Then the water department began reaching out to customers with discussion groups and public meetings. Members of the Surfrider Foundation, an environmental group, reminded residents that almost every municipal wastewater plant practices water reuse anyway, since discharged treated wastewater is reused downstream.

“It isn’t toilet to tap. It’s toilet to treatment to treatment to treatment to tap,” said Belinda Smith, a Surfrider volunteer.

Water shortages and rationing, however, did the most to change attitudes. San Diego’s annual rainfall meets about 15 percent of its needs, and the city’s water managers grew worried that as California reeled from droughts, they could have trouble importing water.

In 2009, the third year of a severe drought, Mayor Jerry Sanders met with biotechnology industry executives who told him that water shortages posed a threat to their businesses. “They were talking about moving away from San Diego,” he said.