(Colorado News Connection) Colorado is one of a handful of states to allow water utilities to purify sewage for use as tap water and cities like Castle Rock are preparing to make it happen within a few years.

Right now, they just use the purified water to recharge the groundwater but said it is clean enough to drink.

Daniel McCurry, assistant professor of civil and environmental engineering at the University of Southern California, predicted at least half of states will adopt wastewater recycling in the next 20 years.

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"Places that you wouldn't normally think of as dry or water-stressed at all are starting to build these plants," McCurry pointed out. "I think anywhere that's primarily reliant on groundwater is going to have water reuse in their future."

California's Orange County Water District, a leader in the field, already purifies 130 million gallons of wastewater per day and sends it down into the aquifer. Other cities use their recycled wastewater to irrigate fields and parks.

Darrin Polhemus, deputy director of the California State Water Resources Control Board, said efficiencies are worth exploring.

"We spend a lot of money and energy moving water from different parts of the state to Southern California, where it's used once and dumped in the ocean," Polhemus observed. "That's maybe not the smartest way to deal with a resiliency question."

David Sedlak, director of the Berkeley Water Center at the University of California-Berkeley, said the "yuck" factor can be challenging, so public outreach is key.

"They have to start building legitimacy from the ground up," Sedlak emphasized. "Sometimes that means changing the culture of transparency and openness. And sometimes that means working with the public and bringing them on board to see and understand it."

Arizona, Georgia, Texas, and Virginia also have large-scale programs to recycle wastewater.

This story is based on original reporting by Matt Vasilogambros and Kevin Hardy for Stateline.