It is now possible to imagine a future in which highly treated wastewater will be plumbed directly into California homes as a new drinking water supply.

On September 8, the State Water Resources Control Board released a long-awaited report on the feasibility of so-called “direct potable reuse.” This means recycling urban sewage flows in a process akin to seawater desalination, then plumbing it directly into a city’s freshwater distribution lines without first storing it in a groundwater aquifer or reservoir (known as indirect potable reuse).

The water board relied, in part, on a 12-member panel of experts from around the world that studied the science and challenges of direct potable reuse for two years. And it concurred with the panel that it is possible to regulate direct potable reuse in a manner that produces safe and reliable drinking water from recycled sewage.

Next comes the process to actually develop those regulations, which the board intends to begin soon. Officials can’t estimate when those regulations will be complete. But there are a number of California water agencies waiting for that to happen so they can begin offering water produced in this way.

No other state has advanced this far with direct potable reuse, making it likely to become another arena in which California pioneers new technology for the world.

“This is a major milestone for California,” said Jennifer West, managing director of the California Water Reuse Association. “I think it has the potential to be a very significant water source for California. Without this report, we wouldn’t even be able to get off the ground.”

The report was required by Senate bill 918, a 2010 law written by California state senator Fran Pavley, D-Agoura Hills. The law required an investigation into the feasibility of direct potable use, but it does not require the state to develop regulations allowing it to move forward. That was left to the discretion of the water board, based upon expert analysis.

Randy Barnard, recycled water unit chief at the state water board, said the agency will begin to draft those regulations, based on the encouraging findings of the experts. “There are agencies all up and down California that would consider a project like this. There’s a lot of interest,” Barnard said. “But they’re just waiting on what the requirements are going to be and what they have to do to move forward.”

The expert panel identified a number of technical questions that must be answered before the state can begin to regulate direct potable reuse. One of the biggest involves the consequences of eliminating the “environmental buffer” that defines indirect potable reuse: blending recycled water with other supplies in a reservoir or aquifer.

For example, Orange County Water District operates one of the largest wastewater recycling projects in America. It is considered indirect reuse because, after the wastewater is treated using microfilters, reverse osmosis and ultraviolet light, the water is pumped into settling basins where it recharges groundwater aquifers. Weeks or months later, it is pumped out to a drinking water treatment plant before delivery to households and businesses.

Even though the water meets drinking water standards when it leaves the Orange County recycling plant, the environmental buffer provides an additional filter and ensures it is blended and diluted with other supplies. It also provides a kind of psychological buffer, Barnard notes, that the public finds appealing.

The process of direct potable reuse would involve all these same steps – and possibly more – except the environmental buffer would be eliminated. The treated water would flow directly into a water treatment plant or even straight into a city’s water delivery pipes.

“If we remove that environmental buffer, the expert panel has told us we have to come up with other processes – engineered processes – that would accomplish the same thing that this environmental buffer does to protect public health,” Barnard says.

The state needs to decide what those steps should be. Then it must figure out how to put them into enforceable regulations that produce measurable results to ensure public health.

Another area of research involves “contaminants of emerging concern,” a broad category of water pollutants – such as pharmaceuticals and chemicals – that are not removed by traditional wastewater treatment practices. The water board must decide which of these contaminants should be regulated as part of direct potable reuse, and what treatment steps should be imposed to control them.

Other requirements include making sure treatment plant operators have the proper training to handle recycled water in a direct potable reuse setting, and defining new water-quality monitoring methods to swiftly detect when there’s a problem with the recycled water.

But the water board has already made a crucial decision in this regard: It is not going to wait for research to answer these questions before developing regulations. Instead, it will begin to develop regulations concurrent with the research, which it will help direct through advertised requests and, in some cases, funding.

West said a number of industry groups have already begun research projects to answer the unknowns. She notes, however, that direct potable reuse won’t be right for every community. For one thing, it is expensive – though not as costly as seawater desalination, largely because the energy requirements aren’t as great. But in many cases, direct potable reuse may be the state’s second-most expensive water source.

Other communities may simply decide they’re not comfortable – despite all the safeguards and treatment steps – with plumbing treated wastewater straight into the drinking water system.

Yet public acceptance of recycled water has grown significantly in recent years. California’s ongoing drought helped, given that many communities opened fill stations where residents could collect free recycled water for landscape irrigation.

Also, many water agencies have safely delivered treated wastewater for years in special “purple pipe” systems for landscape irrigation.

One example is the Santa Clara Valley Water District, which operates a purple pipe system. And in 2014, it opened an “Advanced Water Purification Center” that treats wastewater to drinking water standards. It operates much like Orange County’s system, except instead of discharging to groundwater, the treated water is put into the purple pipe system to improve the quality of other treated wastewater sources.

The Santa Clara district is now planning a project to recharge groundwater with this highly treated recycled water supply, and it is interested in pursuing direct potable reuse once the state adopts regulations.

San Diego is working on a similar project that will pipe treated wastewater to San Vicente Reservoir. There, it will mix with imported water from Northern California and the Colorado River before treatment in the city’s regular drinking water supply system.

Jim Fiedler, chief operating officer at the Santa Clara Valley Water District, said direct potable reuse would be a natural extension of these efforts, because the same water systems that feed recycled water into a groundwater recharge project or a reservoir can just as easily feed a drinking-water treatment plant.

Fiedler served on a separate advisory group of local government and water agency officials that provided input on the water board’s report.

“We’re seeing this potentially as being a raw water source similar to other water sources,” said Fiedler. “When you first ask a person about this, their attitude is pretty negative. But once you start explaining what goes on with the treatment methods, you find this is something they would be more accepting of.”