In a first, Southern California water reuse pilot plant will test carbon removal process

Design work has begun on a Southern California pilot plant that is intended to test the viability of pairing carbon-capture technology with an advanced water treatment facility. Believed to be the first such pairing in the world, the demonstration facility will ensure that the planned water treatment processes comply with California’s regulations for indirect potable reuse. Meanwhile, the facility also will evaluate the technical and economic feasibility of using brine that is generated by reverse-osmosis (RO) membranes as part of a process to capture carbon dioxide.

If successful, the demonstration facility could usher in a novel, cost-effective solution for managing difficult-to-dispose-of brine waste while facilitating the removal of carbon dioxide on a large scale. Because the carbon-capture process also creates water and valuable byproducts, it has the potential to improve the sustainability of advanced water treatment operations and foster a more circular economy.

Pilot testing in Palmdale

In mid-April, the board of directors of the Palmdale Water District (PWD), in Palmdale, California, approved a memorandum of understanding (MOU) with Capture6, a company that focuses on approaches for capturing carbon that rely on converting carbon dioxide into carbonate. 

The MOU allows Capture6 to test its carbon-capture process at the Pure Water Antelope Valley (AV) Demonstration Facility, the pilot plant that the PWD intends to use to evaluate the performance of the advanced treatment technologies that will comprise the centerpiece of its planned Pure Water AV indirect potable reuse program.

To provide drinking water for its 127,000 customers, the PWD relies on imported water provided by California’s State Water Program, local surface water, and groundwater. “Each of these supplies has challenges and constraints that have led PWD to evaluate new, locally controlled water supply opportunities,” according to the district’s website for its Pure Water AV program.

One such opportunity involves the treatment and indirect potable reuse of tertiary effluent from the nearby 12 mgd Palmdale Water Reclamation Plant, which is owned and operated by the Los Angeles County Sanitation Districts. This tertiary effluent will provide the source water for the PWD’s planned 5 mgd Pure Water AV facility, which will employ microfiltration, RO filtration, and disinfection by means of ultraviolet light with advanced oxidation. Injection wells will inject the finished water into the local groundwater basin, where it will remain for at least two months before it then may be retrieved and conveyed to the PWD’s distribution system.

However, before it can undertake the design and construction of its full-scale advanced water treatment facility, the PWD is required by the State of California to operate a pilot facility for a year to demonstrate that its planned treatment processes will comply with the state’s regulations for indirect potable reuse. Scheduled to open in 2024, the 5,500 sq ft demonstration facility will have a treatment capacity in the range of 80 to 120 gal/min and is expected to cost $15 million, says Scott Rogers, the engineering manager for the PWD.

Disposal dilemma

Assuming all goes well with the demonstration facility, the estimated $155-million full-scale facility is scheduled to begin operations in 2027. As a result of its RO process, the Pure Water AV facility is expected to generate approximately 700,000 gal/d of brine, presenting a disposal problem for the PWD.

Because Palmdale is located several miles inland, ocean disposal of RO brine is “not possible,” says Zakir Hirani, a vice president and the water reuse sector leader for the consulting firm Stantec, which is providing program management services for the Pure Water AV project. “The only solution is to evaporate the brine, and that's expensive,” Hirani says. “Just to get rid of the brine could be three times the cost of the osmosis process itself.”

As part of its original plan to dispose of the RO brine by means of evaporation, the PWD expected to have to acquire, construct, maintain, and operate approximately 2 miles of pipeline and more than 70 acres of evaporation ponds, according to an April 17 news release from the district.

‘Perfect synergy’

Enter Capture6. Stantec introduced the carbon-capture company to the PWD because of the complementary nature of the operations and goals of the two entities, Hirani says. The PWD aims to boost its available water supplies, but must do so using energy-intensive processes, he notes. “We want to minimize the impact to the environment [from the water treatment processes], and this carbon-capture technology helps us do that,” Hirani says. “It's perfect synergy.”

Luke Shors, the co-founder and president of Capture6, agrees. “Fundamentally, we need a source of salt,” Shors says. “So, we look to places where there's an excess of salt.” And salt, of course, is a main constituent of the RO brine that the PWD will have to dispose of. 

“Carbon dioxide is an acid,” Shors explains. “It's well known that if you expose it to a strong base, it will bond with that base. The base that we create is sodium hydroxide.” To this end, Capture6 employs nanofiltration to remove salt from brine. “Then we split the water molecule through electrochemistry, and that gives us hydrogen and hydroxide,” he says. 

The sodium that is removed from the brine is added to the hydroxide to form sodium hydroxide. Capture6 then employs cooling towers that “expose the sodium hydroxide on a large surface area to the ambient air,” Shors says, facilitating the capture of carbon dioxide that is “embedded in a water treatment facility.”

Despite the novel purpose to which they are used, the technologies employed by Capture6 to capture carbon dioxide are “known and widely commercialized,” Shors says. As for the cooling towers, “we're researching how to make [them] more effective for this purpose,” he says. “But that's an effort to improve the efficiency of a known system. It's not a reinvention of the wheel.”

That said, the pairing of Capture6’s carbon-capture technology with water treatment processes is a novel approach. “This would be the first installation for a municipal potable reuse application,” Hirani says.

Promising potential

Under the terms of its MOU with the PWD, Capture6 will cover the costs of designing, installing, operating, and maintaining its carbon-capture technology as part of the demonstration facility. Stantec is designing the water treatment components as well as the carbon-capture aspects of the demonstration facility, which will be located beside the PWD’s headquarters to facilitate public outreach and support, Hirani says.

Although the design of the water treatment components of the demonstration facility is at the 30-percent level, Stantec only recently began work on the carbon-capture elements, Hirani says. “We are hoping to still complete the design by early next year at the latest,” he says. “Then you would have the construction bidding and award, which may take a couple months. Then the construction itself may take a year.”

The results of the demonstration facility will be reviewed by an outside expert and an independent panel that has been convened through the National Water Research Institute.

The technical feasibility and economic viability of combining Capture6’s technology with the PWD’s advanced water treatment processes remain to be borne out by the demonstration facility. However, initial analyses are promising, Rogers says. 

Using the Capture6 approach in place of evaporation ponds is expected to result in “about a twenty to forty percent savings from an operational standpoint,” Rogers says. “From a capital standpoint, it would probably save us somewhere along the lines of ten to fifteen percent of the total capital project costs of the actual facility if we built those evaporation ponds.”

The possibility of other synergistic benefits could make the partnership between the PWD and Capture6 even more rewarding. For example, after removing the salt from the RO concentrate, Capture6’s technology is able to return leftover freshwater to the PWD. “We are producing more freshwater than the facility would produce without our technology,” Shors says.

Meanwhile, the carbon-capture process also results in the creation of such compounds as sodium carbonate and hydrochloric acid, both of which are commonly used in water treatment.

“We get a really nice circular economy integration, where we are utilizing the water treatment facility’s trash to effectively produce the solvent we use to capture carbon dioxide,” Shors says. “And our outputs of that process are chemicals that water treatment facilities are buying. Not only do we reduce the need to purchase those chemicals, but actually the chemicals that we can supply back are low-carbon versions of those chemicals, because many of them are produced through fossil fuels. We eliminate the use of fossil fuels to produce those inputs to the water treatment system, and then we also can eliminate the transportation of those chemicals to the water treatment facility.”

For Capture6, removing carbon dioxide on a large-scale also holds the potentially lucrative promise of selling carbon credits. “There's hundreds of companies that have made net-zero [carbon emission] pledges, and even after they power all of their facilities with renewable energy, usually they're left with some residual emissions,” Shors says. “Those companies are procuring carbon removal credits to help them meet their net-zero targets.”

Capture6 would be especially well positioned to serve this market, Shors says. “What those clients want principally is, they want carbon removal that can be easily verified and is permanent,” he says. “Carbonate can be weighed on a scale, and there is a known relation between the weight of that carbonate and the amount of carbon dioxide that was absorbed to create it. That very, very simple chemical math is easy to audit and verify.”

Gigaton-scale carbon removal

Integrating carbon-capture technology into advanced water treatment processes offers a means of removing carbon in a way that is more cost-effective than stand-alone carbon-removal projects that remove carbon directly from the atmosphere, Shors says. “It’s expensive to remove carbon dioxide from the air,” he notes. 

“The only way that we can really have a chance of staying within the comparative safety of a [world that warms no more than 1.5℃] is if we develop gigaton-scale negative emissions technologies,” Shors says. 

With demand for safe drinking water only expected to increase in the future, capturing carbon dioxide as part of the water treatment process offers a “very reasonable way to scale carbon dioxide removal to large levels,” Shors says. “If this can be proved to be of value in water treatment, then suddenly it gets very possible globally to get to that gigaton number for tons of carbon removed.”