A California water district documents long-standing success with indirect reuse of recycled wastewater to bolster the drinking water supply.
While not accurate, the term “toilet to tap” makes great headlines. The use of recycled wastewater to supplement drinking-water supplies is in the news, especially in drought-prone states like California, which has invested $1 billion in the technology.
In reality, the concept is not new. The $481 million Groundwater Replenishment System (GWRS) in Fountain Valley went online in January 2008 as a jointly funded project of the Orange County Water District and the Orange County Sanitation District. It replaced the original Water Factory 21 that went online in 1976 and pumped treated wastewater into injection wells to prevent saltwater intrusion into the aquifer.
The GWRS water purification plant uses microfiltration, reverse osmosis (RO) and advanced oxidation with UV light and hydrogen peroxide to treat wastewater from the Orange County Sanitation District, says Bill Dunivin, director of water production for the water district. The plant converts 70 mgd of treated wastewater into drinking water. A $142 million expansion will increase capacity by 30 mgd by 2015.
Better than the standards
In a traditional scenario, water would flow from the wastewater treatment plant to the ocean and be lost to the system. Instead, after treatment in the GWRS, about half recharges the groundwater supply by percolating through sand and gravel basins. The rest is sent to injection wells for the Talbert Gap seawater barrier, some of which migrates to the groundwater.
Orange County receives about 10 inches of annual rainfall. “We are a desert and in a drought, so water management is a big part of everyone’s day,” says Dunivin. “All the greenery we have is because of water pumped from the groundwater basin or imported from the Colorado River or from northern California through the Metropolitan Water District of Southern California.”
At the GWRS, microfiltration with 0.2 micron hollow fibers removes suspended solids, protozoa, bacteria and some viruses RO. Through semipermeable polyamide, polymer membranes filter out dissolved chemicals, viruses and pharmaceuticals. Disinfection with UV light and hydrogen peroxide destroys any trace organic compounds.
The treatment results in water that is nearly distilled, requiring the addition of lime to replace calcium, magnesium and other minerals. “We do the same thing as a bottled water company — add minerals to give the water a better taste and make it less corrosive,” Dunivin says.
“We analyze the water far beyond what our permit requires. We like to show the world that we are putting all the latest technology to use to make this a safe alternative. What we’re doing is very reliable. The parameters we meet every day exceed drinking-water standards by several times.”
Even though it has been doing indirect potable reuse for more than 30 years, the water district spent 10 years building support for the new plant. “We had a very proactive campaign,” says Dunivin. “We went out into the community speaking to city councils, civic clubs, business groups, churches and any place where people would listen.”
Focus groups revealed that the public was well aware of RO because many people have small RO systems in their homes. As they learned about the plant’s technology, the public became comfortable. Dunivin sees proof of that during hundreds of tours the plant provides every month, ending with samples of finished product.
“They say, ‘Wow, this really tastes good,’” says Dunivin. “Because of our outreach, the public has a sense of confidence in what we’re doing — not only management of the groundwater basin, but doing our part to supplement the drinking-water supply.”
Dunivin acknowledges that indirect potable reuse may not be economical in areas with abundant drinking water: “Our other source is imported water. The Metropolitan Water District charges about $920 an acre-foot, roughly equivalent to a football field covered with 1 foot of water. The water we produce costs about $435, and the price of imported water is expected to go up over the next several years.”
The GWRS product would still be cheaper than imported water if the water district had to pay for the entire plant instead of receiving $196 million from the sanitation district and $89 million in state grants. “If we paid for everything, the water we produce would cost about $825 an acre-foot,” Dunivin says.
Efficient energy use is one way of controlling production costs at the GWRS plant. “We used the most efficient motors and pumps and connected those to variable-frequency drives,” Dunivin says. The pumps automatically adjust with increasing pressure as fibers and RO membranes become fouled during use.
“When we started Water Factory 21, we ran at about 600 psi and the RO membranes had to be cleaned monthly,” says Dunivin. “Our new units run at about 125 psi and may have to be cleaned every six months to a year. Membrane chemistry is changing to allow us to be even more efficient and eventually our membranes may not need cleaning for two or three years.”
Eyeing direct reuse
The technology is already cleaning wastewater to levels better than drinking-water standards, which means there is no technical reason why water from the GWRS plant couldn’t go directly into the drinking-water supply.
Dunivin says public confidence in that approach would be bolstered by more data. “What level of instrumentation do you need to give us and the public the complete satisfaction that there is nothing getting through?” Dunivin asks. “We need direct online analytical methods for getting that data immediately instead of taking it to the lab.”
Dunivin expects such instrumentation to be available within about five years. He observes, “Even though we’ll still be doing indirect potable reuse, we’re going to be one of the first to use those instruments to demonstrate the reliability for direct reuse.”