Waste Not, Want Not
Recycling Wastewater for Potable Use
As climate impacts and population growth put pressure on existing resources, a new study explores wastewater renewal.
The earliest evidence of wastewater reuse dates back 5,000 years to the Minoans, an ancient civilization that occupied modern-day Crete. Their sewage and stormwater systems fed water to their farmlands, keeping crops watered and fertilized during dry spells.
While technology and sanitation have drastically changed over the course of the past 50 centuries, water reuse and recycling remain important issues today.
Last spring Whit Wheeler, the director of Raleigh Water, approached Emily Berglund and Tarek Aziz, professors of civil and environmental engineering at NC State University, to explore potential approaches that would make Raleigh Water more resilient to drought. Now, they have partnered to identify possible solutions for the city.
Defense Against Exceptional Drought
Past threats to water resources sparked this collaboration, according to Berglund. “From 2006 to 2008, the southeastern region of the United States faced intense droughts,” she says.
For North Carolinians, the drought reached its most extreme in the fall of 2007. August of that year was the second warmest and driest the state had ever seen, which led to more than 60% of the state receiving an “Extreme Drought” classification by the start of September.
That October, nearly 40% of the state experienced extreme water shortages, widespread loss of crops and pastures, and a high risk of wildfires. The U.S. Drought Monitor placed these areas under the most severe “Exceptional Drought” classification.
By December 2007, 14 water systems had less than a 100-day supply of water in their reservoirs. The City of Raleigh’s solution, according to Berglund, was to increase the allocation from Falls Lake for its drinking water supply.
“It was a good solution,” says Berglund, “but it’s a Band-Aid. It has a limit.”
Current models predict a new water supply will be necessary between 2045 and 2067 — which is why Raleigh Water is already studying alternatives that include a new reservoir on the Little River in eastern Wake County and a withdrawal from the Neuse River.
In addition, the research team is working with Raleigh Water on a new Community Collaborative Research Grant project specifically to identify secondary water sources.
“If one source fails, or there’s a shortage, then we can rely more heavily on the other one,” Berglund says.
Reuse in Raleigh
Water reuse is not new to North Carolina. The Neuse River Resource Recovery Facility (NRRRF) treats an average of 50 million gallons of wastewater per day — the equivalent of about 100 Olympic-sized swimming pools. The facility takes in wastewater from municipal sources — households, businesses, and other community establishments — and conducts “advanced wastewater treatment.”
The primary stage of treatment filters out physical contaminants like debris, grease, and oils. Following this primary treatment the water passes through more advanced processes, during which it cycles between zones with and without aeration, creating the right conditions for microorganisms to clean the water.
These microorganisms consume organic matter and convert harmful ammonia-nitrogen to nitrogen gas, which dissipates freely from the water. After the nitrogen dissipates, the microorganisms are settled to the bottom of a tank, removing them from the treated water. Finally, in the “tertiary” stage, water passes through fine sand filters to further clarify it and then under ultraviolet lights for disinfection.
The NRRRF “treats the wastewater to a high standard, but not a drinkable standard,” Berglund says. Water from this facility is usable for non-potable purposes — such as flushing toilets, washing cars, irrigation, and filling decorative fountains and ponds.
The NRRRF provides some reclaimed water for non-potable reuse through the Raleigh Reclaimed Water Distribution System but discharges most of the water into the Neuse River.
“Indirect potable reuse” recycles water by introducing treated wastewater to an environmental buffer, such as a river, lake, or aquifer, then withdrawing and treating water again for distribution to a consumer base.
“Indirect potable reuse can provide more assurance in the mind of the consumer that the water is clean,” Berglund says.
But what about direct potable reuse?
Directly Drinkable
In a “direct potable reuse” approach, reclaimed water would directly be treated to drinking water standards and distributed without being stored in an intermediate water body. Once cleaned to the appropriate water quality standard, the water would then reenter the potable water supply.
“On the infrastructure side, direct potable reuse can be efficient.” says Berglund. There is no need for a separate reservoir or a buffer, and unlike non-potable reuse dual systems, “you don’t need a separate pipe system to deliver non-potable water.” Instead, direct potable reuse is conveyed using the existing drinking water pipe network.
One drawback of direct potable reuse, Berglund says, is that the process can be energy intensive, especially for direct potable reuse systems that rely on “reverse osmosis” — an advanced process that uses high pressure to push water through a semipermeable membrane. The membrane acts like an extremely fine version of a traditional filter, allowing only the water molecules to pass through.
The result is extremely pure water. In addition to requiring a lot of energy, filtering water to this level of purity poses another set of issues.
Because it is such an intensive treatment process, reverse osmosis removes both harmful and helpful minerals. Research shows that when people drink large amounts of reverse osmosis and other “low mineral water,” the concentration of minerals in their bodies decreases. Adding supplements back to the water can reverse these impacts, but it also adds more steps to the already complex process.
Berglund says that a direct potable reuse system can be effective when compared with non-potable or indirect potable reuse and that reclaimed water is a good alternative for a secondary source of water. Her team is working to demonstrate how this system would work on a laboratory scale, and Berglund says it will take time and testing to find the treatment approach that will scale up for the Raleigh water supply.
The Pipeline to New Policy
Given the risks to public safety, water reuse regulations and policy do not allow for distribution of direct potable reuse on a large scale, yet.
Although the researchers haven’t found a perfect solution, “there’s a potential pathway for direct reuse to happen in the city of Raleigh,” Berglund says. “Demonstrating that this technology works and demonstrating how much it’s going to benefit the community and our future water supply” is the first step for crafting plans for policy and infrastructure.
“On the east coast, we’re water rich,” Berglund says, “so direct potable water reuse is something we haven’t thought a lot about or implemented.” So, to better understand what reuse could look like outside of the lab, the researchers have turned to policies from other states.
“California, Colorado, and Texas all have potable reuse programs, so we’ve explored what’s in their regulation.” Berglund says. To craft solutions for North Carolinians, the researchers are working to identify the most important and relevant components.
“Since water supply planning happens in decades, it is important that we conduct this research now,” says Wheeler about Raleigh Water’s collaboration with Berglund and Aziz’s team. “I am confident that NC State’s Department of Civil, Construction and Environmental Engineering can lead the way for North Carolina.”
There’s only so much water to go around, Berglund says. “And we need new sources.” New dams and reservoirs aren’t always options, and direct potable reuse can help meet the needs of the present, protect supplies in times of crisis, and preserve resources for future generations.
“Reuse builds resilience,” she says, “not just sustainability.”
More
The New Pioneers: Planning for Wastewater Treatment During Climate Change
The Community Collaborative Research Grant Program receives support from North Carolina Sea Grant and the state’s Water Resources Research Institute, in partnership with the William R. Kenan Jr. Institute for Engineering, Technology and Science at North Carolina State University.
lead photograph credit: Raleigh Water.
Lily Soetebier is a contributing editor for Coastwatch. She is pursuing an M.S. in technical communication from North Carolina State University.
