When a water utility is in a fast-growing area and needs to plan for expansion, it’s good to have something extra in the bank.

Loudoun Water in Loudoun County, Virginia, about 25 miles northwest of Washington, D.C., brought a new water treatment plant online in 2019 to draw water from the Potomac River. At the same time, it is creating a water bank — a stockpile of water in retired stone quarries — to give the system more resiliency and to protect the river during times of low flow.

Drought protection

The idea of using quarries for water storage was first proposed in 1988, but there was no plan to implement it until the Trap Rock Water Treatment Facility in Leesburg was designed. The plant is rated for 20 mgd and produces 10 mgd on average, but it was designed for expansion to 40 mgd.

A retired quarry will provide a billion gallons of storage by 2026. Plans call for other quarries to be added, eventually providing up to 8 billion gallons of storage. The storage is key to the plan known as the Potomac Water Supply Program. In droughts and other emergencies, the utility will be able to draw from the quarries instead of the river, and then refill the quarries when the river’s flow returns to normal.

“In the event of water contamination or a drought, we would be able to pull our operation off the Potomac River and use the water stored in the quarry,” says Jessica Edwards-Brandt, director of water operations.

 Ozone treatment

The storage system is only one example of the utility’s campaign for a sustainable water supply and a clean, energy-efficient operation. The administration building at the Trap Rock Treatment Plant has been certified LEED Gold. The building draws 15% of its power from solar panels, has a water-source heat pump HVAC system and was built using regional material, like the stone in masonry walls.

Loudoun Water also had sustainability improvements in mind when designing the treatment process at Trap Rock. The plant uses a two-step ozone process followed by biofiltration. Water is treated first with ozone (as a preliminary oxidant). After conventional coagulation, flocculation and sedimentation, the water gets another ozone treatment and then passes through biofilters with 48 inches of granular activated carbon. This is followed by Ozonia UV (SUEZ) and chlorine disinfection.

Gerardo Castaneda, plant engineer, says, “We have ozone treatment at the head of the treatment process and also in the middle to target certain contaminants or substances at different stages.

“Following the second step of ozone we also have biofiltration. At the second stage, ozone reacts with organic materials and tears them up into smaller particles. The organic material then becomes more bioavailable to the bacteria that live in the filter. The bacteria will consume that organic matter and remove it from the flow.”

Multiple advantages

The bacteria that consume the partially decomposed organic materials are naturally occurring as long as conditions are right. “We just create the right condition for the biological materials to grow,” Castaneda says. “The biofilters are managed and maintained in order to maximize their capability.”

The two-step ozone process has a number of advantages, Castaneda says. One is that the final product has less organic material that could react with chlorine to create disinfection byproducts in the finished water. Another advantage is that the finished water, because it has less organic material, is more stable and has a longer retention time in the distribution system.

Another potential advantage of the two-step ozone system is that ozone could become the primary disinfectant. “Ozone is a very powerful disinfectant for certain pathogens,” Castaneda says. “We are using it to remove organics, but we hope in the future we would have the option to use it as our primary disinfectant, followed by UV.” Eliminating chlorine for primary disinfection would save on chemicals and energy, but so far the utility’s permit requires chlorine as the primary disinfectant.

Adaptable to conditions

Loudoun Water’s system is adaptable to changing conditions. That’s necessary because the source water is not necessarily consistent. “It’s a river,” Castaneda says. “The water is changing all the time. It changes based on storms. It changes based on seasons. Depending on all these different conditions, we can do a little more of this or cut back on that. This is all part of having many tools at our disposal, not just for now but for what we would need in the future.

“In the end, the goal is to be able to provide safe water to our customers. The better the water quality we put out there, the less concern we have that we are going to run into a problem, or that we’re going to have any customer complaints due to taste or odor issues.”

Loudoun Water serves a population of about 300,000. In addition to the water it produces at Trap Rock, it also draws about 60% of its water from Fairfax Water in neighboring Fairfax County. The source water for Fairfax is also the Potomac River and the treatment process is similar. “The two finished products are very similar,” Castaneda says. “That is intentional so they can blend easily in our distribution system.”