Sitework began in early 2011, and completion is expected in mid-2014. The design-bid-build project exists in two phases: South Burlington, Vt.-based PC Construction Co. (formerly Pizzagalli Construction) won the $70-million contract for site preparation and some of the tankage; Miamisburg, Ohio-based Ulliman Schutte Construction won a $97-million contract for mechanical and electrical work.

Because of the site's proximity to the Potomac River, a 50-foot-deep slurry wall “bathtub” will be built around the new denitrification tanks to avoid groundwater intrusion, says Paul Schlegel, VP and senior program manager for the Columbus, Ohio, office of CH2M Hill. Where the slurry wall stops, an underground layer of clay will act as a sealant and prevent groundwater seepage, he says.

Hawkins says, in recent years, efforts at the plant to reduce nitrogen to about 5 mg/l from about 15 mg/l cost approximately $100 million. However, getting there isn't cheap. “It costs us about a billion. So, one-tenth the improvement at about 10 times the cost,” Hawkins says. “That's where a lot of treatment facilities are at these days—very high costs at the margins as we get better and better at what we do.”

Tunnel Project Reduces Costs

The project would have been even more expensive had the team not used a little ingenuity and developed a solution to enable the plant to treat more water with less tankage, Hawkins says.

Although the plant is equipped to handle 380 mgd, to fully treat high-flow events associated with extreme weather—where the flow might be up to 680 mgd—to the level of nitrogen required under the permit, the plant would need more tanks and billions of dollars, two luxuries DC Water did not have, Hawkins says. “We're simply running out of space,” he says.

The solution? DC Water opted to use the same tunnels that will be built to handle combined sewer overflows to store large peak flows. “Essentially, the cost of enhanced nitrogen removal would have been even more expensive were we not also doing the large tunnel project,” Hawkins says. The tunnel project, which breaks ground in October, will store large amounts of peak flows—flows that currently leave the plant after receiving only primary clarification and disinfection—and gradually release the water to the enhanced nitrogen-removal (ENR) facilities on a metered basis. “By doing that, we are actually reducing the cost of doing ENR by itself,” he says.

During an extreme rain event, the additional storage volume will enable the maximum amount of flow through the plant to be reduced to 555 mgd from 1,070 mgd for the first four hours and, thereafter, to 511 mgd. This approach allows for the construction of smaller ENR facilities than would have been needed otherwise, Hawkins says.

The tunnels are part of DC Water's already underway Clean Rivers Project, which aims to reduce combined sewer overflows by 96%. This fall, excavators will start the project by digging a 120-ft-deep, 132-ft-dia shaft near the plant. In 2012, a large tunnel-boring machine will begin the first four-mile section of the tunnel along and under the Anacostia River, Hawkins says.

The main tunnel will feature 26-ft-dia pipes placed 100 ft below grade. A huge pumping station will push the water up to the plant to be treated, and a 16-story building will be built to store the equipment and machinery needed to maintain the system and the tunnel. The cost for the Clean Rivers Project is approximately $2.6 billion over 20 years.