Photo Courtesy DC Water

The vessels for the Cambi thermal hydrolysis system, the first to be built in the U.S., are up but won't be operational until 2014.

Washington water and sewer utility DC Water is considering using green infrastructure to reduce the size or possibly replace two planned tunnels that would be constructed underneath parts of the city's Georgetown neighborhood and along the Potomac River waterfront.

The decision is part of the "Clean Rivers, Green District" agreement—formalized in December 2012 between the U.S. Environmental Protection Agency and the District of Columbia—to use green infrastructure to reduce wet- weather pollution in the district.

DC Water entered into a federal consent decree in 2005 for combined sewer overflow (CSO) control that is highly dependent on a tunnel system for storing and conveying combined sewage for treatment at the Blue Plains advanced wastewater treatment plant. DC Water has proposed using more green infrastructure to either supplement or replace part of the third leg of the tunnel system, which could require a modification of the consent decree, utility officials say.

The Potomac and Piney Branch tunnels are the final components of the utility's plan to reduce CSOs. About two miles long and 35 ft in dia, the Potomac tunnel would provide 58 million gallons of storage. Conceptual estimates say the project would cost around $400 million; construction would start some time in 2015. The smaller, planned Piney Branch tunnel, which does not require an environmental impact study, would be about a half-mile long and 23 ft in dia. DC Water hopes to reduce the size of both tunnels by using green infrastructure.

Green infrastructure—including green roofs, rain gardens and pervious pavement—offers many advantages over traditional gray infrastructure, says DC Water's general manager, George Hawkins. But the utility will not embark on a multimillion-dollar green infrastructure program blindly, Hawkins says. "We're not willing to make the jump into the green infrastructure before we see how it works," he says. Before altering its plans, the utility first must determine the affordability, sustainability and feasibility of green practices to achieve CSO reduction for improved water quality.

As a result, the utility is taking a multi-step, multiyear approach before making a decision on the Rock Creek tunnel. The first step is the adoption of small-scale demonstration projects. To that end, DC Water announced in May that it would award more than $1 million through a competition for innovative green projects in the District of Columbia.

The contest "provides a platform to generate creative and innovative approaches to managing stormwater and a body of research that will benefit cities worldwide, regardless of whether the district ultimately moves forward with a green solution, gray solution or a hybrid of the two," Hawkins says.

Further, the smaller projects will set the stage for a larger, multimillion-dollar demonstration project to evaluate the viability of incorporating green infrastructure into the CSO plan. Those plans are being discussed and worked out with the Environmental Protection Agency and the D.C. Dept. of the Environment.

As the work on the Potomac and Piney Branch tunnels is not expected to begin until 2015, DC Water has time to fully assess the feasibility of the green-infrastructure alternative, Hawkins says.

DC Water is moving forward with the environmental impact study on the Potomac tunnel as if the project were going forward. That way, if using green infrastructure to incorporate green infrastructure proves to be an unworkable solution, work will begin on the Potomac tunnel as scheduled, Hawkins says.