D.C. System Will Handle Nitrogen and CSO Problems
The District of Columbia Water and Sewer Authority plans to accomplish two goals in one program with its $953-million upgrade of the 370-million-gallon-per-day Blue Plains wastewater treatment plant: treating combined sewer overflows and reducing nitrogen levels to meet more stringent federal water-quality requirements. District engineers say the program is a modification of an earlier plan to address the two problems separately. By coordinating the two projects, D.C. WASA will save money and use fewer resources, says Len Benson, chief engineer. “We’re doing more, faster, with better resulting water quality and [in a] more sustainable [way],” he says.
Treatment works located within the Chesapeake Bay watershed face some of the toughest nutrient-level requirements in the country. Since 2000, states within the watershed and the District of Columbia have worked to set lower limits for nitrogen and phosphorus from point sources to help improve the bay’s water quality, which is severely compromised by stormwater and agricultural runoff.
In 2007, the U.S. Environmental Protection Agency set new requirements for the agency. The modified National Pollutant Discharge Elimination System permit required total nitrogen limits to remain at or below 4.689 million pounds per year. At the same time, D.C. WASA has had existing NPDES requirements for treating wet-weather flows at Blue Plains from the city’s 12,478-acre combined sewer system, which serves about a third of the district’s residents.
The $1.4-billion cost for the two projects using conventional nitrogen removal and excess flow-treatment technology would require more space than is available, prompting WASA to look for ways to blend the two projects, says Sudhir Murthy, research and laboratory manager. “The confluence of events led us to develop a creative solution where we would use enhanced clarification to accommodate peak-storm flows that would help satisfy nitrogen removal as well as our wet-weather goals.”
The program will divert half of the peak wet-weather flow—approximately 521 mgd—through 3.5 miles of new 23-ft-dia tunnels. Most of that flow will be routed through a new enhanced clarification system. The process will use a proprietary ballasted flocculation technique to remove suspended solids and BOD. Enhanced clarification typically adds a ballast such as sand, grit or sludge to chemicals in flocculation basins to help particles bind together—or “floc”—more efficiently. Particles then setttle and exit the treatment stream in tertiary sludge.
The other, remaining 555 mgd will be processed through the existing and expanded biological nitrogen-removal system. The resulting effluent from both systems will have nitrogen levels between 3 and 4 mg/liter, which fall within new NPDES requirements, Benson says. About 31 mgd of wet-weather flow will be held in the new tunnels until capacity becomes available to treat it in the enhanced clarification system.
Englewood, Colo.-based CH2M Hill is designer of the additional nitrogen removal facilities, planned to go online by mid-2014. A designer for the enhanced clarification facility has not yet been selected. WASA plans to advertise in January for site preparation work for the enhanced nutrient-removal project.