Water Treatment Residuals Management Project
The Washington Aqueduct maintains and operates the Dalecarlia and McMillan Water Treatment Plants which serve reliable and safe potable water to the District of Columbia, Arlington County and the City of Falls Church, VA. Historically, river solids removed during the water treatment process were disposed of by returning them to the Potomac River. The reissued version of the National Pollution Discharge Elimination System (NPDES) permit precluded the discharge of river solids or residuals to the river.
The Residuals Management Project Involved the construction of equipment and facilities to collect water treatment residuals from three locations - Dalecarlia Sedimentation Basins, Georgetown Reservoir and the Dalecarlia Reservoir Forebay – convey the residual to a central treatment facility. At the central treatment facility residuals are thickened in gravity thickeners, dewatered by centrifuge, and loaded into trucks for off-site land disposal.
The construction contract was awarded in March 2008 and construction was essentially complete and the last discharge of water treatment plant residuals to the Potomac River was made in November 2012.
Improvements for Disinfection & pH Control (Conversion to Hypochlorite)
Washington Aqueduct made modifications to two components of the water treatment process – chlorine disinfection and control of pH – at both the Dalecarlia WTP and the McMillan WTP to enhance the reliability of the production of safe drinking water and to reduce operational risk.
Bulk liquid chlorine, created by compressing pure chlorine gas, was used throughout the history of disinfection at the Dalecarlia WTP and the McMillan WTP. Due to the hazardous nature of the liquid chlorine, engineering and management controls were employed to minimize risks associated with its handling and use. The conversion from bulk liquid pure chlorine to an aqueous sodium hypochlorite as an alternate form of chlorine provides an inherently safer form of chlorine that is commercially available and widely used in the water treatment industry.
In 2004, in the interest of managing corrosion observed in parts of the District of Columbia water distribution system, the United States Environmental Protection Agency approved a Washington Aqueduct plan to take steps to modify the water treatment process. The initial steps taken were to introduce a chemical corrosion inhibitor and modify the acceptable range for pH in finished water for optimum effectiveness. Studies revealed that using caustic soda for pH control as a supplementary or replacement process for lime, which is currently used at both the Dalecarlia WTP and the McMillan WTP, was necessary to maintain the tight control of pH prescribed by EPA.
The project included:
· Design, construction and operation of bulk sodium hypochlorite storage and feed systems at both the Dalecarlia WTP and the McMillan WTP, with consideration for facilitating the possible installation of on-site sodium hypochlorite generation equipment in the future.
· Design, construction, and operation of a caustic soda storage and feed system in order to trim pH following initial pH adjustment with lime at the Dalecarlia WTP.
· Design, construction, and operation of caustic soda and sulfuric acid storage and feed systems for the control of pH at the McMillan WTP.
The full conversion from bulk liquid pure chlorine to an aqueous sodium hypochlorite and implementation of caustic soda for pH control was complete at both plants in May 2011.