Washington Suburban Sanitary Commission to Upgrade Potomac WFP

Commission Taps Hazen and Sawyer for Analysis and Design of New Treatment Process

Potomac Water Filtration Plant

(Potomac, Maryland – October 5, 2009) – Hazen and Sawyer was selected to analyze the impacts of coagulation with ferric chloride on existing processes at the 285-mgd Potomac Water Filtration Plant (WFP) and to develop biddable design documents for new ferric chloride and caustic chemical storage and feed facilities at that facility. The Potomac WFP is one of two treatment facilities owned and operated by the Washington Suburban Sanitary Commission (WSSC) that provide finished drinking water to the residents of Prince George’s and Montgomery Counties in Maryland.

The Potomac WFP has historically been in compliance with the Safe Drinking Water Act regulations for finished drinking water. However, disinfection by-product (DBP) levels in the distribution system have occasionally exceeded or nearly exceeded the Maximum Contaminant Levels (MCLs) for total trihalomethanes (TTHMs) and five haloacetic acids (HAA5) on several occasions, particularly during the 1999 and 2002 droughts.

The Potomac WFP has used polyaluminum chloride (PACl) as the primary coagulant for more than ten years because of its effectiveness with Potomac River water and its side benefit of low-aggressiveness towards metallic components of treatment equipment. To address the recent DBP issues, however, the WSSC Environmental Group (EG) performed bench-scale and full-scale testing of alternative coagulants to see if improved TOC reduction, and therefore reduced DBP levels, could be achieved. The bench-scale and full-scale testing results indicated that ferric chloride outperformed coagulation with PACl, both with and without acid addition, in regard to percent reduction in TTHM and HAA5 concentrations.

The first phase of the project will entail studying the impacts of ferric chloride use on residuals handling and disposal, distribution system corrosion potentials, and materials compatibility, among other issues. Based on the results of the initial studies, detailed design documents will be prepared for the construction of ferric chloride and caustic bulk storage and feed facilities as well as plant improvements to accommodate the switch to ferric chloride coagulation. The project is being fast-tracked due to the need to have the new chemical feed systems on-line by April of 2012.

Since our founding in 1951, Hazen and Sawyer has focused on two things: providing safe drinking water and controlling water pollution. Our range of services encompasses the planning, design, and construction management of water and wastewater-related projects – from clean water treatment, storage, and distribution to wastewater collection and treatment.

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