CDM Constructors, Inc.

Client:

Bill Dowbiggin - Discipline Leader – Water Treatment

Scope Of Work

To address future water supply needs, protect local communities and promote sustainability, Loudoun Water partnered with CDM Smith to design and oversee construction of a state-of-the-art drinking water treatment facility.

Established in 1959, Loudoun Water provides public water and sewer service for residents of Loudoun County, Virginia. The utility has been providing clean and safe drinking water for decades through wholesale purchase from a neighboring utility which sources their supply from the Potomac River. Forecasts of significant growth in Loudoun County prompted Loudoun Water to embark on a planning initiative called the Potomac Water Supply Program (PWSP), established to ensure a safe, sustainable water supply for the future. The PWSP includes a new raw water intake on the Potomac River; raw water transmission mains; off-line raw water storage using a series of abandoned quarries; a new water treatment plant and finished water transmission. CDM Smith was retained to perform preliminary engineering, final design, bid phase services, and construction management for the 20-mgd state-of-the-art Trap Rock Water Treatment Facility (TRWTF) and finished water transmission system.

“The Potomac River has supported Loudoun County’s growth for decades, but the river requires the additional off-line quarry storage to sustain population growth while maintaining minimum flows in the river,” said CDM Smith discipline leader for water treatment and drinking water expert Bill Dowbiggin, PE, BCEE. “The new facility will provide a reliable and solid foundation for future growth.”

 A Best-In-Class Drinking Water Plant

During the plant’s preliminary design phase, the project team began by analyzing the Potomac River’s water quality to best determine treatment goals for the Trap Rock plant. Targeted bench- and full-scale tests helped to support the development of design criteria for preoxidation and ozone disinfection treatment processes. Components of the facility include:

An ozone generation, delivery and contacting system that includes on-site liquid oxygen vaporization and ozone generation, raw water pre-oxidation using sidestream injection and pipeline flash reactors, and intermediate (settled water) ozonation utilizing sidestream injection and mixing nozzles in contact basins

A two-stage rapid mixing system consisting of a pumped injection flash mixing system and a mechanical vertical shaft mixing system, allowing for sequential addition of treatment chemicals in either or both mixing stages

Parallel three-stage tapered flocculation trains with vertical turbine-type flocculators and variable-speed drives to control mixing energy
 

Conventional sedimentation basins with chain and flight solids collection equipment
 

GAC/sand dual media biological filters with nozzle and plenum type underdrain system

Ultraviolet (UV) reactors for disinfection and/or as an additional treatment barrier 

A finished water pumping station sized to meet the plant expansion capacity of 40 mgd, with three vertical turbine can pumps rated at 10 mgd each for finished water, three backwash pumps and a layout that includes pump slots for installing additional pumps in the future. A 2-mgd sanitary sewer pumping station and the associated 12-inch ductile iron and polyvinyl chloride (PVC) sanitary sewer force main piping from the site to an existing outfall was also constructed

Approximately 5,000 linear feet of 42-inch finished water transmission main, including a tunnel portion and approximately 1,500 linear feet of 24-inch and 16-inch mains for future tie-in

A residuals treatment and recycle system that includes flow equalization, plate settlers, gravity thickening, recycle pumping (designed for full recycle) and emergency overflow storage