Johnston Construction Co.

Owner:

Washington Suburban Sanitary Comimission

Architect:

Whitman, Requardt and Associates, LLP

General Contractor:

Johnston Construction Company

Scope Of Work

Johnston Construction Company, in association with Whitman, Requardt and Associates, LLP, provided design, engineering, construction and commissioning services under a GMP contract for the $5,899,000 improvements to the chemical storage and feed facilities at the 240 MGD Potomac Water Filtration Plant.

The plant is the Washington Suburban Sanitary Commission’s primary water supply facility located along the Potomac River in western Montgomery County, Maryland. The improvements were installed to accommodate the plant’s current minimum flow of 90 MGD; future average day flow of 190 MGD; and ultimate design flow of 285 MGD. The system was seamlessly integrated into the fully operational filtration plant in the midst of an extensive improvement program that was being performed separately through multiple contracts.

The chemicals, including potassium permanganate, sulfuric acid, hydrofluosilicic acid, orthophosphate and sodium bisulfite, are stored in separate buildings throughout the plant. The dry potassium permanganate system was designed to proportionally feed six individual 72 inch diameter raw water intakes. The acid and base liquid chemical delivery systems were designed to feed into raw and finished water prestressed concrete cylinder pipelines ranging from 60 to 84 inches in diameter within the heart of the facility. These circa 1950s pipes were tapped at multiple depths down to 20 feet. A slide-rail shoring system was chosen for the safety, speed, and flexibility it provided in close proximity to multiple utilities. The overall layout of the chemical feed systems overcame many obstacles to accommodate the limited available space.

The potassium permanganate system provides delivery, storage, and feeding of the dry chemical through the use of tote bins and two Carus chemical feed units. The potassium permanganate building exterior complimented the adjacent historic C&O Canal.

The acid and base systems have common features, such as, a truck unloading area, dual storage tanks, a recirculation/transfer pump, redundant chemical feed pumps, metering and instrumentation for monitoring and control from a separate control room. Extensive non-invasive underground utility investigations were performed to locate suitable caisson locations for the acid and base building foundations.

Additional designs for future sodium hydroxide and aqueous ammonia systems were provided and partially constructed.

The project was completed within budget in time to meet the facilities chemical feed demands.