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Upholding a century-long record of clean water

Mott MacDonald is proud to have helped the Commission uphold its record of almost a century with no drinking water violations.

Wilmington Department of Public Works

Wilmington Department of Public Works

Opportunity

In 1912, the District of Columbia complained that waterways in the city were being polluted by waste from Maryland’s Montgomery and Prince George’s Counties. Civic leaders in these counties took action, drawing on the talents of the inventor and scientist Abel Wolman, a pioneer in the field of chlorination for drinking water.

The Washington Suburban Sanitary Commission (WSSC) was established in 1918. By 1920, WSSC had built a million-gallon (3.8 million liters) rapid sand filtration plant in Hyattsville and began to extend the water distribution system to communities like Silver Spring and College Park. Brighton Dam was completed in 1943, creating the Triadelphia Reservoir, and the T. Howard Duckett Dam and Reservoir followed in 1952.

Today WSSC is one of the largest water and wastewater utilities in the nation, with nearly 5,600 miles (9,000 kilometers) of freshwater pipeline and more than 5,400 miles (8,700 kilometers) of sewer pipeline. It serves nearly 1.8 million residents in an area of almost 1,000 square miles.

In its latest Annual Report, the Commission writes, “WSSC’s aging infrastructure presents an ongoing challenge to the Commission and to all of our customers and stakeholders…. We are changing the very culture of our organization as we focus on infrastructure planning and replacement.”

Among the specific challenges faced by the WSSC is a Consent Decree signed with the EPA and four environmental groups to eliminate or reduce sanitary sewer overflows. The WSSC estimates that complying with the Consent Decree will cost over $1.5 billion by the time it is completed in 2017.

Solution

Mott MacDonald has used its expertise to improve a variety of water and wastewater systems for WSSC. Our services have included evaluation of pump stations, treatment plant headworks, and storage facilities, geotechnical reports, soil investigation, cost estimating, surge analysis, stormwater management, microtunneling specification, engineering design, SCADA design, public outreach, permitting, and architectural, electrical, piping, process, and structural services.

Drinking water

Potomac hydropneumatic surge tanks

WSSC retained Mott MacDonald to design tanks that would provide surge protection for water lines of 48 inches (1.2 meters) and 78 inches (1.83 meters) diameter, handling flows of up to 200 million gallons per day (MGD) from the Potomac Plant. We performed a detailed surge analysis to optimize the size of the tanks while providing adequate protection against negative pressures in the system.

Mott MacDonald designed two hydropneumatic tanks 38 feet (11.6 meters) in diameter with air compressors and level controls that maintain the optimal air to water ratio. We found that changing the pump control valves from cone valves to hydraulic-cylinder-operated ball valves would have a positive impact on pressures at a far lower cost than the original concept of adding check valves.

Relocation of Adelphi pressure regulating facility

We provided design services for the abandonment of a pressure regulating facility located along a busy highway and its replacement on a different property. We designed a new pressure regulating valve station and a new regulating valve vault, and prepared plans, specifications, and bid documents.

Replacement of Brightseat valve vault

For this project, Mott MacDonald faced an accelerated schedule as well as the need to design work around an active work site, relocate a bus stop, and provide temporary traffic and pedestrian control. The design included a 42-inch (1.1-meter) gate valve and vault and 32 feet (10 meters) of 54-inch (1.4-meter) ductile iron pipe.

Replacement study for Central Avenue valve vaults

To help restore the required level of service, reliability, and operational flexibility, Mott MacDonald developed and investigated alternatives for the repair and replacement of the valve vaults, as well as related piping and valves. We developed design criteria, performed a hydraulics analysis, and provided a review of constructability and a conceptual-level estimate of construction cost.

Wastewater

Broad Creek Pumping Station upgrade

The Broad Creek Pumping Station near Fort Washington, MD, pumps wastewater to the Piscataway Wastewater Plant for treatment. Heavy rainfall has sometimes caused untreated wastewater to overflow into local waterways. The area has many new residential subdivisions.

As part of its Consent Decree, WSSC was required to eliminate wastewater overflows at Broad Creek. At the pumping station, Mott MacDonald designed mechanical, electrical, and architectural improvements including six 17-MDG-rated dry pit submersible pumps, the largest of their kind in the country.

In a Joint Venture, we designed a new conveyance system that included five miles of pipeline. Sections of the pipeline run through sensitive environmental areas adjacent to parkland in a historic district. To minimize environmental impact, microtunneling was used for 6,400 feet (2 kilometers) of the conveyance system. We prepared a geotechnical baseline report for the microtunnel sections.

Headworks and storage improvements at the Piscataway plant included a new screening facility, a vortex grit removal system, a 5-million-gallon covered concrete storage tank, and a 15-million-gallon (57 million liters) lined storage pond for peak flow storage.

Outcome

Mott MacDonald is proud to have helped the Commission uphold its record of almost a century with no drinking water violations.

Through a variety of services, we have helped WSSC ensure surge protection, eliminate wastewater overflows, protect sensitive environmental and historic areas, ensure public safety, avoid disrupting existing transport and infrastructure, estimate costs, develop cost-saving alternatives, and meet accelerated deadlines where necessary.

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