Situation

New Castle County, Delaware is a suburb of Wilmington with approximately half a million residents. The county sewer system comprises approximately 1,800 miles of gravity sewer and interceptors, 38,000 manholes and more than 150 pump stations. Included in this system is the Brandywine Hundred Sewer System, which comprises 420 miles of the system in some of its most densely populated areas.

 

The Brandywine system contains some of the oldest and most troubled sewers in the county. Groundwater was infiltrating into deteriorating sanitary sewer lines, overwhelming sewage treatment facilities. On more than one occasion, treatment plants were so inundated that water was allowed to bypass the facility and flow into the Delaware River without treatment. This resulted in the closing of public beaches, contamination of drinking water and consent decrees with the U.S. Environmental Protection Agency.

 

To address ongoing maintenance needs in the aging system, the county implemented a 25-year rehabilitation plan for its wastewater infrastructure. A thorough assessment revealed significant infiltration and inflow problems in the Northcrest-Afton neighborhood.

Solution

While the long-term solution to this problem might be construction of additional sewage treatment plants and the replacement of aged and obsolete pipes, the most effective short-term remedy was to line the lateral pipes to prevent migration of rainwater into them through cracks and fissures.

 

Closed-circuit TV inspection of the terra cotta sewer lines revealed that many contained numerous cracks, and some had broken or collapsed. Grouting and lining were identified as the best solutions to the problems. Beginning in July 2006, B. Frank Joy was brought in as the contractor to line 3,800 feet of mainline pipe and lateral lines for 56 homes in the Northcrest-Afton community.

 

During the six-month project, workers found that infiltration was much higher than anticipated. They dealt with the problem by setting up bypass pumps in the mains.

 

B. Frank Joy installed LMK T-liners to at least the edge of the property line, and when the property owner agreed, as close to the building foundation as possible. Impacts on private property were minimized by using smaller excavation equipment with matting.

Results

Engineers hoped to achieve a 34 percent reduction in inflow and infiltration as a result of this rehabilitation project.

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