Resilience and Innovation: Safeguarding DC's Critical Wastewater Infrastructure

In Washington, DC, lies the Anacostia Force Main (AFM), a 108-inch, prestressed concrete cylinder pipe (PCCP), stretching over 33,000 feet. It is a critical asset for both the District of Columbia Water and Sewer Authority (DC Water) and its partner, the Washington Suburban Sanitary Commission (WSSC) conveying approx. 200 million gallons per day (MGD) of wastewater from WSSC to DC Water's Blue Plains Advanced Wastewater Treatment Plant.

The AFM is a vital conduit that, if compromised, could impact customers, residents, commuters, parks, and natural resources. Constructed mostly in the 1970s, it had one failure in 2018 when a section of its crown collapsed. Since then, concerns about other potential failures remained. A condition assessment of the AFM was a necessary precursor for any next step, such as planning redundancies, rehabilitation or even replacement. However, the DC Water team knew that inspections on the AFM would be challenging. Flows in a pipeline may need to be diverted or bypassed to provide a suitable environment within pipelines for visual inspection. Previously planned electromagnetic inspections were rendered futile due to the unknown location of an inline valve, and access issues.

So, how did the DC Water team successfully inspect and assess the condition of the AFM? The resilience of the team comprised of internal DC Water resources and its consultants and their ability to understand the most likely root failure causes, and then deploy the right kind of innovative technologies.

Understanding that biogenic corrosion was the most plausible root cause for a compromised AFM, the team focused on tools that could identify air pockets while successfully navigating the AFM. They also decided to conduct a screening inspection rather than a high-resolution inspection.

The team selected the INGU Pipers tool, a free-floating and neutrally buoyant 3-inch spherical multi-sensor device used for the collection of screening-level inspection data in pressurized potable water, raw water, and wastewater (force main) pipelines. The Pipers tool is suitable for use in pipelines 3- to 120-inches in diameter with an in-service active flow velocity maintained between 0.3 ft/s and 13.1 ft/s. In July 2024, several INGU devices were inserted through an air release valve, and some were recovered at a downstream structure.

The Pipers tool recorded various sensor data during inspections:
Acoustic Data — A passive acoustic sensor detecting ambient sounds as Pipers move through the pipeline. Leaks and air pockets appear as characteristic signals. The intensity of these signals indicates the relative size of the leak or air pocket.
Magnetometer Data — Changes in the earth's ambient magnetic field inside the pipeline indicate the presence of joints, valves, or significant changes to the pipeline wall.
Pressure Data — Pressure fluctuations indicate changes in elevation or flow conditions within the pipeline. These readings may be used to estimate vertical alignment or existing hydraulic grade line (HGL) but can't detect leaks or air pockets in zero-pressure (open channel) flows. Multiple Pipers tools were launched at set intervals, and data from each is analyzed independently before being combined for the final report. Use of multiple data sets minimizes noise and confirms detected features.

The condition findings of the AFM are summarized by the following:
Leaks - No leaks detected.
Air Pockets/Entrained Air - 12 localized air pockets and 7 segments with air pockets were found. Biogenic corrosion due to hydrogen sulfide (H2S) gas may deteriorate the pipe walls and possible wall loss.
Magnetic Data - 12 magnetic anomalies were detected: 7 matched known features, and 5 were not associated with known constructed features.
Structural Condition - The structural condition of the AFM cannot be definitively determined. Areas with air pockets need further inspection to assess biogenic corrosion damage. O&M Condition - HGL values were calculated at 5 locations and found to be lower than theoretical values assuming new pipeline conditions, suggesting reduced flow efficiency.

The information gained from the screening inspection data was invaluable. It revealed that only 3% of the AFM may be susceptible to biogenic corrosion, in areas with air pockets. The data indicated that there are no leaks and no specific areas of debris, thus alleviating immediate concerns regarding blockages or excessive wear. The data also indicated the locations of magnetic features, which could correlate to inline valves. Insights from the screening inspection were then used to conduct a drone inspection at a critical segment with an air pocket with an air pocket to collect high-quality video and LIDAR data.

The implications of the condition assessment are highly significant. The AFM rehabilitation is now expected to be limited to areas affected by biogenic corrosion and identified via the inspection data, thus reducing the planned capital investment by approximately 60%. The inspection team's effort is a prime example of resilience and innovation at DC Water as they continue to explore and deploy innovative technologies. The AFM rehabilitation is scheduled to proceed starting in Fiscal Year 2025, and will remain a critical operational asset, silently serving the capital of the United States, the surrounding region, and its citizens.