Effort launched to quantify microfracture leaks in sewers

Posted October 30, 2020
SCCWRP works with staff from the San Diego County Department of Public Works and others to recover water from a sewer manhole in August for a study aiming to develop a method for detecting potential microfracture leaks in underground sanitation infrastructure. Clean water was pumped into a sewer pipe 291 feet upstream of this site at a controlled flow rate, then collected in a tank at this site to look for differences in the water volume pumped in vs. recovered.

SCCWRP has partnered with the County of San Diego to explore how to accurately detect potential microfracture leaks in underground sewer pipes – the latest step in an ongoing, multi-year effort to identify the origins of widespread human fecal contamination in the region’s waterways in wet weather.

The experimental field test, which kicked off in August in El Cajon, involves pumping a known volume of water at a controlled rate through an isolated section of sewer pipe, then recovering the water using a vacuum pump and looking for a difference in the volume of water pumped in vs. recovered.
Researchers’ goal is to develop a novel, field-based method for reliably and precisely detecting potential, relatively small leaks across a given section of underground pipe. Sanitation agencies, which run robust pipe inspection programs for detecting and preempting raw-sewage spills, rely on cameras and other tools that are not necessarily optimized to detect microfracture leaks.

Development of this leak detection method is part of an ongoing, multi-year study that kicked off in 2018 to determine if fecal contamination – which is widespread during wet weather in rivers, creeks and stormwater conveyances that discharge to the coastal zone – is coming from leaky public sewer systems, from defects in privately maintained sewer lateral lines and septic systems, and/or from humans depositing raw fecal material directly into waterways.

Researchers also are examining whether the microbial community that grows inside sanitary sewer pipes – known as biofilm – is unique to the pipes, which could enable researchers to trace human fecal contamination found in waterways back to specific areas or types of sanitation infrastructure.

The leak detection method was originally developed by SCCWRP in early 2020 using an above-ground, custom-built experimental system. Researchers constructed a 20-foot-long sewer pipe with ultrasonic flow meters at either end, then pumped clean water through the pipe at various controlled flow rates. Upon reaching the end of the pipe, the water was pumped into a tank to measure its volume.

After trial-and-error testing and calibration, researchers were able to reliably and precisely measure water volume at both ends of the above-ground system.

If the leak detection method can be shown to work in the field, researchers’ long-term goal is to use the method to estimate the total potential contribution of sewer leakage, or exfiltration, to fecal contamination across the San Diego River watershed. Pipe microfractures, if present across thousands of miles of underground sanitation infrastructure, have the potential to add up to a significant fecal contamination source.

Sanitation agencies, meanwhile, could begin using the leak detection method to test sections of sanitation infrastructure for microfractures. This infrastructure could be prioritized for testing based on factors like age, construction material, repair history, flow rate and proximity to stormwater conveyances.

Researchers have previously calculated theoretical exfiltration rates for sanitation infrastructure by developing models that weigh factors like pipe age and visible flaws and cracks in pipes. But no one has directly measured sewer leakage across actual sections of underground pipes under typical flowing conditions.

As the field work progresses, researchers also plan to introduce tracers, such as dyes of harmless salts, into the water to confirm exfiltration. If water is escaping from a sewer pipe and reaching a storm drain, the tracer should be detectable in adjacent stormwater conveyances.

For more information, contact Dr. Joshua Steele.

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