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Infiltration and inflow (I/I) impact every community with a sanitary sewer system. I/I is not new, however, as environmental regulations are strengthened and capital is increasingly scarce, I/I is being looked at more closely than ever before. I/I can increase sanitary sewer overflows, which can impact the environment. I/I can also rob precious wastewater treatment capacity from other needs including desired growth. Traditional methods to investigate and identify I/I include closed circuit television (CCTV) inspection, smoke testing and nighttime flow monitoring. These methods have been battle tested and include pros and cons, yet none are perfect.

A new technology to investigate and identify I/I has been developed in Europe and has been recently brought to the United States. The promising technology is Focused Electrode Leak Location. The trade name for the technology is FELL-41. Smoke testing, CCTV and nighttime flow monitoring all have their limitations. The FELL-41 system is another tool in our toolbox which can help bridge these gaps and identify previously undetected sources of I/I.

FELL-41 is unique in that it measures current which shows the potential for water actually to escape from or enter the sewer. The process involves using a specially constructed electrode called a “sonde” to generate an electrical field. The sewer is artificially surcharged with water, and the sonde is pulled through the sewer much like a closed circuit television camera. A surface electrode is placed in the ground near the line segment being tested. The FELL-41 system measures the current flow between the sonde and the surface electrode. The electric field is focused into a narrow disc about an inch long. When the sonde is placed in a sewer segment that is holding water, the current is very small. When the sonde is placed in a sewer segment that is leaking, the electricity follows the leak and the current increases. The FELL-41 system works. FELL-41 can be used for identifying I/I locations, testing of rehabilitation lining operations, and testing of newly constructed sewers.

This paper will highlight the successful pilot testing of the FELL-41 system accomplished by the City of Raleigh in February 2002.

Document Type: Research Article


Publication date: January 1, 2004

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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