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.
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