NEED A NEW TOOL FOR ASSESSING SANITARY SEWERS? FELL-41 MAY PROVIDE SOME LIGHT

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.