Sewer Infiltration & Inflow

Have you ever walked into a room and stepped into a puddle?  If you’re like me, you first try to figure out where it came from.  Did something spill, is the ceiling leaking, is it coming from underneath the floor?  Chances are you should be able to figure out the source of the leak, since you can check out most of the suspected locations.  But consider a sewer pipe that is buried deep underground with many paths for liquids to flow into the pipes, and the entire system creating additional inflow and infiltration that it was never designed to handle.  Locating such infiltration and inflow (I&I) in the system is a process that involves detailed investigation to understanding the primary and any secondary source(s) contributing to it.  This month’s article covers I&I detection and available technologies that can assist in more accurately pinpointing the sources.

Raindrops keep falling in my pipes: Infiltration and Inflow

“I & I” is a common expression for Infiltration and Inflow, two unwanted, and potentially expensive visitors in any sewage system.  Ground water infiltrates poorly constructed or aging pipes through pipe defects such as misaligned joints, cracks, fractures, and holes.  In wet weather, inflow enters pipes of all conditions through manhole covers, catch basins, and rain water connections.   Once in the pipe, excess water must be transported to the plant, reducing the pipe’s capacity to transport wastewater. At the plant, the infiltrated water and inflow must also be treated, adding an unnecessary expense to the affected municipality.

Inspection via Closed Circuit Television (CCTV) is a commonly used method to locate infiltration. In its most basic form, pipes are inspected by trained operators looking for signs of infiltration.  Using NASSCO PACP inspection codes, infiltration defects identified during CCTV inspections are marked as weepers, drippers, runners, and/or gushers.  CCTV inspections can be augmented with smoke testing or dye testing to improve their resolving power. However, these types of inspections capture I & I only at a specific instance of time, and may fail to reveal all the problems. For example, CCTV inspections done during dry weather may not reveal any significant infiltration, yet during wet weather, significant infiltrations will occur. Additionally, CCTV will not reveal any information on pipe condition below the flow line, which is a difficult tightrope to walk when trying to identify infiltration in wet weather.

Flow monitoring — a different technology —  addresses the fundamental problem with CCTV inspection by enabling owner/operator(s) to measure flow continuously,.  Furthermore, monitoring the upstream and downstream of suspected problem areas enables owners to perform differential flow calculations. For example, in a current problem area, if the downstream flow measurement is higher than the upstream flow measurement — in a line with no side connections — infiltration is the primary culprit.

While more comprehensive than CCTV,  flow monitoring only tells half the I&I story, and one must look towards recent advances in pipe inspection technologies for the rest.  Currently,  the most exciting trends in wastewater are comprised of new technologies that can detect leaks within a gravity sewer. Among the most impressive technologies currently available is the Focused Electrode Leak Location (FELL) system. FELL technology uses a probe that is transported through a pipe in conjunction with a surface electrode to identify and localize leaks.  If infiltration is occurring near the probe, a current spike will be registered on the surface electrode, making detection possible. The magnitude of the spike is proportional to the amount of infiltration.  By transporting the FELL through the pipe, it is possible to map out the exact locations and relative sizes of each source of infiltration. This is an important capability that distinguishes FELL from I&I flow monitoring methods measuring in aggregate.

Advanced technologies like the FELL are an emerging but potentially important capability. FELL enables us to root out even the deepest, most difficult-to-identify sources of infiltration and inflow.

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