Long-Term Sewer Repair Solutions

A holistic approach to rehabilitation proves more cost-effective over time.

Long-Term Sewer Repair Solutions

Results of dry weather analysis in one project area shows that reductions for all parameters are very close, indicating the entire diurnal curve has shifted downward, which is a strong indication for the effectiveness of the rehabilitation process.

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Picture a pipe with three cracks in it: Two are small, and only one is large enough to let in a significant amount of water. Many would say, “Fix the large crack because it will be cheaper than fixing all of them, and it should stop the majority of intrusion.”

But after fixing only the large crack, you find that water is still getting in, and even worse, the two previously small cracks are deteriorating even faster than before.

Now scale that concept up to an entire collections system and you have the principal finding of a long-term study in New Castle County, Delaware, by environmental consulting firm Malcolm Pirnie (a division of ARCADIS).

“The overarching rehabilitation philosophy has been one of holistic rehabilitation, to offset the migration of water within the pipe bedding to the next-easiest point of entry,” according to the white paper, Rehab Effectiveness – Why Holistic Rehab is Required for Significant I&I Reductions.

Simply put, if you plug one hole, the water will just go to the next, unless you address all the issues within a given segment.

“Migration of water within the pipe bedding to the next point of entry makes it extremely difficult to get significant reductions by rehabilitating a pipe here and a pipe there,” the paper says.

And that idea doesn’t just apply to individual pipes. The same principle can apply across basins and even entire systems. If only conducting mainline or manhole rehab without addressing laterals, I&I can simply end up impacting another area of the system and causing more damage as a result.

“Experience, review of work by others, and discussions with others in the industry have led us to the conclusion that piecemeal rehabilitation does little to actually reduce flows or volume,” according to the paper’s authors, Paul Batman, Jim Shelton and John Paul Travis. “In fact, it was noted during an early rehabilitation project that rehabilitation of the mainlines and laterals actually increased leakage in the adjacent manholes.”

Mitigation plan

Malcolm Pirnie collected pre- and post-rehabilitation flow data over several years across at least five individual projects and compared them to untouched “control” areas. The study compared projects that addressed singular rehabilitation efforts and combined methods.

New Castle’s reduction program covered the 400 miles of sanitary sewer main in its Brandywine Hundred Basin — nearly a quarter of the overall system. Such a large endeavor required careful consideration of which methods to use and when.

They hired Malcolm Pirnie to develop the rehab program and do the evaluation, with the goal of assessing early projects to increase efficiency as they continued working through the system. They performed a life-cycle cost analysis to optimize funding, which meant using minimal repair methods in pipes that were in good shape overall, despite leakage.

“The objectives of the rehabilitation effectiveness monitoring for these projects are to confirm that completion of each rehabilitation project results in significant I&I reduction, and also to quantify the effectiveness of the rehabilitation methods selected, in order to better predict flow reductions over the long term of the program, and optimize the selection of rehabilitation methods for future projects,” the paper says.

They determined that over the life of the project, holistic rehab was the best way to achieve significant reductions and provided the best bang for their buck.

For example, many I&I mitigation projects ignore private-side infiltration. While the paper acknowledges that there are times when the logistical challenges are insurmountable, it also found that private-side projects are often the most cost-effective.

“The reductions from sump pump disconnections are very high, particularly considering the low costs involved compared to the cost of pipeline rehab,” the paper states. “For a system exhibiting significant inflow sources that aren’t found by smoke testing, an intensive sump pump identification and disconnection program is likely a very cost-effective solution for I&I reductions.”

Going holistic

One significant change that New Castle County made during this time was to alter its bidding approach. Instead of hiring a single contractor to manage the entire scope of an I&I mitigation project, they decided to break their contracts into individual specialties. This gave them the ability to demand more stringent specs for the work and allowed for greater oversight.

“Most rehabilitation contractors specialize in one or two technologies and are not equipped to properly manage the number of subcontractors necessary to perform a wider variety of technologies,” the paper says. “It has forced rehab contractors, who are typically used to just ‘doing it like we always have,’ to pay much closer attention to their workmanship in the field, to provide quality and consistency in the products.”

The paper also says prices have increased slightly to reflect the new procedure but that so far it has been worth the cost, and contractors have “learned to meet the expectations, and are providing a higher-quality product as a result.”

A subbasin dubbed Talleybrook best demonstrates the efficacy of holistic rehab — it was rehabbed under the new technology-specific contracts, which allowed Malcolm Pirnie to compare methods as they were performed in isolation. They found a statistical indication that mainline grouting performed without lateral connection grouting had little impact on I&I reduction, compared to the combined effect.

The paper concludes, “For those planning or considering rehabilitation for I&I reduction, it is important to stress that the application of rehab technologies should be done systematically and holistically in order to achieve the best reductions.”


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