Jill Rossini, Water Resources Engineer, Kleinfelder
As a member community of the Massachusetts Water Resources Authority (MWRA), the Town of Canton (MA) owns and operates its own sewer collection system that contributes to the MWRA treatment system. The town’s sewer system was first constructed in the 1930s and further built out through the 1960-1980s and is comprised of approximately 96 miles of separated gravity sewer, 6 town-owned pump stations, and 3 miles of force main. The majority of collection system pipes range in size from 8- to 14-inch in diameter and are either asbestos cement (AC), predominant material used in the 1970s, or polyvinyl chloride (PVC), indicative of newer pipe. In 2020, the town developed a Risk Model to help guide their sewer asset management program. The risk model characterized a pipe’s risk by considering its likelihood of failure along with the consequence to the system and community should that failure occur. The likelihood of failure is often determined from the overall condition and capacity of a pipe and, in the absence of actual field condition information, the surrogates of age and material can be used to estimate a pipe’s potential for failure. In the 2020 risk model for the town’s system, condition data was only available for approximately 10 to 15% of system’s pipes; therefore, the risk model relied heavily on pipe age and material to determine a pipe’s likelihood of failure rating (i.e., score from 1 to 5, 5 being the highest). Using only pipe age and material characteristics to determine likelihood of failure scores resulted in approximately 60% of the system’s pipe assets having a score of 4 or 5 suggesting a potentially high overall system risk.
Another factor that can contribute to the failure of sewer pipe assets is capacity of the pipe to carry the necessary flow. To consider this further, a review of the town’s FY2020 MWRA Annual Infiltration and Inflow (I/I) Reduction Report indicated Canton was seemingly experiencing high contributions of I/I during high groundwater periods of the year compared to drier periods. This finding, coupled with a lack of reliable condition information for much of the town’s collection system, spurred the town to perform a town-wide flow monitoring program in Spring of 2021. The flow monitoring program consisted of installing 28 flow meters for a period of 10 weeks and included 56 groundwater gauges, four rain gauges, and 260 manhole inspections. The I/I analysis performed using the results of the flow monitoring program indicated six priority meter areas that were either high infiltration contributors, as defined by MassDEP’s 4,000 gallons per day per inch of diameter per mile of pipe (gpd/idm) threshold and/or were cost effective to address. From here, targeted flow isolation and closed-circuit television (CCTV) inspections were conducted within those priority meter areas to refine the problem pipes to be rehabilitated or replaced. A phased sewer rehabilitation program was developed based upon the outcomes from the overall I/I analysis to target I/I reduction within the first 5 years of the program.
Concurrent with the I/I analysis, the town continued to progress its condition review of pipes throughout its system. By August 2023, CCTV inspections had been collected for roughly 50% of the system’s pipe segments, a vast improvement over the 2020 status. Using this newly collected data, the risk model was updated and the results compared back to that original 2020 model. From the comparison, it was apparent that the prior model had overestimated the overall risk of the pipe network – the town’s sewer pipes were actually in better condition than anticipated, a large portion of assets originally scored as a 4 and 5 (high likelihood of failure) were reallocated to ratings of 1 (low likelihood of failure). This outcome pointed to the importance of using field collected data to assess condition and remaining useful life rather than relying on pipe characteristics alone, leading to more data-driven sewer system management decisions.
Using the results from this updated risk model, an Asset Management Plan was developed including recommendations for both sewer inspection and rehabilitation. A 4-year comprehensive pipe inspection plan was developed by reviewing the 2023 Model for areas considered to be higher risk and coupling those areas with flow metering program results as well as looking for pipes across the system of higher likelihood of failure (e.g., easement pipes). The goal of the inspection plan is to visually inspect the remaining 50% of the sewer system using both in-house and contractor CCTV inspection services to further refine the risk model and rehabilitation plan. Additionally, a 5-year rehabilitation plan was recommended that focuses first on those areas deemed high priority for I/I reduction and continues with high-risk areas determined through the town’s inspection and risk-based planning efforts.
The design of Phase 1 of the Sewer Rehabilitation Program commenced in mid-2023 for Meter Area (Subarea) 27; the rehabilitation included approximately 16,000 linear feet of 8-inch, 10-inch, and 15-inch cured-in-place pipelining (CIPP) with associated manhole rehabilitation. When comparing flow meter data against the flow isolation data, Subarea 27 demonstrated relatively high I/I values in both field tests. Additionally, no flow isolation was collected for the pipes within easements or abutting Reservoir Pond due to site constraints (see Photo 1). Ahead of the rehabilitation work, the easement pipe was inspected and included in the rehabilitation project as a likely infiltration contributor. A critical component of the rehabilitation work in Subarea 27 included CIPP of a pipe that crosses under an active Massachusetts Bay Transportation Authority (MBTA) track and right-of-way; early coordination with MBTA was key to keeping this project on schedule and getting the appropriate permits to complete the work.
The Phase 1 Sewer Rehabilitation project received favorable bids, and as such, an additional subarea, Subarea 7, was added to the overall project scope; Subarea 7 included an additional 3,000 linear feet of 8-inch CIPP with associated manhole rehabilitation. The total project cost was approximately $1M and was completed largely throughout 2024. During construction, easement access was determined to ensure stream-cured CIPP was a feasible trenchless methodology for the rehabilitation. Other priorities for construction included minimizing traffic impacts and effectively communicating with the public and homeowners.
After the successful completion of the Phase 1 Sewer Rehabilitation project, the design of Phase 2 is now underway for Subareas 16 and 22. These two subareas were selected as the next priority due to their high infiltration contributions. During CCTV inspections, a substantial infiltration gusher (see Photo 2) was identified in the easement pipe near a brook crossing which will be addressed through design, further validating the data that lead us to this project.
When the town of Canton began the assessment of their sewer system, it seemed that significant resources would be needed to address the needs of their aging infrastructure. However, using a risk-based management approach, the town has been able to collect important and relevant field data to help them identify and prioritize areas of greatest concern and highest risk and funnel their limited resources to resolve issues that have a high return on investment such as I/I reduction and replacement of critical pipe infrastructure. Canton has also developed an asset management process that will help them to sustainably maintain their system well into the future.