The Private-Side Fight Against Inflow

Comparison study shows that private-side inflow is the biggest contributor to overall inflow and infiltration problems.

The Private-Side Fight Against Inflow

Infiltration removal comparison from comprehensive rehabilitation of the public wastewater system.

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If you want to effectively mitigate inflow and infiltration, you need to have a plan for the private side of your system.

Engineering firm CDM Smith has found across a number of I&I mitigation programs that private properties contribute an estimated 50 to 70 percent of total I&I for any given municipality. Their findings were confirmed during a comparison study in Revere, Massachusetts.

Following a consent decree negotiation in 2010, Revere set out to remove 40 to 50 percent of total I&I. CDM Smith was hired to analyze the 99 miles of sewer main supported by 12 pump stations. To develop a plan for the substantial I&I reduction, CDM Smith had to determine where the city would get the most bang for its buck: the private or public side.

In the end, to achieve their goals and complete the consent decree, they formulated a comprehensive mitigation plan to address both sides of the system, with a focus on cured-in-place pipe and service lateral connection lining, manhole rehabilitation and, most important, private inflow removal.

Studying the problem

Before embarking on any mitigation efforts, the city had to assess the system and find out where the I&I was coming from.

“Because of the aggressive timeline and the magnitude of the work to be conducted, the city elected to perform flow isolation during the nighttime hours in dry-weather conditions to determine specific areas with infiltration rates in excess of 4,000 gallons per day per inch-diameter mile (GPD/IDM) to narrow down the scope of future investigations,” according to the study.

Those isolated portions of the system were evaluated using CCTV inspection, smoke and dye testing, manhole inspections and house-to-house inspections.

Revere’s collections system consists of predominantly 100-year-old sewers and service laterals, and their main interceptor is an egg-shaped brick sewer constructed in 1904. Finding areas of infiltration by targeting aging parts of the system would be ineffective.

Instead, they used the evaluations in conjunction with estimated flows by type of source with a computer-modeled simulation.

Flowmeters and groundwater gauges were also used to determine the magnitude of wet-weather flows. With all this information, they developed a hydraulic model of the system, following a standard used by the Environmental Protection Agency.

After analyzing the results, they determined that 60 percent of the I&I in Revere was coming from private properties. Considering their goal to eliminate 40 to 50 percent of the city’s I&I, they would have to address both sides in a comprehensive plan to complete their consent decree.

Addressing private inflow

Improper sump pump discharge was believed to be the biggest contributor to private inflow, and so the city initiated the Sump Pump Amnesty Program to encourage participation and identification of illegal discharges. The city also amended local ordinances, requiring building owners to redirect illicit sources discharging into the sewer system.

Outdated drainage systems such as roof, yard, driveway and patio drains were another issue and included in the amended ordinance.

As part of the amnesty program, residents were given until the end of the 2015 calendar year to notify city officials that there was an illegal sump pump or allow an inspection. If they did so, the city would redirect the discharge at no cost to the homeowners. Beyond that, once the program expired, property owners would be responsible for the full cost and subject to possible fines for ordinance violation.

“Gaining access to private properties is the single greatest challenge when conducting a private inflow removal program. As in any private inflow program, the success of the program is directly related to the success rate of entering the private homes/private property in order to perform an inspection,” the study notes.

Using a low-lying area as a baseline for their study, CDM Smith found that after inspecting 47 percent of properties in the area, approximately 30 percent of known private inflow sources were removed.

“Thus, through extrapolation, assuming all inflow sources identified to date are eventually removed, and the flow contributed from each of those removed remains the same, we could anticipate a total percent removal of inflow of approximately 43 percent,” the study indicates.

Knowledge wins

Discovering and acknowledging the role of private-side I&I in their system was key for the city of Revere in reaching its ambitious mitigation goals.

“The goal of 50 percent I&I removal could not be achieved by simply rehabilitating the public sewer system,” the study says. “This did not come as a surprise to the city of Revere or CDM Smith; however, the amount of I&I attributed to private inflow was certainly more than originally anticipated and resulted in a need to focus intently on private inflow removal.”

While public mitigation via CIPP lining, manhole rehabilitation and service lateral connection lining were also important in meeting the consent decree — removing as much as 22 percent from the test area — it was the cooperation with private landowners that made the difference.

The city is still working toward its consent decree, but with the combination of these public and private programs, they estimate reaching 65 percent I&I removal — well over the consent decree requirements.


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