Budgeting for Infiltration and Inflow: The St. Louis Solution

I&I is a complex mix of factors, which can make budgeting tricky. If you’re looking for best practices, see how St. Louis, Mo., tackles the topic with its comprehensive program.

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How do you budget for infiltration and inflow? Not an easy question to answer because I&I is linked to so many other issues, such as precipitation, flooding, customer connections and age and condition of sewer lines.

Ultimately, utilities need to consider separating combined sewers, stemming the flow at the source or expanding sewer and treatment systems to handle increased flows. However, each of those cost money, require detailed planning and can take years to complete.

Although the St. Louis, Mo., Metropolitan Sewer District is larger than most at 525 square miles and more than 9,000 miles of collection and trunk sewer and force mains, its current program to combat I&I and prevent overflows features several best management practices.

A recent conversation with Rich Unverferth, director of engineering, outlined how the St. Louis MSD is tightening up its system.

MSW: How did you develop your plan to address I&I issues?

Unverferth: System characterization, monitoring and modeling formed the basis for our entire capital program to meet our consent decree. We performed smoke testing and dye testing. We used flow meters and rainfall gauges to see how our system responded to wet weather events, and we transmitted the data back into models for our sewer system.

At the same time, we were cleaning our system extensively, using CCTV to identify the sources of I&I. We used the data to develop future operations and maintenance activity within our asset management program.

MSW: What did the monitoring and modeling reveal about I&I?

Unverferth: Within the city, sewers are combined, and outside the city, they are separated. However, in the fringe areas in between, we discovered a lot of cross connections, downspouts and yard drains getting into the sanitary-only sewer system. We spend a lot of time disconnecting downspouts and drains, and using CIPP to line sewer mains and lateral lines.

Smoke testing doesn’t always reveal everything. When we inspected homes, we found other connections. We would walk around the house, identifying potential connections from downspouts and drains from patios and driveways. Then, we’d validate it with a dye test and try to estimate the amount of water getting into the system and plug that information back into the model. We’d notify the homeowner and offer to pay for the disconnection as part of an MSD project, give them the option to disconnect on their own at MSD’s expense, or if they were not interested, we would let them know that it would still be required later on, but they might have to pay for it themselves.

MSW: How have customers reacted to this program?

Unverferth: This approach — one on one communications — really marked a change in our customer relationships. We’re still putting resources in place to support that. We have a universe of 104,000 connections, and we’re getting good participation. We’ll probably inspect 100,000 to 120,000 homes and ultimately disconnect a quarter of those. We want to stop all cross connections at some point and get that flow off the system.

MSW: What about the bottom line? What is your budget, and how has the program affected rates?

Unverferth: The total cost of our capital project is $4.7 billion, in 2012 dollars. On average, we’re spending close to $100 million per year on I&I, and 99.99 percent comes from ratepayers.

We take the $4.7 billion and divide it into four-year buckets. Every four years, we meet with our rate commission and lay out our program. Then, they decide how to fund the projects, whether through rates or bond issues. We monitor our rates, which are in line with other large metro districts. On average, rates have been around $38 per month. It’s a very public process, and we don’t shy away one bit from discussing the budget with ratepayers.

We also maintain a special Infrastructure Repair Fund to address roots or collapsed lines. We set aside about $5 million each year for that.

MSW: What are some of the lessons you’ve learned through this process?

Unverferth: One of several things we’ve learned is that lining the mainline sewer will not prevent all stormwater from entering the system. Water tends to travel alongside the rehabilitated sewer line and enter the line at a connection point. We’ve used CIPP top-hat sleeves to prevent that.

The other lesson is that cleaning, inspecting and smoke testing to identify sources of I&I might not catch everything.


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