Cold War

Small town puts ice pigging technology to the test on an ailing force main and brings home honors.
Cold War
Treatment Plant Superintendent Bob Wells works to unclog a blocked sewer. (Photography by Oliver Parini).

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When the Town of Middlebury, Vt., faced a significant reduction in capacity of a critical sewer force main, it gambled – and won – on the use of a novel cleaning technology. Ice pigging restored original pipe capacity, increased system efficiency and reduced the potential for combined sewer overflows.

Middlebury is a town of fewer than 9,000 people, located about 50 miles southwest of the state capital Montpelier. Its sewer system is relatively young; most of it was constructed in 1960.

Gravity sewer pipes are made of a variety of materials, including clay and PVC between 6 and 24 inches in diameter. Most new installations employ PVC. The town’s force main is made of both ductile iron and PVC.

“Overall the pipes are in fair condition,” says Bob Wells, treatment plant superintendent for the Town of Middlebury. “We do a lot of repair work in-house and we keep a stock of critical items so that our downtime is minimal if something fails.”

In-house equipment includes a rodder, a hoist truck and a loader. The town has also contracted such technologies as directional drilling to replace a sagging pipe in 2013, and has previously used sliplining on a problem force main.

Sewer rodding is routinely conducted in-house, while an outside company is contracted yearly to perform a solid week of sewer line cleaning.

“When you think you’ve seen everything that people will put into the sewer, there always seems to be something that tops the last item,” Wells says.

Flow monitors detect leaks

System leaks are detected by monitoring pump hours against rain events, adjusted for the season. In addition, two flow monitors are employed as needed in areas of suspected infiltration.

“Last year, we hired an engineer to assist in nighttime flow monitoring,” Wells says. “Several spots have proven to be prone to infiltration and each is put onto a capital improvement list that will be addressed as funds become available.”

One procedure that has been paying off incrementally is the town’s policy for buildings either going up for sale or being refinanced.

“We inspect the property for any type of illegal hookup to the sewer system,” Wells says. “Items like sump pumps discharging into the sanitary system can put a lot of groundwater into the sewer when there may be an alternative that would be beneficial not only to the land owner but also to our system.”

In 2000, a new wastewater treatment plant was constructed, replacing the old plant. Wastewater is pumped to the plant through a force main via the Middlebury Main Pump Station. The plant employs sequential batch reactors and ultraviolet disinfection. Solids are sent to two sludge holding tanks and then to two 1.5-meter Komline-Sanderson presses. The solids are heat pasteurized and applied to agricultural land as Class A biosolids. Treated effluent is released to Otter Creek, which passes through town on the way to Lake Champlain.

Sewage treatment is also provided for adjacent sections of the neighboring town of Weybridge.

Reduced pump capacity risks CSOs

Like many communities, Middlebury continues to meet the challenge of controlling CSOs during wet weather. While the overflows often represent small volumes, they’re taken seriously. The most recent concern involved reduced pumping capacity at the main pump station, which conveys wastewater through 12,000 feet of 16- and 18-inch ductile iron and PVC force main to the wastewater treatment facility.

The pumps were originally designed to discharge wastewater at a rate of 6.2 million gallons per day, but pumping rates decreased over time. During some wet-weather conditions, the pump station could not keep up with incoming flows, and sewage was discharged to Otter Creek.

“Everything checked out fine, from pumps to air release valves, so we realized the problem was obstruction in the force main,” says Wells. “Regardless of the type of work we were going to do on the force main, the State of Vermont’s CSO policy required us to construct a larger wet well to hold any additional volume of wastewater while the work was to be done.”

Middlebury contracted Aldrich + Elliott, PC, an engineering firm located in Essex Junction, Vt., to assist with the design of the pipe remediation job.

“The traditional method of getting the force main back to original condition would be through the use of a mechanical pig,” says Wayne Elliott, vice president, Aldrich + Elliott. “However, solid poly pigging was ruled out due to size changes, 90-degree bends, wyes, and a lack of insert or retrieval stations. With a force main 12,000 feet long you would also need to shut off the pumps for quite a while. If the pig became stuck, it would take even longer. Most of the guys who traditionally do this sort of work wanted nothing to do with this project.”

The ice pigging alternative

Both the town and the engineering firm were previously aware of the offerings of Utility Service Group (USG), an Atlanta-based company that holds the rights to ice pigging for sewer and water applications in North America. The technology employs saltwater ice slurry, which is injected into the pipe, scouring the interior like a glacier and absorbing impurities as it passes through. In European projects and a few test runs in the U.S., the technology has proven effective at clearing sewers of sediment, sand, debris, sludge, and fats, oils and grease.

After receiving approval from the Public Works Committee and the town’s selectboard, USG was contracted to pig the force main.

“While sewer ice pigging has been used on force mains, it had never been attempted on a line this long anywhere in the world, so we made the town an offer – if they didn’t see noticeable improvement after the first few segments had been treated, we would pack up and they wouldn’t owe us a cent,” says Paul Treloar, ice pigging project manager with USG.

USG’s ice pigging rig consists of a brine tank and chiller that cools a 4.7 percent brine solution to between 22 and 26 degrees. An operator controls the chilling process, which may require as many as 36 hours before the slurry is ready for injection. Unlike a mechanical pig, the ice pig is driven by the existing pressure inside the force main.

“The job needed to be divided up so that we could insert the ice from our 10-ton delivery rig to scour a certain length of pipe before it melts,” says Treloar. “We were able to break the job down into nine sections, with air valves to be used as ice insertion points.”

Wells and his crew assisted USG with confined-space entries and tapped the force main in two additional places to provide optimal injection points.

Elliott notes that the temperature of the wastewater is critical to effective ice pigging.

“In this case, we found that Middlebury’s wastewater is warmer during the early morning hours, so we found it made sense to delay the procedure a few hours each day,” he says.

In water main ice pigging, water samples will show impurities that gradually clear up as the process is completed.

“In force main ice pigging you can’t really put a bucket at the end of the pipe to see what you’ve cleared,” says Elliott. “The real test is to determine if we get an increased flow rate in the pipe at the same pump horsepower – and we were definitely seeing that during the first two applications.”

Pump capacity increased

The entire force main was scoured over a period of about 10 days, with pumping efficiency improvement following most applications.

After USG completed the procedure, pump capacity increased by more than 640,000 gallons per day, returning the system to its original specifications.

“Ice pigging has a place in the market as another tool in the toolbox for line cleaning applications,” Wells says. “Assess the job to be done to see if traditional or ice pigging is more appropriate for you. Ice pigging certainly worked best for us with this particular job.”

The project was recognized by the American Council of Engineering Companies of Vermont with a 2014 Award of Excellence presented to Aldrich + Elliott in association with the Town of Middlebury and USG.


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