In August 2012, Danbury, N.C., became the first U.S. city to formally contract ice pigging service to clean its water mains. The technique employs the friction of ice slurry to remove impurities such as biofilm, iron and manganese sediment from water pipes, without the use of mechanical systems. A local success, ice pigging is now being mobilized across the country.

Danbury is a town of fewer than 200 people located close to the southern border of Virginia, about 50 miles northwest of Greensboro. The water system is only 40 years old and was acquired by Stokes County in 1978 and leased to the town. Contractors operated the system until 2008, when the county assumed full responsibility for its operation.

The water system, in good condition due to its recent vintage, consists of about 2.5 miles of 6-inch PVC pipe. Stokes County staff members are responsible for water sampling, testing, flushing and valve exercising. Equipment is pooled and shared among neighboring communities. The town relies, for example, on nearby Walnut Cove to offer its trailer jetter from Sewer Equipment Company of America as needed. All work requiring excavation, from construction to heavy maintenance and repair, is outsourced.

In terms of system responsibility, the buck stops at the desk of Stokes County Public Works Director Mark Delehant, who took on the top job in 2008. He acts as water and sewer system manager for Stokes County, which owns several other systems in different areas of the county.

“The water system faces two major challenges,” says Delehant. “It offers only about 100,000 gallons of storage and the town’s two wells provide a combined output of only about 80 gallons per minute. With limited supply and storage, all leaks must be addressed immediately or we run the risk of draining the town water supply.”

While the system serves only 100 connections, some of those connections require more water than others. As the county seat, Danbury hosts a hospital, county jail, courthouse and administrative buildings.

Water quality an issue

The presence of iron and manganese in the water has also presented an ongoing problem for customers.

“Danbury customers regularly experienced iron that on occasion would result in dingy looking water and even stained clothes,” says Delehant. “Customers would come by my office and hand me a bottle of discolored water and complain.”

The water utility’s attempts to flush the pipes clean of deposits proved counterproductive.

“A surge of water would loosen only some of the coating, enough to discolor the water further,” says Delehant. “With only 100,000 gallons of stored water, we never had enough water volume to properly scour the pipe.”

Delehant next began to look at traditional “pigging” — sending a mechanical device, or pig, down the pipe to scrub it clean.

“I learned that mechanical pigging of water mains involved considerable cost, downtime and risk, such as pipe damage and getting the pig stuck inside the main,” he says. “One contractor said his pig could do 90 degree angles, but if it got stuck, we would have to dig it out on our dime.”

Discovering ice pigging

Further investigation led Delehant to ice pigging, a process which uses icy brine to scour pipe interiors. While the technique has been used in Europe and elsewhere, ice pigging was being offered in the U.S. only by Utility Service Group of Atlanta, Georgia, the exclusive license holder for ice pigging for water and wastewater applications in the U.S. and Canada.

“I was familiar with the company,” says Delehant. “Once I learned more about the process it seemed a perfect fit for my problem. There was no price premium and ice pigging required very little water system downtime, no excavation, utilized very little water and carried no risk of harming my 30-plus-year-old pipes. The only real risk appeared to be that it wouldn’t work.”

Once the contract with Utility Service Group was signed, Delehant examined the system map to locate fire hydrants and inline valves to identify segments of line that could be isolated, with an injection point at one end and an extraction point at the other. The points had to follow the direction of flow away from the water supply tank, since ice slurry is pushed through the line using only existing system pressure.

“Another consideration was how many feet of line could be cleaned with the amount of ice their machine would produce,” says Delehant. “To clean the entire mainline in four segments we installed two 2-inch blow-offs so they could be used as injection/extraction points.”

The project design consisted of 18,500 feet of 6-inch line. The four segments measured 8,000 feet, 5,000 feet, 4,000 feet and 1,500 feet.

Delehant next scheduled service dates and notified customers of the times their service would be interrupted. He also identified a staging area for equipment in close proximity to a fire hydrant.

First in America

After initial trials in other cities using a demonstration rig, the first official American application of water main ice pigging began with the shipment of full-sized equipment from the U.K. to North Carolina. Delehant notes that the delivery of the equipment raised a few eyebrows. “Some of the residents thought it looked like we were taking delivery of a brand new liquor still,” he jokes.

Overseeing the Danbury project were Paul Treloar, ice pigging project manager, and Andy Tillman, water systems consultant, both with Utility Service Group. Treloar witnessed one of the earliest applications of ice pigging while working for the city of Bristol about 10 years ago.

“The technique was developed by Professor Joe Quarini from the Faculty of Engineering at the University of Bristol,” says Treloar. “I wasn’t with Bristol Water but I saw that ice pigging was going to succeed.”

Treloar oversaw the successful ice pigging of 66 miles of water mains coated with biofilm in Dordrecht, Netherlands and also supervised projects in Germany, Australia and other countries.

“The ice pig itself is slushy brine,” says Treloar. “It acts as a solid but can be pumped like a liquid. It doesn’t so much bulldoze the impurities as scour the inside of the pipe and then soak up impurities like a sponge. It’s an exceptionally low-risk process because, unlike a mechanical pig that might get stuck in the main or damage butterfly valves, an ice pig will melt quickly.”

Concerns about damage to water mains suddenly exposed to freezing temperatures have also proved groundless. Treloar says that stress gauge tests have shown that the contraction of water mains is negligible and won’t lead to pipe breakage.

Time to chill

Ice pigging equipment consists of a brine tank and a chiller that cools the brine to about 22 to 26 degrees F. Since ice pigging requires about one-third the water used in traditional flushing, Danbury relied on its own water reserves. The water was converted to a 4.7 percent brine solution — the equivalent of seawater — using food-grade table salt certified by the National Sanitation Foundation International. The chilling process is controlled by an operator and may require 36 hours before the slurry is ready for injection into the system.

“Initially, we had the ice-making machine on one truck and the holding tank on another, but we now have everything mounted on a single trailer,” says Tillman.

Two operators oversee the pigging process, which typically takes three hours to complete. The pig is driven by system pressure alone — no additional pumps are required. Once operators know that ice is on the way, they pull water samples from the hydrant every 30 seconds to monitor progress.

“We ice pigged our first segment of 8,000 feet at night, beginning at 6 p.m.,” says Delehant. “That allowed for easier traffic control and with cooler temperatures we felt we would maintain the consistency of our ice for what would be our longest run. Once the pig was launched, I positioned myself at the extraction point so I could see the results firsthand. At first I saw my dingy water turn from light brown, to dark brown, to black and finally to clear.”

The first segment required 1,600 gallons of ice and 18,635 gallons of water. The salt water is disposed of according to local regulations. In some cases it’s captured in a truck tank, while in others it’s drained to the sanitary sewer system.

Since the successful Danbury project, Utility Service Group has added a second ice pigging unit to its equipment lineup and completed other projects across the country, including the conversion of a 50-year-old raw water line to a distribution main in Bedford, Pennsylvania.

Delehant’s souvenir of the experience is a pair of pipe core samples, showing before and after conditions.

“I’ve since recommended ice pigging to other utilities that have heard about our project,” says Delehant. “I can’t speak to how successful ice pigging might be in addressing other issues, but it was very successful in getting rid of the iron and manganese deposits from the interior lining of our PVC pipe.”