Traditional “pigging” involves sending a mechanical device — or pig — down a pipe to clean or inspect it. Its descendant, ice pigging, substitutes abrasive ice brine slurry for the mechanical pig, allowing it to scour the interiors of water and sewer pipes. 

The technique was developed in England by Professor Joe Quarini of the Faculty of Engineering at the University of Bristol, to clean pipes in the food processing industry. From there, the process was adapted for water mains at Bristol Water, the utility serving the city of Bristol. 

Ice pigging was ultimately patented and is offered in North America by Utility Service Group (USG) of Atlanta, Ga., the license holder for water and wastewater applications in the U.S. and Canada. 

Clean sweep

“The ice pig doesn’t so much bulldoze as scour the inside of the pipe like a glacier and then soak up impurities like a sponge,” says Paul Treloar, ice pigging project manager with USG. “It’s an excellent treatment for biofilms, iron, manganese, sediment and grit. Unlike a mechanical pig that might get stuck or damage butterfly valves, however, an ice pig melts quickly.” 

Treloar notes, however, that ice pigging is not recommended for tuberculated pipes, because the abrasive slurry can remove too much tuberculation at once, risking pipe blockages. 

Ice pigging equipment consists of a brine tank and a chiller that cools the brine to about 22 to 26 degrees F. Food-grade table salt converts the water to a 4.7 percent brine solution, roughly the same salt content as seawater. It may take as many as 36 hours of chilling before the slurry is ready to inject into the system. 

Two operators oversee the pigging process, which takes up to three hours to complete. The injected slurry must follow the normal direction of water flow, since it’s pushed through the line using existing system pressure. 

Salt water is later disposed of according to local regulations, either captured in a tank, or drained to the sewer system. 

The process is currently used to clean pipes ranging from 1/4 to 24 inches in diameter. The slurry must fill about 20 percent of the pipe’s capacity. 

Minmial downtime saves money

In 2012, Stokes County, N.C., became USG’s first formal U.S. customer for water main ice pigging in the town of Danbury. Mark Delehant, water and sewer system manager for Stokes County, says he first investigated mechanical pigging to deal with customer complaints about iron and manganese impurities. 

“Mechanical pigging involved considerable cost, downtime and risk,” he says. “Ice pigging required very little water system downtime, no excavation, utilized very little water and carried no risk of harming my pipes.” 

The Danbury project represented a typical ice pigging procedure. First, the water system was analyzed to locate fire hydrants and inline valves that would delineate pipe sections that could be isolated, with an injection point at one end and an extraction point at the other. 

In Danbury, 18,500 feet of 6-inch PVC main were targeted in four segments. Slurry was injected into the system as operators pulled samples from a hydrant every 30 seconds to check water quality. 

“I saw my dingy water turn from light brown, to dark brown, to black, and finally to clear,” Delehant says. 

The first pipe segment of 8,000 feet required 1,600 gallons of ice and 18,635 gallons of water. 

Pressure is on

USG has also used ice pigging to clean and restore capacity to sewer pipes, removing fats, oils, grease, sediment, grit and debris.

“As with water mains, the slurry must be driven through the pipes by system pressure, so ice pigging works only with forcemains,” Treloar says. 

Recent sewer projects include Dallastown Borough, Pa., and Middlebury, Vt. In Middlebury, USG scoured almost 12,000 feet of 18-inch ductile iron and PVC pipe, the largest diameter forcemain serviced to date. 

“We’ve also employed ice pigging to scour siphons, since they’re also subject to pressure,” Treloar says. “However, we had to use inflatable plugs on either end of pipe sections, then pulled the plug on the far end to resume siphonic action.” 

Treloar says the company is currently examining ways to increase the volume of ice available for each operation, in order to service larger diameter mains. 

“The larger the volume of brine, the more difficult it is to maintain the consistency of the slurry, because ice wants to float,” he says. “We’re currently at 10 tons, but we’re looking to the UK where they’ve had some experience with 15-ton trucks.”