All water and sewer pipes need to be cleaned once in a while. Biofilm develops along the walls of water pipes, iron and manganese flake off into potable water and discolor it, and grit and sediment creep in. In sewer mains, flow may become restricted due to depressed sections or corners of bends that collect deposits. These buildups may reduce water flow, leaving water quantity inadequate for both firefighting and increased residential or commercial building.
Flushing and pigging are the common techniques to clean out pipes and mains. Pigging? Not familiar with the term? It began in the oil and gas industry where metal discs connected by a rod were moved through the lines to remove buildup of paraffin wax on the internal wall of the pipe. The action of metal on metal made a squealing sound like a pig. There are also pigs made of other materials; typically pigs are propelled along by the pressure of the material in any given pipeline.
Soon the technique and the name were adopted by water and sewer utilities where the pig is launched into a line and clean water is used to push it through the line.
There are different types of pigs: gauging pigs used after constructing a pipeline and profile pigs with multiple gauging plates used to help map the inside condition of the pipe walls.
Also, there are magnetic cleaning pigs used to pick up ferrous debris left in the pipeline. Transmitter pigs or detector pigs can map out pipeline locations or help locate a stuck pig. Round spheres or sponges can be used to negotiate 90-degree elbows, irregular turns, or bends.
Pigs can get stuck because of a blockage of the flow, or fluid bypassing the pig and not pushing it. The line pressure and flow rate can be increased to push it through the blockage, or pressure can be released and the line drained back toward the launcher, allowing the pig to relax to its normal shape and back up. The line can then be repressurized in an attempt to drive the pig through the obstruction. Alternately, pressure from the opposite end of the water line can be applied, using a pump or pumper fire truck.
However, a pig getting stuck is the greater danger, and the line may have to be dug up. This becomes especially problematic if the wastewater main travels under rivers, railroad tracks, highways, or aqueducts where an inverted siphon, or depressed line shaped like a V, counts on pressurized volume to move the wastewater through it. In this case, the literature says, the sewer system operator would have to bore another pipe.
Flushing involves forcing high-speed water through the pipes to carry away particulates, but it uses very large volumes of water and it may be impossible to get the required velocities in large diameter pipes.
Unidirectional flushing involves flushing clean water through a dirty pipe and out, working in one direction and one segment at a time. By cutting off other flows, scouring velocities of 5–10 feet per second or more are achieved, compared to 1–3 feet per second with conventional flushing.
Ice Pigging is Invented
To address the problem of stuck pigs, a professor at the University of Bristol in the United Kingdom developed ice pigging technology in 2000. By 2010, the university and Bristol Water had commercialized the technology. It was introduced in the US in 2012 when Utility Service Group obtained an exclusive, 10-year license.
The technology is rather simple—it uses system pressure to push an ice slurry into a main through a hydrant or a two-inch fitting and down to the other end of the pipe and out through another hydrant or fitting.
Ice pigging can clean any material pipe, says Paul Treloar, Utility Service Group (USG)’s product manager for ice pigging. He says the technology was designed for potable water mains and adapted for sewer force mains.
Ice pigs cannot be used in gravity fed sewers (they need the force of pumped water to drive them forward). “Ice pigging will remove iron, manganese, and biofilm buildup as well as fats, oils, and grease, but not hard tubercules,” he says.
The types of problems found in pipes vary, says Treloar. For example, where water is sourced, whether it is ground or surface, will produce different kinds of sediments and biofilm buildup in the pipe, and the types of treatment will vary. “If you don’t use Chlorine, there could be biofilm problems,” he says.
Treloar says the traditional metal pigging requires cutting pipes open to get the pigs in if pigging launch stations have not been installed. Cutting pipes open can create problems with contamination, he says. Then, once it is closed, it should be disinfected.
Foam sponges or swabs are inserted through hydrants and also use water pressure to get pushed through, says Treloar. But they can get lost. However, ice pigs melt if they get lost and the problem disappears without further intervention, he says.
Ice pigging equipment deployed
Danbury Finds the Solution
The North Carolina town of Danbury is the county seat, but it is still a small town with only 100 water connections and 2.5 miles of 6-inch PVC water pipes that are approximately 40 years old. Mark Delehant, public works director for the Stokes County Water District, says the pipes had serious iron and manganese buildup on the linings, due to the well water that provides the potable water to the town.
The Danbury water system was originally privately owned, says Delehant. The county acquired it in 1978 during a system expansion project and then leased it to the Town of Danbury, who hired contract operators until 2008. That was when the county took it back, and it continues to operate it today.
The two wells that supply the town’s water have a combined output of 80 gallons per minute, and the water system has only 100,000 gallons of storage. As a result, all leaks must be addressed immediately, says Delehant, “or we run the risk of draining the town’s water supply.”
Maintenance of the system, says Delehant, involves sampling, testing, flushing, and valve exercising. All maintenance and repair activities requiring excavation are contracted out.
Delehant began working for the utility in 2007 and became director of the water system in 2008 when the Stokes County Public Works Department took over its operations. Danbury water customers regularly had to deal with iron in the water that produced dingy-looking water, and even stained clothes. He says customers would come to his office with a bottle of discolored water and complain.
System flushing only made matters worse, says Delehant, since a layer of iron deposits had built up on the interior lining of the water main. With the limited water storage, “We did not have enough water to properly scour the pipe.”
Delehant discovered ice pigging after searching for water line cleaning methods on the Internet. In the search he learned that mechanical pigging involved considerable cost, downtime, and risk. Further, he learned that the ice pigging technology was new, and offered by USG—a company he was familiar with.
Further research convinced Delehant that ice pigging was a perfect fit to solve his problem. It would use very little water, require very little water system downtime, carry no risk of harming the PVC pipes, and there would be no excavation.
In order to get approval from the County Manager and elected officials, Delehant told them that while the water coming out of the wells is clear, the discoloration was picked off the interior lining of the pipes as it passes through the pipes. The Public Works Department received the votes it needed to sign a contract with USG and proceed with ice pigging.
The department and USG then identified fire hydrant locations and inline valves to determine segments of line that could be isolated with an injection point at one end, and an extraction point at the other. These points had to follow the direction of flow away from the water supply tank that would inject the water, under pressure, to push the ice pig through the line.
Furthermore, the team had to decide how many feet of pipeline could be cleaned with the amount of ice the USG ice machine could produce. Delehant says they chose the 18,500 linear-foot, 6-inch main that was part of the primary distribution system that could be cleaned in four segments. “We actually installed two, two-inch blowoffs so they could be used as injection/extraction points.”
Danbury, although very small, hosts a hospital and the county jail, and these cannot go without water for an extended time. Once they determined that the four sections of the pipe were cleaned, dates and times were set based on when USG’s ice-making machine would arrive. Customers were then notified when their service would be interrupted. Delehant arranged for the local volunteer fire department to handle traffic control.
The first and longest segment, 8,000 feet of 6-inch line, was ice pigged at night, beginning at 6 p.m. The advantages of working at night were lower traffic and improved control, and the lower temperatures allowed the ice to maintain its consistency.
To view the results, Delehant stationed himself at the extraction point. He saw dingy water come out first, and then as more water exited, it turned from light brown to dark brown, to black, and finally to clear. He compared a piece of pipe from a pre-ice pigging repair and compared it to a piece of pipe after ice pigging. He says the results were better than expected.
Work on each segment took about 2.5 hours, and all work was completed at night over a two-day period. USG reported that it used a three-man team plus a supervisor to complete the project. Equipment used included a 10-ton ice delivery tanker, a 10-ton ice production unit powered by a portable diesel generator, and a Ford truck carrying a “Flow Analysis System.”
Ice pigging process hooked up
Siphons Were the Challenge
The small town of Diablo Grande in the hills near Patterson, CA, lies approximately two hours south of San Francisco. Western Hills Water District operates the town’s water and sewer system. The 10-year-old main sewer line runs by gravity over 6 miles down to a wastewater treatment plant in Patterson.
Roughly 10,000 linear feet of sewer pipe with inverted siphons—or depressed pipe—splits into a 12-inch diameter high-density polyethylene and 14-inch diameter HDPE pipe passing beneath two aqueducts—the 135-foot-wide California Aqueduct and the 100-foot-wide Delta Mendota Aqueduct, as well as the Interstate 5 highway.
At each aqueduct, the main splits into two pipes, one at a slightly higher level than the other to allow for peak flows. The line is designed to allow the contents to pass under the aqueducts in siphonic action.
The staff noticed a reduction in the flow capacity and believed it was due to a buildup of sediment, grit, and sludge at the lower point of each of the inverted siphons, probably due to the main not working to its full capacity. It had been designed to handle large flows that future development would bring.
The main was designed with mechanical pigging launch stations at the high end of each inverted siphon. However, the district’s general manager was reluctant to use this method, given the chance the pig might get stuck in the main, which would require new pipes to be bored below the aqueduct in an emergency situation.
Built into the design is a cross connection to the raw water supply that provides the community with water from the California Aqueduct. The connection was used regularly to inject large volumes into the sewer line for flushing purposes. However, the flushing had not been effective to clear the buildup that was causing the partial blockage in the inverted siphons.
Western Hills was already a customer of Utility Services Group, and once it was aware of the problem, ice pigging was suggested as a solution. The general manager recognized this solution would be more effective than flushing and safer than using a mechanical, or hard, pig.
However, the water supply for the pipe was 6 miles away. The water had to be shut off to drain the pipe to the injection point at the cross connection, then the crew turned on the water supply and timed the injection so the ice didn’t melt before the water arrived.
USG’s Paul Treloar filled in the details. Normally, ice pigging will not work in gravity-fed systems because pressurized water is needed to push the ice slurry through. But in this case, a small section of the main that went into the inverted siphon could be pressurized, he says. “We installed flow-through plugs at the end of the line to plug off the main.” The ice pig had then been inserted at the pigging launch station and the plug was kept shut.
When the crew turned on the water supply—it took about two hours for the water to arrive at the cross connection—a man waited at a nearby manhole to tell crew members when it arrived there. Once the signal was given, the plug was pulled out allowing the ice slurry to start flowing and when the water arrived, it pushed the plug forward. “We’d never done it before, and it was an absolute success,” says Treloar.
About 2,500 linear feet of pipe were successfully cleaned during each of four runs, lasting about an hour each day. Large amounts of sediments and fats, oils, and grease were removed, leaving the HDPE pipe clean. Flow capacity returned to normal.
Pumping Capacity Was Failing
Middlebury, VT, built a new wastewater treatment plant in 2001 and converted its original treatment plant to a main pumping station with about 12,000 linear feet of new 16-inch and 18-inch ductile iron and PVC force main to convey wastewater to the new treatment plant.
By 2005, wastewater staff discovered that the pumps, originally designed to discharge 6.2 million gallons per day, capacity with two pumps operating, had decreased to less than 5.0 MGD. In addition, the wet well (to store wastewater) was undersized with limited operating volume and emergency storage. This meant that during extreme wet weather the pump station could not keep up with incoming flows, and combined sewer overflows (CSOs) were discharged to Otter Creek, which runs through town.
Phelps Engineering evaluated the wet well, pumping capacity and force main that year, and concluded that grit accumulation in the wet well and force main was the primary cause of reduced pumping capacity.
In 2007, in response to its Vermont discharge permit renewal, the Clean Water Act’s CSO Control Policy required Middlebury to monitor rainfall and overflow frequency duration and volume until 2011.
Middlebury passed a $1.8 million bond vote in October 2009 that included $900,000 in Federal ARRA funding for CSO abatement improvements. Engineering firm Aldrich & Elliot completed the wet well expansion and a grit removal system, according to Robert Wells, Middlebury’s Wastewater superintendent.
However, the force main needed to be cleaned to return the pump station capacity to 6.2 MGD. Other goals were to improve pump efficiency and save energy. Wells says his staff, along with Aldrich & Elliott, evaluated different cleaning techniques such as conventional and unidirectional flushing (UDF), and traditional pigging using metal or plastic pigs.
Costs of the different types of cleaning techniques were illustrated in an AWWA distribution system issues webinar in April 2012. UDF was estimated to cost $0.98 per linear foot, ice pigging was estimated to cost $1.70 to $5.50 per linear foot, swabbing was estimated to cost $5.70 to $9.10 per linear foot, and traditional pigging was estimated to cost $16.10 to $21.00 per linear foot.
In evaluating each of the different cleaning techniques, the Middlebury staff concluded that flushing would not have enough velocity; it required resource allocation and a lot of water. Traditional pigs and swabbing would be incompatible due to the pipe size changes, bends, and no insert or retrieval stations. Launching and receiving pigs would require excavation and customers would experience long water supply cutoffs.
On the other hand, ice pigs present an exceptionally low risk since they melt if they get stuck, they use a minimal amount of water, they are much more effective than flushing, they go around bends and changes in diameter, and the process is fast.
But, as noted above, ice pigging cannot remove heavy corrosion and tubercles, remove hard water deposits, or rehabilitate pipes. Ice pigging can put off rehabilitation costs and remove mineral buildup, organic matter, and loose materials.
What Middlebury Did
Once it chose ice pigging, Middlebury worked with USG and Aldrich & Elliott to create a schedule and plan for nine ice pigging insertion points. The number and location of insertion points were based on pipe diameter and length and a cool wastewater temperature to make sure the ice pig slurry would hold together as it traversed each pipe segment. Use of seven existing air release/cleanout manholes for insertion points saved project costs.
Ice was inserted every other day to allow ice to be manufactured during the off days. The pumping capacity of the pumps was checked via drawdown testing after each cleaning to document results. The contract had an out clause if a measurable improvement was not being observed.
The ice pigging successfully cleaned the force main and pump rates returned to about 6.2 MGD. Removal of accumulated deposits increased capacity of the pump station by more than 640,000 gallons per day. The increased efficiency lowered pump run times, saving energy and water.
The pump station now operates at full capacity and eliminates combined sewer overflows, protecting public health and the environment.
The project was completed on schedule and within budget with no field changes or change orders. Wells says this experience proves large-diameter force mains can be cost-effectively and successfully cleaned by ice pigging.
REFERENCES
The author is grateful to the following organizations and their publications for much of the background information in this article:
American Water Works Association, Opflow, April 2014, The New Ice Age: Pigging
Effectively Cleans Water and Wastewater Pipelines.
National Environmental Services Center Tech Brief, Line Pigging, Spring 2007.
PVC Pipe Association: Handbook of PVC Pipe Design and Construction, Industrial
Press Inc, 2012.
University of Bristol, Bristol Water plc, Investigation and development of an
innovative pigging technique for the water supply industry.
WaterWorld, Unidirectional Flushing, by Dave Lewis, Wachs Water Services,
June 2013.
