24 minute read
Ice Pigging: Award-Winning, Advanced
Paul Treloar
Ice pigging, a sustainable cleaning method for potable water distribution mains and wastewater force mains, was developed in the United Kingdom and introduced in the United States in 2012. The method involves pumping a slurry of ice into a main through a hydrant, or other existing fitting, and using system pressure to push the ice pig downstream to exit through a similar fitting. The ice slurry, filling 20 to 30 percent of a pipe’s volume, cleans with shear force—between 100 and 1,000 times greater than with water alone—providing more-effective cleaning and using significantly less water than traditional flushing methods.
Introduction
An ice pig works like a glacier does. Rather than bulldozing sediment and biofilm, it incorporates them into the ice. Because the ice pig enters and exits through a hydrant, specialized launch and retrieval stations are not required, as with mechanical pigging or swabbing; customer service isolation usually is not necessary either.
Sediment; fats, oils, and grease (FOG); and debris accumulation in wastewater collection systems clog force mains and siphons, causing pipeline restrictions. Theses restricted flows can cause increased energy use and sanitary sewer overflows and can lead to needed capital improvements, including increased pumping capacity and force main replacement. Other technologies, like flushing and water jetting, are inefficient and sometimes ineffective. In addition, these processes use a lot of water, which may not be readily available.
Background
Developed by the University of Bristol in England, ice pigging is an innovative, low-risk, advanced pipe cleaning technology to clean drinking water pipes, sewer force mains, and siphons.
The ice slurry can be inserted and removed through line taps, air valves, and other existing fittings, so expensive excavations are not required. Ice pigging harnesses the characteristics of a semisolid material that can be pumped like a liquid, but behaves like a solid once the pig is formed in the pipe
Because ice pigging relies on the natural glacial effect of ice to pick up unwanted sediment, it uses approximately 50 percent less water than standard water flushing and takes significantly less time. Typically, the section of main being cleaned is out of service for no more than 60 minutes.
A central feature of ice pigging is that it cannot get stuck. If for some reason that were to happen, time would be allowed for the ice to melt and flush it from the main. Pipe bends, changes in diameter, or butterfly valves can all pose problems for swabbing or pigging, yet ice pigs can easily negotiate these obstacles.
To launch and receive traditional pigs, excavations may be required to allow the installation of launch and reception stations. This can mean extensive and costly interruptions to any system and may require the installation of bypass pumping or a temporary water supply. Ice pigging is far less intrusive to any system it’s used on.
The Benefits
Ice pigging represents a sustainable best practice and unique approach to pipe cleaning. The advantages include: S It’s efficient, rapid, and environmentally friendly. S Combines operational benefits of flushing with the impact of solid pigging. S Ice slurry injects through existing fittings. S System pressure pushes the ice. S Suitable for pipes of all sizes and materials. S Effectively removes biofilm, iron, manganese,
FOG, grit, and sediments. S Produces quantifiable results. S Exceptionally low risk.
Figure 3. Typical potable water main setup.
Methodology
To maintain the correct consistency of the ice pig, a freezing-point depressant is used; in most cases, food-grade, fine table salt is used, which is approved by the National Science Foundation (NSF). This is dissolved in potable water, which is always sourced from a public water supply. The current maximum batch capacity is 2,700 gallons.
The brine is made in a 316-stainless steel delivery tanker and hose connections are made to the ice machines that are mounted on a separate trailer (Figure 1). The brine is fed into the ice machines which, in turn, freeze the liquid and return it to the delivery tanker. This cycle continues until the ice slurry is at the thickness known as ice fraction, which measures the amount of ice crystals as a percentage of total volume. Ice fraction is related to the cooling capability of the slurry compared to pure ice (100 percent); this is known as the calorimetric value.
Ice pig operators use a simple French press coffee plunger (Figure 2) to test the “ice fraction” (or the ice thickness) onsite prior to pumping it into the main.
Typically, the thickest ice is used on plastic and sound concrete-lined pipes, as well as asbestos cement, but when older unlined cast iron pipes are cleaned a thinner ice slurry is used that does not clean as aggressively. The thinner ice slurry will not disturb the buildup of tuberculation, which could damage the integrity of an old and heavily corroded unlined cast iron pipe.
Ice Delivery
Setup for delivery varies slightly for each different application; a typical setup for a potable water main is shown in Figure 3. The delivery rig connects to the inlet hydrant or other suitable fitting (2 inches or greater tapping with valve Continued on page 30
Figure 4. Taking samples.
Continued from page 29 control) and at the outlet, a flow analysis system is connected. This system measures and records the flow, pressure, conductivity, turbidity, and water temperature as the water and ice are discharged. Once set up, the main is flushed briefly to note and record preflush readings. The main is then isolated by the owner’s operators and the required amount of ice is pumped into the main.
At the same time, the outlet hydrant is opened to create a flow and allow water to be displaced as the ice enters the main. With careful control between the inlet and outlet, the flows are balanced to allow slightly more ice into the main than the amount of water being displaced. This has the effect of the ice forming as a pig against a pressurized wall of water.
Once the required amount of ice is in the main, the delivery pump is turned off and the upstream valve is opened to allow the system flow and pressure to “push” the ice pig along the main toward the outlet hydrant. The flow rate is controlled by the outlet operator at this time.
As the ice pig approaches the outlet, the conductivity reading will rise as the salty water of the melting pig arrives in front of the pig. The monitoring equipment will show the water temperature falling and conductivity rising as the ice arrives.
At this stage, the operator may collect samples of the ice at regular intervals for later analysis (Figure 4). The temperature and conductivity will return to preflush levels when all the ice and salty water has flushed out of the system and the flushing continues briefly to allow the turbidity levels to return to preflush levels (or lower) according to instructions from the owner. The main is then returned to normal service. No disinfection is necessary.
Sanitary Sewers
The setup for sewer force mains and siphons is similar to the water main setup detailed previously, except no monitoring equipment is used on the outlet. Instead, the ice is pumped to a gravity main or the wastewater treatment plant (WWTP), as shown in Figures 5 and 6.
The delivery rig will connect to a suitable fitting for ice insertion (Figure 7). This may be an existing fitting, such as an air release valve (ARV), or a lift pump bypass fitting. In the event there are no existing suitable fittings, a 2-inch or greater tap and control valve can be installed.
On a typical force main, the lift pump will be isolated, and the wet well will be allowed to fill to near high water level while the ice is pumped in. The ice can only travel in a direction away from the pumps due to the check valve at the pumping station. The ice will form as a pig against the head of water existing in the main. Once the required amount of ice has been inserted, the pump is turned on to give the pressure and flow to “push” the pig along the main. The main is returned to service immediately.
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With siphons, inflatable packers are used to create a pressurized environment in the siphon; ice is then pumped into the siphon via a valve-controlled flow-through pipe. Once the desired amount of ice is in the line, the packers are deflated and pulled and the siphonic action takes place as gravity takes over and the ice flows through the siphon, taking any sediment or other debris with it, leaving a clean pipe.
Case Studies
Western Hills Water District – Diablo Grande, Calif. Sewer Siphon – September 2013
Diablo Grande is a small community in the hills near Patterson, Calif., approximately two hours south of San Francisco (Figure 8). The water and sewer system is run by Western Hills Water District (WHWD). There is one main sewer that runs by gravity over 6 miles down to a WWTP in Patterson. It was designed to cope with the large flows that future development will bring. The sewer passes under two aqueducts: the California Aqueduct and the Delta Mendota Canal.
At each aqueduct the main splits into two pipes, one at a slightly higher level than the other, to allow for peak flows. It’s designed as a siphon to allow the contents to pass under the aqueducts by means of a siphonic action. The WHWD had noticed a reduction in the flow capacity and believed it to be due to a buildup of sediment, grit, and sludge at the low point of each of the siphon.
Although the main was designed with mechanical pigging launch stations at the high end of each siphon, the district engineer was reluctant to use this method in case the pig should get stuck in the main. Designed and built into the siphon is the ability to flush the line with raw potable water from the California Aqueduct. This connection can be used to inject large volumes of water into the sewer line for flushing purposes.
Unfortunately, the flushing had not proved effective on the buildup, causing a partial blockage of the siphons. The WHWD determined that ice pigging may be the solution, rather than traditional pigging, to eliminate the risk of getting a pig stuck under the aqueduct where excavation for retrieval is not an option.
The theory was that if the siphons could be plugged off at the lower end, then the siphon could be allowed to fill naturally from residual flow back up to the higher-end point where the ice would be injected. The stations were already in place for mechanical pigging, so these were adapted for ice injection. Once the siphon was full, it could be isolated and the residual flow directed into the second bypass siphon. Ice could then be injected into the full siphon, while the inflatable plug at the lower end would allow water to be displaced via the flow-through pipe in the plug. This took very precise communication between the operators at the injection end and the contractors operating the flow-through plug at the “outlet” end.
Prior to this, a backup supply of over 10,000 gallons of raw water was pumped into the sewer at one of the flushing points 6 miles away at the community treatment works. This was the water that was to “push” the pig through the siphon. It was estimated that it would take approximately three hours for the backup water to arrive at the siphon once it was released.
Once the full tank of ice was injected, the flow-through plug was isolated, thereby holding the ice pig suspended in the first section of the siphon. It was then a matter of waiting patiently for the backup water to arrive.
The timing of this was very crucial so as not to allow the ice pig to melt before being able to clean the main. After a few tense moments, the backup water arrived; simultaneously, the plug was pulled, and the flow diverted into the siphon containing the ice pig. Again, a few more moments of waiting; this time it was for the ice pig to arrive at the lower end of the siphon.
Finally, the water started to darken in color and lumps of sludge and debris passed though the manhole at the lower end of the siphon. The water turned darker (signs of the melted front end of the pig), and then, thicker. The decreasing temperature was monitored using a thermal laser thermometer. Eventually, the ice was visible in the manhole and a huge slug of ice squeezed out of the main. Once the main body of the pig passed, the fluid quickly turned clear, indicating the main had been thoroughly cleaned. The siphon was returned to service and full flow was resumed. This concluded the world’s first known ice pigging of a gravity sewer siphon (Figure 9).
Statistics
Delta Mendota Canal
• Type of main: Gravity sewer siphon • Length of main: 2 x 1,400 feet • Diameter and material: 12-inch and 14inch high-density polyethylene (HDPE) pipe • Ice quantity: 2,700 gallons • Ice fraction: 90 percent • Time main out of service: None • Results: Siphon returned to full flow
California Aqueduct
• Type of main: Gravity sewer siphon • Length of main: 2 x 3,151 feet • Diameter and material: 12-inch HDPE • Ice quantity: 2,700 gallons • Ice fraction: 90 percent • Time main out of service: None • Results: Siphon returned to full flow
Middlebury (Vt.) Main Pump Station Wastewater Force Main – October 2013
The Middlebury Main Pump Station conveys wastewater through 12,000 linear feet of 16-inch and 18-inch ductile iron and 18inch polyvinyl chloride (PVC) force main to the wastewater treatment facility (WWTF). During some wet weather conditions, the pump station could not keep up with incoming flows and raw sewage was discharged to the Otter Creek (combined sewer overflow [CSO] events). The pumps were able to discharge 6.2 million gallons per day, with two pumps running during the first few years of operation (as designed), but pump rates decreased by more than 10 percent (620,000 gallons per day) over time as the force main collected grease, grit, and sediment.
The objective of what turned out to be
Figure 9. Ice discharged from the siphon.
Figure 10. Utility Service Group winners of the ACEC grand award for engineering excellence on the Middlebury project. Figure 11. Drawdown tests show a steady increase in flow after each operation.
an award-winning project (Figure 10) was to clean the force main by pigging to regain the lost pumping capacity and eliminate CSO, improve pump efficiency, and save energy. It was determined that “industry standard” solid poly pig techniques would not work due to the changes in pipe size, no available insertion and retrieval stations, bends and wyes in the force main that would have restricted travel, and the difficulty of handling the volume of water that would back up into the pump station wet well if the poly pig got stuck. Because of this risk, a local conventional pigging contractor would not even provide a quote. Ice pigging was offered as an exceptionally low-risk solution, and following evaluation, it was considered to be the best solution, given the conditions.
Calculations were made to determine the number of pipe segments to be pigged and the location of insertion points based on the pipe diameter, pipe length, and temperature of the wastewater to make sure the ice pig slurry would hold together as it traversed the pipe segment. The force main was divided into nine segments with nine insertion points. Of those nine points, seven were located in existing air release or cleanout manholes, saving both time and money. The force main was exposed and taps were installed for the other two insertion points. The project was completed on schedule over a three-week period.
This project was the first use of the ice pigging technique to clean force mains larger than 8 inches in diameter in North America. It was also the longest continuous run of sewer force main (12,000 linear feet) successfully cleaned with ice pigging. The project demonstrates that large-diameter force mains (both ductile iron and PVC) can be cost-effectively and successfully cleaned by ice pigging, avoiding other moreexpensive and invasive pipe cleaning and repair methods.
The ice pigging successfully cleaned the force main, and force main capacity was returned to 6.2 million gallons per day, based on daily drawdown tests at the pump station after each day of pigging (Figure 11).
Through ice pigging, accumulated deposits were removed, decreasing friction loss and increasing capacity in the force main by more than 640,000 gallons per day. Pumping efficiency was increased, lowering pump run times and saving energy and wear. The success of ice pigging was evident each day when sand, grit, organics, and grease discharged at the WWTF.
The increase in pump capacity should eliminate sewer overflows, protecting public health and the environment. The Confirmatory factor, or C factor, analysis, which is used to determine the factor and factor loading of measured variables, has shown that the friction loss in the pipe is now typical of that of a new pipe.
After determining pumping velocities for different pump speeds, the town engineer was also able to recommend programming changes in the pump cycles and pump speeds to increase the velocity of flow through the force main during pumping to achieve a “scour velocity” that should greatly reduce buildup of sediment in the future.
Middlebury should be able to operate the pump station at full capacity, saving energy and eliminating sewer overflows for many years to come. Capital improvements to increase pumping capacity or replace the existing force mains were avoided.
Statistics
Middlebury (Vt.) Main Pump Station
• Type of main: Wastewater sewer force (pumped) main • Length of main: 11,772 feet • Diameter and material: 18-inch PVC and ductile iron pipe • Ice quantity for each run: 2,700 gallons • Ice fraction: 85-90 percent • Time main out of service: One-hour maximum during each run • Results: 15 percent increase in flow
Dallastown Borough – Dallastown, Penn. Wastewater Pumping Station – December 2012
The first sewer force main in the U.S. to be cleaned by ice pigging was performed at Dallastown Borough, located in south central Pennsylvania (Figure 12). The borough was experiencing an underperforming wastewater pumping station and consulting engineers discussed capital upgrades to the pumping station to meet the current demands. After being introduced to the ice pigging technology, the borough agreed to an ice pigging cleaning project as one last attempt to put off any expensive capital improvements. No other options were considered because of the long disruption to service and cost of required enabling works. This wastewater force main project of 1,200 Continued on page 34 Florida Water Resources Journal • May 2022 33
Continued from page 33 linear feet of 4-inch-diameter unlined cast iron took approximately two hours to complete using 600 gallons of ice slurry. The ice was injected in two batches to allow a primary partial clean, followed by a secondary clean, which cleared out any remaining sediment. This was done to avoid any potential heavy buildup of sediment in the small 4-inch pipe.
The entire operation took just two hours and the ice pigging technology removed an obstruction in the main, increasing the pump flows by almost 30 percent. The borough could abandon the capital expenditure and put the money to good use elsewhere.
Statistics
Dallastown Wastewater Pumping Station
• Type of main: Wastewater sewer force main • Length of main: 1,200 feet • Diameter and material: 4-inch unlined cast iron pipe • Ice quantity for each (of two) runs: 300 gallons • Ice fraction: 80 percent • Time main out of service: 30-minute maximum during each run • Results: 30 percent increase in flow capacity
Town of Danbury – Stokes County, N.C. Distribution Network
The water system for the Town of Danbury is over 30 years old and is supplied by two wells, both having some iron and manganese that, over time, had resulted in a buildup on the interior lining of the system pipes. Regular customer complaints about discolored water made it necessary to search for a solution. Having limited water production capabilities and only 100,000 gallons of storage, flushing was not a viable option.
A number of calls were made looking for a company that had experience in pigging water lines. After some research, the Stokes County Public Works Department learned that ice pigging had many advantages over the moretraditional cleaning techniques, such as minimal interruption of service, up to 70 percent less water required, and no digging necessary. The department identified the need to clean 18,500 feet of 6-inch PVC potable water mains, with the aim of removing as much sediment and manganese matter as possible to improve water quality and reduce customer complaints of discolored water.
A desktop study was carried out using the water maps provided by the department to measure out the lengths of pipe to be cleaned in order to determine ice quantities and set out a proposed schedule of work. This was backed up by a detailed site survey to determine the suitable insertion/extraction points.
The objective of the project was to provide a service that was a sustainable best practice method of cleaning the water pipes using minimal amounts of water, providing the most- effective results, and with minimal disruption to the water supply for the client’s customers.
The project team consisted of three people, supervised by the ice pigging project manager, and the project equipment included a 10-ton ice delivery tanker (Figure 13), a 10-ton ice production unit powered by a portable diesel generator, and a Ford F-250 carrying a flow analysis system.
The project features included: S Existing hydrants used to insert and extract ice. S Existing fitting in a pressure-reducing valve (PRV) pit that was used for ice insertion on one run. S Entire project carried out in a total of four runs over two days. S Maximum supply interruption time was two hours on each run. S Ice samples were collected for further analysis. S Waste tanker was used to capture and dispose of the discharged ice.
Statistics
Town of Danbury Distribution Network
• Total length of mains cleaned: 18, 500 feet • Average time taken during run: 2 hours, 20 minutes • Average volume of water used: 1.6 x pipe volumes • Average amount of sediment removed: 87.6 pounds per mile of pipe
Summary
As of February 2022, over 1,000 miles of pipe in the U.S. have been cleaned using ice pigging across 43 states and over 410 projects (Figure 14).
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How much salt is used in ice pigging and what effect does it have on a wastewater treatment plant?
The process uses a brine solution with a salt percentage similar to seawater. The salt used as a freezing depressant is food-grade, NSF-approved table salt. The effect on a wastewater plant needs to be considered as the salt can harm the good bacteria used in the treatment process; it’s a simple matter of dilutions and a question of what quantities the plant takes in a typical day. Generally, the ice quantities are insignificant compared to the capacity of the treatment plant.
Is it effective on cast iron pipes that have heavy tuberculation?
The ice is effective on any pipe material. A certain amount of care is required when applying it to heavily tuberculated cast iron. The ice slurry is prepared with a lower ice fraction; therefore, it’s runnier and less aggressive. This allows the pig to give an effective clean, removing all the loose sediment, biofilm, and manganese buildup without breaking off too much of the tuberculation.
What pressure is required to push the ice through the main and will it require excessive force?
The ice flows through the main using the normal system flows and pressures; there will be no undue pressure applied to the main. Prior to ice insertion, the static pressure is tested so that the bar is set when inserting the ice. The operators have the experience and skill to control the pressure by adjusting and balancing the flows as they inject the ice. Is the equipment clean, or is there a risk of cross contamination?
The equipment is disinfected prior to every new project and at the end of every workweek. All hoses are disinfected, capped, and stowed in clean boxes ready for use. There is a separate set of hoses clearly marked for potable water and wastewater. No hose is ever used on a main for which it's not designated.
How is the discharge disposed?
Once the ice is delivered into the main, it becomes the property of the pipe owner; disposal will be according to the owner’s instructions. Public sewer is the preferred choice, but in the event of a sewer not being suitable or available, a waste disposal tanker can be arranged. A last resort would be to discharge to the ground, but only after written approval from the state is obtained by the customer.
How does it perform in the heat?
Extreme temperatures are not the ideal situation, although ice pigging can still be effective in these conditions. Ice quantities would normally be increased to allow for the expected higher water temperature. This may add to the cost to the customer, so the work could be done in the cooler periods of spring, fall, or winter.
Will the cold ice cause the main to break?
No. Tests were conducted on an exposed pipe that was ice pigged in the usual manner. Strain gauges and temperature sensors showed no undue stress on the main at all when the ice passed through the main.
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Figure 14. Map of the United States shows the green-shaded states where ice pigging has been performed. Continued from page 34
Pipes ranging from 2 to 42 inches in diameter have been cleaned and the maximum length cleaned in one pass in the U.S. to date is 2.7 miles on an 8-inch PVC main in Sutcliffe, Nev.
Ice pigging is currently being used on potable water, raw water, and sewer force mains, and sewer siphons, all with successful results.
Ice pigging is also being adopted as a cost-effective method of pipe cleaning in many countries around the world. The experience gained has shown that the technology offers an opportunity to make real cost savings by reducing energy bills. More importantly, large capital expenditures on new pumps, pipelines, and structures can be avoided with a system that provides the owner with a rapid, environmentally friendly, and effective solution with exceptionally low risk.
Acknowledgments
Figure 10 is used with permission of Aldrich + Elliott, PC Water Resource Engineers.
Paul Treloar is head of business development at American Pipeline Solutions in Merritt Island. S
