All drinking water pipes, in all water distribution pipes around the world will, over time, require cleaning due to potential buildup of sediment, biofilm, iron, manganese and trihalomethanes (THMs).
Although many forms of sediment may not initially cause any harm to the quality of the water or the end user (the general public), added to the fact that disinfection treatment can help to kill off any bad bacteria, nevertheless over time biofilms are likely to grow exponentially, forming a slimy substance within the pipe. In turn, THMs (which are a biproduct of the use of chlorine in potable water systems), may become an issue. Depending on the source of the water supply, manganese or iron may be quite prevalent in the water causing discolored water issues and complaints.
For most water companies and municipalities, the preferred solution to eradicate these issues, is flushing.
Flushing is a term used in the water sector, whereby, a water operator or technician will go from hydrant to hydrant systematically opening the valves one at a time and allow the water to discharge, usually to waste down a nearby storm water drain. The operator will do this for a certain amount of time, usually based on the flow rate, to allow a pre-determined amount of water to be discharged, in the hope that the velocity of the rushing water will be high enough to pick up any loose sediment that has built up in the line.
This is where the problem lies, because the simple fact is that water alone provides a relatively low shear on the inside of the pipe wall. In other words, the scouring power is poor and, therefore, the water must be allowed to travel at high velocity to increase the shear on the pipe wall. Quite often though, these velocities are not achievable due to several factors, including limited system pressures, tuberculation or scale build up restricting the flow, or, simply, insufficient volumes.
A technique called Uni-Directional Flushing, (UDF) is an enhanced means of flushing, often thought to be more effective than “standard” flushing. These UDF programs are designed to maximize the potential velocities and thereby, remove greater amounts of sediment from the mains. However, the fact remains, that water provides low shear (or scouring power) and so even the best thought out UDF plan is often insufficient to remove all the sediment from the system.
Even when the velocity in a main is at an acceptable level, there is then the (very serious) factor of water wastage. It is generally thought by water consultants around the world that at least four to five volumes of the pipe being cleaned, needs to be flushed out of the line before the flushing is at all effective. To put that number into perspective, that represents almost 69,000 gallons of water in just one mile of 8-in. water main (about 10 blocks), or 108,000 gallons (enough to fill five- to six-average size in-ground swimming pools), in a mile of 10-in. water main.
Many smaller communities or municipalities don’t even have the luxury of large storage reservoirs sufficient to provide the required amount of water and then in many states in the United States, severe droughts have led to heavy restrictions on the waste of precious water. In any case, ask yourself, “Why would you pay to collect water, pay to store it, pay to treat it and then just flush it away before it gets to the end customer?”
This is where a relatively new, advanced pipe-cleaning technique called ice pigging comes to the forefront.
Using Ice Pigging
Ice pigging uses a slush ice — or ice slurry — injected into the main, where it then flows through the line using normal system flows and pressures and is discharged out at a pre-determined point. As it flows through the main, the ice crystals pick up the sediment by scouring the pipe walls, carrying it in the body of the ice and out of the line at the discharge point, where it can be safely disposed.
Simply by turning water into ice crystals, you can achieve up to 1,000 times more shear (scour power) on the inside of the pipe wall than flushing plain water. The ice even has the ability to do this at much slower velocities than water, so for those situations where the desired velocity is not achievable, ice pigging will obtain a far superior clean.
Although it is inevitable that some water may be wasted during an ice pigging operation, it will be up to 70 percent less wastage than a similar flushing project and yet the cleaning efficacy will be so much superior.
Let’s Talk Money
Although ice pigging may initially appear to be more costly, a recent study carried out in Australia found that while ice pigging is more expensive on a ‘$ per foot’ basis, when using the indicator of ‘$ per lb. of sediment removed’, it is approximately six times cheaper. The study also found that ice pigging removed up to 17 times more sediment from the mains compared to flushing, while using half the amount of water.
It is hypothesized that the superior clean achieved by ice pigging will result in less discolored water complaints and, therefore, lead to cost savings in the reduction of unscheduled flushing programs and the frequency of mains cleaning of water supply zones (P. Dang et al., 2014).
Another finding from the study indicated that ice pigging a water supply zone improves penetration of chlorine residuals.
It should be noted that due to the success of this study, the water company involved commenced a full-scale program to ice pig approximately 250 miles of water mains each year for the following five years.
Benefits of Ice Pigging over Flushing
- Lower velocities required
- Smaller volumes required
- Superior clean
- Drought Friendly (Less waste)
- Shorter downtimes
- Cheaper when using the indicator of $ per lb. of sediment removed
Case Study to Explain a Bit
In 2022, a comparison test was carried out at an ice pigging project for a large Idaho city. This included certain ‘problem’ areas that were flushed by means of a UDF program followed the next day by ice pigging, with the aim of determining how much additional sediment was able to be removed. You can clearly see from the photo below of samples taken at the ice discharge hydrant, how much sediment was actually removed.
Note the ‘clear’ sample in each picture is a sample of the discharge before the ice arrives (essentially flushing water), then how the samples darken with the amount of sediment suspended in the ice slurry, a clear indication of the superior cleaning effect of the ice pig.