The objective of this project was to demonstrate the removal of biofilms and sediment buildup in drinking water distribution systems using a pipe cleaning technology known as ice pigging. Effective removal of biofilms will reduce the need for periodic water system flushing events, which waste significant quantities of water.
The main benefits of this technology stem from the water savings resulting from reduced flushing that is currently performed routinely to maintain water quality. Ice pigging technology is an innovative pigging technique that would be more readily acceptable to Department of Defense (DoD) utility and energy managers to implement into their routine Operation and Maintenance (O&M) protocols.
Ice pigging is a technology that combines the operational advantages of traditional flushing with the cleaning impact of soft pigging, with minimal interruption to water services during pigging. Ice Pigging overcomes operational limitations commonly found in traditional cleaning methods. A main feature of Ice Pigging is that it does not get stuck in the pipeline or appurtenances; if for some reasons the ice pig would get stuck, operators would allow the ice to melt and flush it out from the main. Pipe bends, changes in pipe diameters, or butterfly valves can all pose problems for swabbing or pigging, yet ice pigs can easily overcome these obstacles. To launch and receive traditional pigs, excavations have to be made to allow the installation of launch and reception stations. These can result in very costly and extensive interruptions to water system operations, and can require the installation of bypass pumping to provide water supply.
Demonstration results showed significant improvement to the operation of the water system post pigging. Sediments removed by ice pigging ranged from 8.3 to 81.8 lbs per mile of pipe cleaned whereas conventional hydraulic flushing did not remove any sediment. Residual chlorine did not change significantly before and after ice pigging and was steady at a level above 1.0 mg/L. Water used for hydraulic flushing was reduced from 5.5 million gallons per year before ice pigging to 2.3 million gallons per year post-ice pigging. Ice pigging can be financially justifiable at water rate of $0.008/gallon or higher due to this water saving. Sodium hypochlorite consumption for re-chlorination was reduced by 465.1 gallons per year post-ice pigging. There was no total trihalomethane (TTHM) violation after ice pigging, whereas four violations were recorded the year prior to ice pigging when only conventional hydraulic flushing was performed for system maintenance.
Results of the bacterial community analysis study in water distribution pipelines using genome sequencing analysis sequencing procedures showed that ice pigging removed entrenched biofilms and bacterial species highly resistant to the disinfectant, whereas conventional flushing only removed bacterial species closer to surface of biofilms in contact with water. Also, in pipelines with extreme levels of chlorine, highly resistant bacilli predominated, whereas, under less extreme conditions, proteobacteria formed the predominant species. Biofilms can exert chlorine demand and generate THMs, and hence the effective cleaning provided by ice pigging can help maintain distribution system water quality for a longer time horizon.
Table E-1 shows the comparison of conventional hydraulic flushing and ice pigging performance. Although the overall results are positive, they cannot all be attributed to ice pigging alone because the site has also implemented other improvement measures, such as the addition of a booster pump station to lower the water ages of parts of the distribution system, and a recirculation system to improve water circulation in the water storage tanks. It is a combination of the cleaning of water mains and the water age reduction measures that were undertaken that have resulted in the improvements.
Ice pigging technology has been commercialized, and SUEZ is the only company that can perform ice pigging as the sole licensee of the technology. Implementation is typically achieved through standard contracts to procure ice pigging services. Ice pigging is an effective water main cleaning technique that might help in improving the operation of DoD drinking water distribution system. Water systems may not need water main cleaning when routine conventional hydraulic flushing is sufficient to attain adequate disinfectant residuals within the water system as well as attaining compliance with Safe Drinking Water Act water quality standards. Water main cleaning by ice pigging would be a good option to consider when water systems require aggressive hydrant flushing in order to comply with water quality standards. It could have a cost saving effect from reduced water wastage for flushing if the water rate is $0.008 per gallon or higher or if water rates are expected to escalate rapidly due to droughts. For water systems where free chlorine is used for disinfection, typical water quality issues arise from the loss of disinfectant residuals, and from violations of the TTHM Maximum Contaminant Level requirement. For water systems using chloramine as disinfectant, nitrification (elevated nitrate/nitrite levels and low disinfectant residual) is the main water quality issue. Water systems experiencing stagnant water issues should implement other measures to reduce water age first because water main cleaning will not increase water demand or cause the water to move faster to reduce water age.
The risk of damage to water pipes is very low since ice pigging uses water systems’ own water pressure for pigging. A water distribution system planned for ice pigging should be in fair operating condition. Water mains meeting requirements for normal water distribution system operations are suitable for ice pigging.
Furthermore, based on lessons learned from this project, installations should consider the issues listed below when planning for ice pigging.