Research has Established that Common Drinking Water Disinfectants Degrade Polyethylene (HDPE & MDPE) Pipe: Research dating back to the 1950's documents oxidation of polyethylene polymers1,2,3 It is understood that the amount and type of oxidants, that are added to disinfect drinking water, attack polyethylene pipes and can cause premature aging – and failure. Reports of HDPE water pipe failures have linked the failures to oxidative degradation. The two largest water utilities in the world, Veolia Environnement and Suez Environnement experienced a rash of HDPE service line failures – which were initially thought to be ductile in nature (warmer soils can soften plastic pipe causing bursts). Upon closer examination – the failures were brittle in nature – indicating chemically induced embrittlement. Veolia and Suez spent millions of dollars and eventually built a pressure testing laboratory that could simultaneously test approximately 400 samples under various pressures, temperatures and disinfectant regimes. They also exhumed over 200 samples of in-service HDPE pipe from around the world (including North America). Analysis of the laboratory testing and subsequent correlation with the forensic examination of field samples has been published in respected peer review journals.4,5,6,7 Veolia's and Suez's findings are consistent with others in the world of polymer research. It is now readily accepted that HDPE pipes are subject to degradation and therefore pose a threat for possible premature failure in water disinfectant environments – particularly in warmer environments. 8,9,10,11,12,26
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Failed HDPE Water Line, France– Suez Environnement 2009 | 6” Failed HDPE Water Line, USA – Duvall, Edwards 2010 |
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Edwards, et al. Failure Analysis of Polypropylene Used in Hot Water Environments – Effect of Different Stabilizer Systems, ANTEC 2007. |
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Rozental-Evesque, et al. A Reliable Bench Testing for Benchmarking Oxidation Resistance of Polyethylene in Disinfected Water Environments, Plastic Pipes XIV, 2008. |
Further Forensic Analysis of Field Exhumed Pipe in the United States (US) Has Confirmed HDPE Pipe Oxidation: In 2007, Jana Laboratories (Toronto) published a paper for the ANTEC plastics conference that examined a number of exhumed HDPE pipe samples and reported, “..it would appear that the failures are generally consistent with the Mode 3 Oxidative Initiation-Mechanical Propagation type of failure”.15 In 2009, Engineering Systems Inc., led by Dr. Donald Duvall (a noted polymer expert and former technical manager of one of the largest HDPE pipe manufacturers) conducted a study that examined over 50 HDPE pipe samples from 13 US water utilities which found varying degrees of oxidation induced degradation in the samples (23 were from failure sites).13, 14 Failures which are suspected to be related to oxidation have occurred and continue to occur across the US. Las Vegas Valley Water District is in the process of replacing 86,000 HDPE service lines due to pre-mature aging and failure.16 HDPE service line failures in cities such as Mesquite, NV; Pomona, CA; Henderson, NV; Bakersfield, CA; Maui, HI; Hamilton, OH; Laughlin, NV; HB & TS, TN have all been linked to oxidative aging in potable water service.13,14
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Scott, Charles, Forecasting Pipe Replacements Using Weibull Analysis, Society for Maintenance and Reliability, Annual Meeting, 2007. |
Service Temperature and Type of Disinfectant Influence the Degradation Phenomenon: The problem of oxidative degradation in HDPE pipes has been particularly severe in areas with higher water temperatures that use either chlorine dioxide disinfectant or chlorine (hypo-chlorite) disinfectant.6 Accordingly, some HDPE pipe manufacturers have modified their warranties to limit coverage in oxidative environments.17 The Australian trade association, PIPA (Plastics Industry Pipe Association of Australia), printed an advisory in 2010 that stated, “Especially at service temperatures above 20°C, chlorine dioxide will shorten the service life of polyethylene pipes. For this reason chlorine dioxide water disinfection should not be used with polyethylene, polypropylene or polybutylene (i.e. polyolefin) pipes.” 18
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Duvall, et al. Oxidative Degradation of High Density Polyethylene Pipes from Exposure to Drinking Water Disinfectants, 2009. |
Appropriate Design Guidance is Lacking, but Needed: There is a real need for practical, quantitative guidelines for prospective HDPE pipe users and their design engineers. Outside of some very general warnings, the only guidance currently being provided by the manufacturers of HDPE pipes is in the form of very general chemical compatibility charts that do not consider pressurized environments or long term exposure and express only vague warnings on unspecified “pressure reduction” or “service life reduction”.20,21,22 A recent JANA Laboratories paper, sponsored by the polyethylene pipe trade association – Plastics Pipe Institute, looked at some chlorine and chloramine (did not include chlorine dioxide) disinfected waters and reported that,
“… a methodology has been developed for characterization of the Mode 3 (oxidation) long-term aging mechanism. This methodology for forecasting long-term aging for this mechanism shows that performance is a function of the water quality, water temperature, and operating stress and varies by utility.” 23
However, the quantitative details for applying the methodology are not disclosed and are not available for use by designers. Certainly, one of the greatest challenges in applying any methodology, once released, will be the difficulty in being able to accurately anticipate all of the potential changes in water quality, temperature and stress that could take place in the coming 50 to 100 years for any system.
Finally, Carollo Engineers reviewed the existing literature in 2007 and their investigations also found that premature aging of HDPE in potable water lines was a failure mode which was not well understood in the US, but should be addressed by the relevant pipe standards.19 Carollo found that HDPE oxidation in water disinfectant environments is a problem not currently addressed by the HDPE pipe standards. ASTM and AWWA are silent on guidance for engineers and owners who require guidance in the area of prevention of premature aging of HDPE pipe.
In Summary –Polyolefin Materials such as Polyethylene (HDPE) are Inherently Susceptible to Oxidation by Chlorine Based Water Disinfectants. Pipe Industry Design Guidance to Account for Factors that Impact the Rate of Oxidation Induced Aging such as Temperature, Disinfectant Concentration, Pressures in Isolation or more Importantly in Combination Do Not Exist:
Polyethylene is a polyolefin family polymer – just like polypropylene and polybutylene. As a polymer family, polyolefins are more susceptible to oxidation through a free radical mechanism than other plastics. Anti-oxidants are added in polyethylene pipe formulations to extend their service lives (the anti-oxidants are preferentially attacked by the free-radicals and “sacrificed” – as long as they are available in the pipe). However, premature failures of HDPE water pipes have demonstrated that under some operating conditions the antioxidants are depleted and serious degradation of the pipe occurs. This is why PVC, not HDPE, pipes are typically used for swimming pool piping and manifolds where chlorine contact with plastic is required. Polyolefin pipes are not recommended for service in highly oxidative environments, which includes some drinking water systems. The idea that polyolefin pipe materials are subject to oxidation induced failure is not new to the pipe industry. In fact, the $1.1 billion Polybutylene Pipe Settlement Fund was the result of a legal class action that centered around the susceptibility of polybutylene pipes to fail in home plumbing systems where warm chlorinated water contributed to premature embrittlement and cracking.25 Another class action settlement featured polypropylene water heater “dip-tubes” where polypropylene dip tubes failed prematurely due to oxidative attack and resulting embrittlement. 24 While the polyethylene pipe industry has sponsored a study that positions “100 year life” as expected for polyethylene pipelines in water disinfectant environments (chlorine, chloramines – not chlorine dioxide), it remains the only study that comes to that conclusion. 23 On the other hand, the vast majority of research studies (both exhumed pipe forensics and controlled laboratory studies) point to premature oxidation induced aging as an important failure mode in polyethylene pipe systems. Despite this set of facts, pipeline designers still have no meaningful guidance from third party sources that can reconcile the various factors (pressure, temperature, disinfectant type, disinfectant concentration) which have been shown to affect HDPE pipe degradation in oxidative potable water environments.
End Notes: