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Is High-density Polyethylene (HDPE) a Good Choice For Potable Water?


Rapid Response Question: Metal pipe and plumbing materials have historically had issues with corrosion, sediment build-up, pressure resistance, thermal conductivity, and chemical resistance. Cross-linked polyethylene (PEX) piping is one alternative, but the available sizes are too small for major commercial installations. Is high-density polyethylene (HDPE) pipe a good environmental choice for potable water applications?

Initial request by: Sellen Construction (2012).

Updated June 2015.

* DISCLAIMER: PPRC does not endorse any specific products or manufacturers mentioned herein.

Background

HDPE-Pipes stackPlastic pipes and tubes are widely used to convey gases and liquids of all types. Plastics may be preferred over metal due to inherent advantages. They are lighter weight, do not require open flame to join, and they’re flexible, which can simplify installation and reduce breaks due to freezing.

Plastics are typically lower in cost, and resist the corrosion and scaling that plague metals in some applications.

Plastics used to contain food and water are receiving heightened scrutiny due to concerns of chemical contaminant migration. Much recent research and media has focused on endocrine disrupting chemicals such as bisphenol-a, found in polycarbonate baby bottles, and on phthalates, found in vinyl toys and other products.

Plastic Pipe Resins

In potable water applications, plastics have created some controversy. Polybutylene plumbing materials, introduced in the 1970s, led to unacceptable leakage problems culminating in a large class-action lawsuit. Polyvinyl chloride (PVC) and chlorinated-PVC (CPVC) pipe are common. However, some environmental groups have suggested that the risks associated with PVC production and pipe disposal outweigh the advantages of these materials (1).[1]

Cross-linked polyethylene (PEX) has been used since the 1980’s for radiant heating systems, and has become popular for potable water in recent years. PEX or PEX-lined pipe has wide code acceptance across the country, but PEX requires special fittings and is not recyclable. The chemical crosslinking required to produce PEX adds expense, and increases the potential for contaminant migration from plastic to water. For example, when PEX piping is used underground, the piping can come in contact with ground water. During the California state code approval process, Reid (in 2005) provided testimony that in areas where groundwater has been contaminated by petroleum products, the gasoline additive methyl tertiary butyl ether (MTBE), or pesticides, may permeate through the PEX pipe (2). he final environmental impact report suggests that while chemical migration is an issue, contaminant levels rapidly decline over time to safe levels. Opponents argued for more thorough testing of polymer formulations and chemical leachates (3).

High-Density Polyethylene (HDPE) pipe has been used for decades in non-potable water applications. In particular, HDPE pipes are often preferred for their welded joints[2]. While special equipment is required to form the weld, welding eliminates the need for separate fittings, a common source of leaks and contaminant infiltration. HDPE is very flexible and can endure harsher site handling than more brittle polymers like PVC. Flexibility also allows turns in the piping system without the need for additional joints.

For potable water, HDPE was initially limited to cold water service applications, as early formulations were not strong enough for the high temperatures of hot water systems. Suppliers then developed cross-linked polyethylene (PEX) with superior strength and high temperature performance. PEX is common in radiant floor heating applications and, increasingly, in domestic hot/cold water systems. But, as noted above in the initial question, the available pipe sizes are too small for larger commercial installations. Both HDPE and PEX are polyethylene (PE), but because of their different properties care should be taken to not confuse these two very different materials.

HDPE can be used for hot water as a liner in multilayer pipe, where the strength is provided by another pipe layer, such as aluminum, but multilayer pipes don’t offer all of the performance advantages of plastic alone. Over the past decade or so, new HDPE formulations, for example Dow’s PE-RT (polyethylene of raised temperature resistance), have been available for high temperature use, including domestic hot water.

Do Chemicals Migrate or Leach from HDPE Pipe in Presence of Potable Water?

All plastics contain some residual of the chemicals required for their manufacture. These may include one or more catalysts that assist the polymerization reaction, as well as traces of unreacted raw material. A number of additives are typically compounded along with polymer resin prior to forming the final product. These may include stabilizers, UV-blockers, plasticizers, antioxidants, colorants, etc., to enhance both processing and performance characteristics (4). These additives may not be disclosed by the company producing the piping, so the risk of chemical migration must be evaluated for any material that comes into contact with potable water, food, or beverages.

When chemical contamination occurs, it is usually due to migration of these non-polymer additives, or possibly from residuals of the manufacturing and installation procedures. For example, cutting of the pipe may leave some dust or particles inside the pipe, however most of this is flushed out after installation and before first use for drinking.

Below are annotations from several independent research reports relating to HDPE resin and HDPE pipe and chemical migration, bacterial growth, or permeation in potable water applications.

At first, research in contaminant migration from plastic pipes seemed to focus on taste and odor issues, rather than chemical hazards from plastic pipe (5). In addressing these sensory characteristics, a 2003 study by Skjevrak, identified a wide variety of leachates from HDPE pipe.[3] Odors associated with these leachates were above acceptable levels set by USEPA’s non-mandatory quality standards (6).

Skjevrak found that the dominant source of contamination was likely due to breakdown-products of common polymer antioxidants. While these contaminants are not of remarkable toxicity, a host of other minor contaminants were identified, including benzene and xylene. Similar aromatic hydrocarbon contaminants occurred in all samples, but at sub-part-per-billion levels, well below the maximum contaminant levels set by the USEPA for safe drinking water (7).

A number of studies on chemical migration from HDPE to water are reviewed in 2005-6 research by Monique Durand (8; 9). Durand states that chemicals identified as contributing to taste and odor “originated from 1) alteration or degradation products, produced from original additives during the extrusion step (200-250 C) in the pipe manufacturing process and 2) compounds were by-products or impurities resulting from synthesis of pure phenolic additives.” The phenolic additives mentioned are, again, common polymer antioxidants. Chlorinated water and high temperature seem to accelerate leaching. Over time, chlorine can break down polymer antioxidants leaving the pipe more vulnerable to chemical attack. To reduce migration, manufacturers have investigated methods of binding antioxidants to the polymer matrix and reducing impurities in the antioxidant additives.

Durand suggests that of the common plastics, HDPE generated more intense odors than PEX or CPVC plastic material. CPVC and copper were the least odor-causing materials. Durand also reports that organic leachates were low in CPVC and HDPE and somewhat higher in some PEX materials. No claims were made about health risks associated with these leachates.

In addition to contamination from leaching processes, chemicals can enter drinking water by permeation of contaminants through the pipe wall from contaminated soil around the piping (3). In most cases, permeation problems have involved plastic materials and diesel or petroleum-product contaminated soils in industrial areas, for example, near a gasoline station. Plastics should be avoided where soil contamination by organic liquids is likely (10).

According to a 2011 study by Yang, et al., which conducted laboratory extraction on many different plastic resins and products, most plastic resins showed detectable levels of estrogenically active (EA) compounds (11). This test was not conducted on plastic pipe, but HDPE product. This testing shows that the EA compounds are present in most plastics, including HDPE, however it does not prove that any of these compounds migrate during normal use (such as plumbing applications) as compared to lab extraction with EtOH or saline.

Researchers under the first phase of a three-year National Science Foundation project, conducted by the Whelton Group at Purdue University and others, are conducting field tests of PEX and HDPE and other plumbing. Findings, including HDPE chemical leaching and potential for bacterial growth, were presented at the American Water Works Association annual conference in Boston, Massachusetts, in 2014. Whelton has conducted previous research on chemical contamination in plumbing pipes after oil or other fluid leaks have contaminated drinking water. At this time, the Whelton Group research papers were not found online. Visit the Whelton Group website (12) for additional background and contact information.

A 2012 assessment report, prepared for the HDPE Municipal Advisory Board of the Plastics Pipe Institute, presents an engineering methodology for calculating the benzene, toluene, ethylbenzene, and xylene (BTEX) permeation through HDPE water pipe, based on several lab experiments for HDPE pipe of one inch wall thickness. The report sought to develop a methodology to be able to predict the level of BTEX from contaminated soil surrounding the pipe that could permeate HDPE pipe and potentially contaminate a water supply. One conclusion may be of interest to engineers designing water pipe material systems. The researchers surmise that water flow in HDPE pipes significantly reduces BTEX contamination, to safe drinking levels. The report states: “The example calculations show that the presence of BTEX contamination in soil along an HDPE water pipe does not necessarily mean that the drinking water in the pipe will exceed regulatory limits” (13).

While there have been lab and field studies on HDPE pipe, a review by Stern and Lagos points out the complexity of risk assessment of plastic water pipes (14). Which chemicals migrate from any given pipe depend not only on chemical formulation, but also on pipe material characteristics, and possibly even the surrounding ground or fill material around the installed pipe. Plastic formulations can vary from supplier to supplier and over time. Migration may change with water quality and usage environment. Little is known about some contaminants, while others are known to be harmful, especially to vulnerable populations. Considering such dynamic conditions, assuring safety is a challenge.

The Role of Regulation and Third-Party Certification

Under the Safe Drinking Water Act, the United States Environmental Protection Agency (USEPA) establishes regulations for contaminant levels in drinking water distribution systems. These standards mainly address water quality where it enters the distribution system and do not address changes in quality from contamination downstream, such as in building plumbing. Drinking water standards include a long list of contaminants and their maximum allowable level (maximum contaminant level, or MCL) for drinking water.

Building water system components are predominantly governed by local codes. Many code agencies rely on third-party certification, especially ANSI/NSF Standard 61, as a minimum requirement for safety of materials that come into contact with drinking water. NSF 14 is another certification particular to plastic pipes. These standards are widely recognized and at least 36 states have adopted them as requirements for residential plumbing. The Uniform Plumbing Code requires that plastic materials for potable water meet both ANSI/NSF 14 and 61 (15).

Certification provides a basic level of protection from chemical migration. For NSF 61 approval, plumbing materials are placed in contact with various test water samples (typically a three-week exposure), including waters with typical post-disinfection chemical characteristics and a range of acidity levels to simulate different “in-use” conditions (16). The contact water is then tested for the presence of over 300 chemicals and compared with “safe” levels.[4] Unfortunately, the list of contaminants monitored is not widely available, so purchasers have only a yes/no certification result, with little additional information to allay their concerns.

NSF certification is not accepted by all as sufficient protection. In California, a prolonged battle occurred over whether the state should approve PEX tubing in the state building code. Opponents suggested that there were chemical migration concerns not fully addressed by the NSF 61 certification process (17).

European governments and safety agencies have a patchwork of regulations governing water quality. Most of these address issues of chemical migration and require some type of certification testing. According to manufacturers’ promotional material, some standard and high-temperature HDPE formulations have been approved for potable water applications across Europe (18).

 

Conclusions

By and large, HDPE is reported to be one of the “good” plastics, safe for use with food and water. A common plastics memory aid can be found in various sources: “One, four, five and two, all of these are good for you.” This rhyme refers to the recycle code numbers found on plastic containers, where one is PET, two is high-density polyethylene (HDPE), four is low-density polyethylene (LDPE) and five is polypropylene (PP). PPRC found no evidence of any widespread health problems related to the use of HDPE in food and beverage or potable water applications.

Independent studies have shown that organic contaminants leach from HDPE pipe into water. While contaminant levels are likely “safe” by USEPA drinking water standards, there are some who will doubt the safety of any level of contamination. The chemical exposure risks cannot be fully elucidated, given the complexity of material types, changing formulations, and varying application circumstances.

While commodity costs have driven contractors toward increased plastic use, states and municipalities have been slow to add new material types to their building codes. CPVC was added to California code only in 2007 and PEX in 2009. HDPE pipes are widely approved for use in potable cold water applications in Europe and the United States. Additional approvals for some high-temperature formulations have satisfied the standard chemical migration requirements of ANSI/NSF 61 and some European standards organizations. Unfortunately, a search of NSF listings suggests that only two hot-water HDPE formulations have been approved to date, and code acceptance of high-temperature applications may take some time in the US.

Several green building organizations recommend polyethylene as a good alternative to other piping materials, though this is probably due more to the desire to eliminate PVC (19; 20) than to the proven safety of polyethylene. Industry groups have also highlighted the favorable life-cycle impacts of plastic versus copper and the high potential for recycled material sourcing for some piping applications (21).

There are pros and cons to any choice of piping materials, but it seems likely that the risk of chemical contamination from copper piping is lower and better understood than for plastic pipe. On the other hand, plastic offers reduced cost, lower flow resistance, and reduced breaks and leaks. For anyone anticipating use of HDPE for potable water, it is critical to choose materials that have passed ANSI/NSF 14 and 61 certification requirements. Furthermore, it seems reasonable to question the vendor regarding potential for chemical migration, odor and taste issues, and high-temperature performance. For example, new installations may require special flushing protocols to be sure contaminants are below USEPA MCL thresholds.

Regardless of pipe material choice, a significant risk of contamination still exists from some plumbing fixture types (22). Furthermore, due to on-going and legacy pollution, introduced chemicals, pharmaceuticals, pesticides, fertilizers, industrial chemicals, etc., are returning to us through surface water (23). As a result, drinking water may be contaminated prior to building supply and distribution. Plumbing materials may simply add another source to this ongoing contamination.

Key Findings

  • HDPE is widely approved by both standards organizations and code agencies for potable cold water applications. High-temperature HDPE formulations have been widely used in Europe for some time, but there are only a few materials ANSI/NSF certified for domestic hot water in the United States.
  • Independent research studies have identified that chemical contaminants do migrate from HDPE pipe materials to water, and can permeate certain plastic pipes when in contact with contaminated soil. However, these studies are inconclusive as to human health impacts of these contaminants.
  • Those anticipating the use of HDPE, especially in hot water applications, should ask vendors for data and certifications regarding chemical migration, taste and odor, and high-temperature performance.
  • Those most concerned about chemical contamination may prefer to forgo the use of plastics entirely, but plastic piping offers some significant installation, use, cost and environmental advantages over copper.

 

Sources

  1. Thornton, Joe. Environmental Impacts of Polyvinyl Chloride Building Materials. net. [Online] 2002. http://www.healthybuilding.net/pvc/Thornton_Enviro_Impacts_of_PVC.pdf.
  2. 2005. “Re: Comments on California Department of Housing and Community Development Consideration of the Use of PEX as Potable Water Pipe.” Letter to Thomas Enslow. [Online] Retrieved from http://www.documents.dgs.ca.gov/bsc/pex/exhibit_b_reid_pex.pdf.
  3. Final Environmental Impact Report: Adoption of Statewide Regulations Allowing the Use of PEX Tubing. [Online] January 2009. www.documents.dgs.ca.gov/bsc/pex/…/PEX%20FEIR_01-08-09.pdf.
  4. Science News. Plastic Water Pipes Affect Odor And Taste Of Drinking Water. Science News. [Online] August 28, 2007. http://www.sciencedaily.com/releases/2007/08/070823141100.htm.
  5. United States Environmental Protection Agency. Secondary Nuisance Chemicals: Guidelines for Nuisance Chemicals. Ground Water & Drinking Water. [Online] http://www.epa.gov/ogwdw000/consumer/2ndstandards.html.
  6. Basic Information about Benzene in Drinking Water. Drinking Water Contaminants. [Online] http://www.epa.gov/ogwdw000/contaminants/basicinformation/benzene.html.
  7. Durand, Monique. Disinfectants and Plumbing Materials: Effects on Sensory and Chemical Characteristics of Drinking Water. Virginia Polytechnic Institute. [Online] November 16, 2005. http://www.scholar.lib.vt.edu/theses/available/etd…/ThesisMoniqueDurand2.pdf.
  8. Durand, M and A.M., Dietrich. Water Quality Changes Associated with New & Standard Domestic Distribution System Piping Materials. Florida Water Resources Journal. [Online] December 2006. http://www.fwrj.com/TechArticle06/1206FWRJtech4.pdf.
  9. Office of Ground Water and Drinking Water. Permeation and Leaching. United States Environmental Protection Agency. [Online] August 2002. http://www.epa.gov/SAFEWATER/disinfection/tcr/pdfs/whitepaper_tcr_permation-leaching.pdf.
  10. Yang et al. 2011. Most Plastic Products Release Estrogenic Chemicals: A Potential Health Problem That Can Be Solved. Environmental Health Perspectives 119(7): 989-996. [Online].   http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3222987/
  11. Whelton Group. 2014. PEX Pipe, West Virginia MCHM Plumbing System Interaction Results [Online]. http://wheltongroup.org/?p=4447
  12. 2012. Assessment and Calculation of BTEX Permeation Through HDPE Water Pipe – Final Report. Prepared for: HDPE Municipal Advisory Board, Plastics Pipe Institute®. [Online]
  13. Plastics Pipe Institute. Handbook of PE Pipe, Second Edition. [Online] https://plasticpipe.org/pdf/permeation-report.pdf
  14. Stern, B.R. and Lagos, G. Are There Health Risks from the Migration of Chemical Substances from Plastic Pipes into Drinking Water? A Review. Human and Ecological Risk Assessment. 2008, Vol. 14, 4.
  15. Brown, Jeremy. Personal communication. NSF International. February 2010.
  16. Water Science and Technology Board – National Research Council. Drinking Water Distribution Systems: Assessing and Reducing Risks. National Academies Press. [Online] 2006. http://books.nap.edu/openbook.php?record_id=11728&page=R1.
  17. California Building Standards Commission. PEX (Cross-linked Polyethylene) CEQA Review. [Online] http://www.bsc.ca.gov/pex.htm.
  18. Dow Chemical Company. DOWLEX PE-RT. Plastic Pipes Europe, Middle East & Africa. [Online] http://www.dow.com/plasticpipes/cert/dowlex.html
  19. Build It Green. HDPE (High Density Polyethylene Pipe). org. [Online] http://www.builditgreen.org/attachments/wysiwyg/22/HDPE-Pipe.pdf.
  20. Harvey, Jamie and Lent, Tom. PVC-Free Pipe Purchasers’ Report. net. [Online] http://www.healthybuilding.net/pvc/pipes_report.pdf.
  21. ppfahome.org. [Online] http://www.ppfahome.org/greenbuilding/index.html.
  22. Raloff, Janet. Faucets Destined for Brassy Changes. Science News. [Online] 31 October, 2008. http://www.sciencenews.org/view/generic/id/38233/title/Faucets_Destined_for_Brassy_Changes.
  23. Environmental Working Group. Over 300 Pollutants in US Tap Water. org. [Online] http://www.ewg.org/tap-water/home.

 

[1] Standards organizations: ANSI – American National Standards Institute, NSF – National Sanitary Foundation

[2]The vinyl chloride monomer used to produce PVC is a carcinogen. Production of vinyl chloride and incineration of waste containing PVC lead to the release of dioxin and other toxins.

[3]Welding is usually done by electric heating, aka electrofusion, though some codes may require fittings or other methods.

[4]The specific formulations used in these Norwegian tests are not known. Presumably European formulations are standardized to some degree, as in the United States.

[5]For example, with the USEPA listed organic contaminants, the ANSI/NSF 61 standard requires that contaminant levels be no more than one-tenth of the maximum level allowed in water by the USEPA or other regulating authorities.

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