The Role of Hydraulic Models in Risk Assessment Analysis
Most hydraulic models offer a ‘water age’ option that allows users to calculate the residence time of water within the pipe network. Although this is a standard water quality indicator used in practice, it does not address the problem when we need to analyze the contact time with pipes of specific materials such as polyvinyl chloride (PVC).
Why is it important to analyze contact time with a material like PVC?
Vinyl chloride monomer (VCM) – a synthetic chemical that presents itself as a colorless and odorless gas at ambient temperature and pressure – is one of the reasons for the need for such analyses. Typically, VCM is fully polymerized into PVC, with only a very low content of residual monomers capable of migrating from pipes to drinking water. Vinyl chloride in water usually evaporates rapidly when it is near the surface and has a low solubility in water. However, VCM in its unchanged state is a carcinogenic substance to humans and is controlled in drinking water as well as food and air.
VCM is potentially released through PVC glue in pipes manufactured between the 1960s and 1980s subject to poor polymerization. Since dangerous levels of migrating substances can be exceeded for water that is in contact with the material for more than 48 hours, water officials would benefit from knowing the accumulated time of contact between the water and such pipes. Water flow through new PVC pipes usually adds very small amounts of vinyl chloride to the water and is not recognized to pose any risk. However, new pipes from other materials such as polyethylene (PE) can also cause migration of other problematic substances.
Despite approval plans in various European countries, many pipe materials are found to leak other unwanted substances. An example of this are some of the degradation products of the antioxidants used, many of which are thought to be endocrine disruptors (substances that can interfere with the human hormone system).
protection of public health and safety
The presence of hazardous substances migrating from drinking water installations, including PVC to VCM, depends on the manufacturing process as well as the quality of the raw materials. As a result, pipe quality varies according to suppliers and cannot be assumed. The European Commission’s Drinking Water Directive 98/83/CE specifies a limit value of 0.5 microgram vinyl chloride/liter in drinking water, which is the maximum level today for all EU member states.
In the WHO guidelines for drinking water, a threshold limit for VCM has been set and has been reduced over the last 25 years and today, the guideline value in drinking water is set at 0.3/l. However, for many substances, no maximum limit has been set, neither in the EU Directive nor in the national requirements. Most countries have quality standards for new installations but not for all relevant substances. With regards to older pipes, it most likely has never been rated.
A 2011 decree passed in France states that random sampling on the network should be carried out by the Agence Regional de Sainte (ARS – regional health agencies of France) to determine the level of VCM in water. When the level exceeds 0.5 µg/l, the network owner, ie the city, is required to replace its pipes and eliminate sanitary intoxication within three months. If this condition is not met, they can stop all water distribution.
To protect public health and safety, the ARS strongly recommends implementing a model to determine the exposure of VCM in drinking water, especially for cities where a carcinogenic molecule has been detected.
A new computational method for analyzing cumulative contact times
To analyze the contact time with pipe of a specific material such as PVC, a new computational method was introduced that specifically supports residence time accounting for plastic pipe. The simulation results are shown in the example below where the pipe is color-coded, corresponding to the time the water was in contact with the PVC pipe.
Pipes marked as green/blue are pipes where the contact time with PVC material is less than 48 hours. Pipes in orange/red are pipes where there is a risk of unhealthy concentration of VCM released from the PVC pipes. © DHI
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About the Author
Senior Project Manager and Hydraulic Engineer, DHI Czech Republic
Petr is a water supply system specialist at DHI with over 20 years of professional experience. Throughout his career, Petr has focused on the development and application of hydraulic models to support water supply and distribution. He has managed several global projects and provided technical direction and troubleshooting on various modeling efforts.
Project Engineer, DHI France
Holding a double engineering degree between France and Tunisa, Murtada is a subject matter expert on the modeling of drinking water supply networks. He is particularly knowledgeable about the risks of vinyl chloride monomer (VCM) and has prior experience in roles in relation to water, the environment and hydraulics.
Lise M Moller
Senior Scientist, DHI Denmark
Lise is specialized in exposure and toxic risk assessment of chemicals in industrial products and consumer goods. His expertise also includes endocrine disruptors, environmental risk assessment of chemicals and development of environmental quality standards. Liss has over 15 years of experience in assessing the health effects of polymeric materials exposed to drinking water.