Effect of temperature on the growth of opportunistic pathogens

This project is investigating how the heating up of drinking water affects the growth of opportunistic pathogens in the mains system. We are using a pilot array that simulates the normal conditions in the system. Pathogens are added to the drinking water in that array and the water temperature is set and maintained at various levels. The next step is to look at how well the pathogens grow. The information from this study will help drinking water utilities to answer the question of the extent to which higher drinking water temperatures and exceedances of the 25°C standard lead to undesirable risks with opportunistic pathogens. On the basis of the results of this study, the utilities can determine whether measures will be needed to mitigate the risks and they can engage in discussions with other stakeholders and government authorities.

Climate change is pushing up the temperature of drinking water in the mains system. For example, in warm years such as 2018, the 25°C standard for drinking water was exceeded much more often than in cooler years. In addition, drinking water temperatures are expected to rise further in the years ahead due to climate change and the energy transition. The energy transition means, for example, that drinking water utilities will get questions more and more often from a range of organisations and agencies about the structuring of the subsurface: they want to install underground heating networks, for example, or other hot spots are being created, and drinking water utilities are being asked how much drinking water could heat up as a result. All these factors are expected to result in even higher temperatures in drinking water systems in the future.

Opportunistic pathogens in the mains network

The potential risk associated with these higher temperatures is that opportunistic pathogens could multiply in the distribution system and affect public health. With more knowledge in this domain, the drinking water utilities themselves can justify decisions to maintain the statutory legal 25°C threshold, or not, and whether to enforce it. They will also learn more about the effect of temperature on the risk of propagation of opportunistic pathogens in the mains system.

Warmer drinking water may result in a risk of the growth of opportunistic pathogens

The effect of a constant drinking water temperature (15 to 30°C) on the growth of opportunistic pathogens in semi-stagnant conditions has been investigated in the past as part of the Joint Research Programme of KWR and the water utilities (BTO) (BTO 2020.036). That research demonstrated that, as drinking water temperatures rise, the numbers of Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Mycobacterium kansasii and Aspergillus fumigatus increased in the biofilm. It was therefore concluded that rising drinking water temperatures, particularly above 25°C, could exacerbate health risks. However, that study was conducted on the basis of a worst-case scenario with optimal growth conditions for the opportunistic pathogens. Nevertheless, conditions in the mains network, including the pipe material, temperature and hydraulics, are normally different. It is therefore still unclear to what extent higher temperatures will result in the growth of opportunistic pathogens under normal conditions in the mains system.

Testing the effect of drinking water conditions on the growth of opportunistic pathogens

The first step in this project will be to conduct an inventory of the drinking water temperatures in the mains network over the past ten years, and variations during a day and in different seasons. The conditions for the experimental tests will be selected on the basis of these data and the results from temperature models. These experiments will determine the effect of temperature, pipeline material, flow rates and type of drinking water (biologically stable v. less stable, sourced from groundwater and/or surface water) on the growth of five opportunistic pathogens (A. fumigatus, S. maltophilia, M. kansasii, P. aeruginosa, L. Pneumophila) and L. anisa (belongs to Legionella spp.). A trial setup is being used for this purpose in which the conditions will resemble the normal conditions in the mains system more closely. These results will be subjected to a risk analysis in order to formulate their significance in terms of the public health risk associated with the heating up of drinking water.

Understanding risks, possible management measures and policy rationale

The aim of the project is to determine the impact of the heating up of drinking water – for example due to climate change and the energy transition – on the growth of opportunistic pathogens under normal conditions in the mains system. It also looks at the impact on the health risk for consumers. This will provide drinking water utilities with a picture of whether management measures will be needed in the future and, if so, which measures can be used to limit the growth of opportunistic pathogens in the drinking water system. The drinking water utilities can then use this knowledge for discussions with the government and other stakeholders about the maximum thermal impact of heating networks on the mains network and possible adjustments to the statutory standard for drinking water temperature