Drinking water biological stability in relation to temperature

The law establishes that drinking water temperature must be under 25°C. However, there aren’t any, or hardly any, measures available to actually keep the water temperature below this limit. It is expected that the temperature of drinking water will rise under the impact of climate change, urbanisation and the energy transition. Since it is not clear what the 25°C limit is based on, or what the possible future microbial risks are, the project’s first step is to examine the research already done on the subject (within and outside KWR), and to set it within the framework of possible short-term risks and developments.

The steps involved include conducting risk analyses based on close consultation between KWR and the water utilities. In addition, research will be done to discover at what temperature different species of microorganisms can grow in drinking water and in drinking water biofilm, and whether this presents a potential risk.

Drinking water warming

Drinking water in the Netherlands is distributed without post-disinfection (chlorine). This means that the microbial activity during the distribution must be kept at a sufficiently low level to prevent any hazards to public health. One of the conditions involved concerns the water temperature which, as stipulated by law, must be under 25°C. However, there aren’t any, or hardly any, operational measures available to actually keep the water temperature below this limit. The water temperature in the distribution network depends on a large number of factors, such as the weather, as well as surface and subsurface heat sources. During hot summers, tap water temperatures above 25°C are already being recorded on an incidental basis. The expectation is that the temperature of the drinking water will increase as a result of climate change, urbanisation and the energy transition (more heat networks, heat exchangers and electrical cables).

Microbial consequences of warmer drinking water

Microorganisms grow faster at higher temperatures. In theory, some microorganisms that are pathogenic (opportunistic pathogens) for humans with weakened immune systems can also start growing at higher temperatures. At the moment this hardly ever occurs, but it is important to know the temperature to which the drinking water can be safely warmed, without microbial problems arising.

Culture plates with bacteria and fungi; from left to right: Aspergillus fumigatus, Aeromonas, Legionella, Mycobacterium.

Research question

With this research we will discover whether, and at what temperature, different opportunistic pathogens can grow in drinking water and in drinking water biofilm.

In addition, all previous research conducted into this subject (within and outside KWR) will be examined, and then set within the framework of possible short-term risks and developments. This will involve, among others, carrying out risk analyses based on close consultation between KWR and the water utilities.

Applying knowledge in practice

By quantifying the possible microbial risks of supplying drinking water with a higher temperature, and acquiring a clear picture of where potentially hazardous situations will arise in the future, possible management strategies can be developed. For example, the avoidance of lengthy periods of warming in specific network sections, or the acceptance of temperature increase (temporary or not) up to a specific critical level or limited duration.