Quantifying the effects of anthropogenic heat sources on the water temperature in the drinking water distribution system

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The temperature of drinking water is an important parameter, affecting physical, chemical, biological and aesthetic properties of the water. Currently, measurements exceeding the legal limit of 25ᵒC at the tap are exceptional. However, there are reasons to believe that the number of exceedances will increase due to climate change and urbanization. So far the effects of sunlight hours and soil coverage on drinking water temperature have already been quantified quite well. Still lacking is the exact effect of anthropogenic heat sources on these temperatures. In this study more insight is given in the extent of this effect for Rotterdam, and measures are discussed to mitigate or to prevent drinking water warming. First, potential anthropogenic heat sources are identified. By analyzing data on tap temperatures and old soil temperature measurements, several anthropogenic heat sources emerged. A map of the statistically warm drinking water temperatures in Rotterdam, appears to have a lot of resemblance to the city heating system whereabouts. Furthermore, smaller indications point at high voltage power cables and metro lines to be important anthropogenic heat sources. Soil temperature measurements around these potential heat sources confirm some of the expectations. Around high voltage power cables, a limited elevated temperature of 0.6ᵒC is found. Much more alarming results were encountered around the city heating mains. In autumn, primary heating mains warm-up the soil by 4.9 – 5.3ᵒC and secondary heating mains do so by 2.9ᵒC. A temperature elevation of 1ᵒC is measured to a distance of 3.8 - 5.25m and 2.3m, respectively. However, these results are highly dependent of several parameters, like the temperature differences between the air and soil, the thermal properties of the soil, and the depth and state of the anthropogenic heat sources themselves. Several strategies to mitigate the warming of drinking water in the distribution system are applicable. In a general sense, it is most sensible to create a distance between any heat source and the distribution system. By placing drinking water mains in the shade, below grass and away from anthropogenic heat sources, warming of the drinking water can be prevented in a relatively inexpensive manner. Measures targeted at temporary local hotspots can be focused on restraining the heat flux from the heat source to the soil, the heat flux through the soil, and the heat flux from the soil to the drinking water.