Drinking water surfacing from the ground usually surfaces at a chilly 8° C, but by the time it has reached the tap, water around the city can be as warm as 16 to 18° C making it unpleasing to the palate and potentially unhealthy.
Water from the cold tap preferably should not climb over 12° C, said Solveg Nilsson, head of the Danish Association of Water Works, (FVD) to daily newspaper Urban.
The warmer water could pose a health risk resulting from bacteria that thrive in the higher temperatures.
'Warm water is a prime breeding ground for legionella bacteria which can make people ill,' said Nilsson.
A higher level of coliform bacteria has also been measured in some water supplies. While coliform generally do not pose a health risk, they are used as an indicator of water quality as they are easier to detect than the pathogens that do pose a health risk. Higher levels of coliform usually go hand in hand with increased levels of dangerous bacteria. Nilsson said the higher levels could also be a result of more advanced measuring instruments.
'We are worried about the water quality when the temperature climbs too high which is why we test the water some 60 times a year,' Peter Madsen, a hydro geologist with Esbjerg public water supply in western Jutland, told daily newspaper Jyllands-Posten. Madsen emphasised that the water is safe to drink as bacteria have not been found in the tepid waters.
The abnormally warm water is a result of warmer ground temperatures and poorly insulated pipes.
'If water is to be kept at the recommended 12° C, pipes need to be laid much further into the ground than the 1.2m here in Esbjerg,' said Madsen. 'That doesn't happen because it is quite costly to dig deeper.'
Madsen said once water reaches customers on the fringes of Esbjerg's service area, it has often above the recommended temperature. He said the situation is made worse once the water sits in the pipes inside of houses as they are often not insulated, leading the water temperature to regulate itself with that of the house.
Nilsson said housing blocks have warmer water in the cold taps all year round, and especially this summer with the lingering heat wave. She said mounting a cooling device or insulting pipes would help the water temperature stay at the optimum 12° C.
Both experts noted that filling bottles with tap water and refrigerating them is the best way to ensure tasty and safe water while the heat wave continues.