After Saturday’s heavy rain, the water quality along several of the country’s coasts has deteriorated. Coasts in both Aarhus and Copenhagen are among the affected areas, according to badevand.dk. The problem arises when large amounts of rain mix with wastewater, leading to an increased risk of bacteria in the bathing water.
Gitte Normand Andersen, department head of Nature and Environment in Aarhus Municipality, explains that the high pressure on the sewer system during heavy rain showers can cause wastewater to flow into the waterways and further out to the bathing coasts. “Therefore, we raise the red bathing flags and warn against bathing after heavy rain showers,” she says.
Saturday’s rainfall was exceptionally heavy, with Hald north of Randers as an example, where 60.4 millimeters of precipitation were measured. This is almost equivalent to the total amount for the month of July, which is 65 millimeters according to DMI. The large amounts of rain overload the sewer system in many places, as rainwater ends up in the same pipes that carry wastewater from toilets to treatment plants. To prevent flooding, sewer outlets are opened, resulting in wastewater mixing with rainwater and flowing into lakes and the sea.
Normand Andersen explains that it typically takes a day or two before it is safe to swim again. The problem is well known and occurs every time there is a lot of rain. To solve the challenge, municipalities and utility companies are working to separate wastewater and rainwater into separate pipes instead of using common sewer pipes.
In Aarhus Municipality, a long-term strategy for separate sewer systems has been adopted, spanning 60 years. “We are slowly working our way through the city. Each time we separate an area, the risk of overflow is reduced year by year,” says Gitte Normand Andersen.
Similar steps are being taken in various parts of the country to minimize the risk of water pollution, but it is a time-consuming process. In the meantime, it is important to follow the warnings from municipalities and avoid swimming after heavy rain showers.