After industrialization began along Akerselva in the 1840s, the town's population increased sharply in the second half of the 19th century. As early as the 1860s, Akerselva was so polluted by emissions from businesses and sewage from households that the city's water intake had to be moved to Maridalsvannet. In the 1930s, truncated sewers were built in the lower part of the river, but the pipes were moved so far from the river that large volumes of sewage still flowed directly into Akerselva. Work continued after the war and intensified from the late 1960s. An important milestone was the separation channel to the Bekkelaget sewage treatment plant. Completed in 1971, it collected sewage along the Akerselva from Maridalsoset to Sandaker.
At that time, Akerselva was heavily polluted and had a high content of organic material and nutrient salts. Oil spills and leaks from buried oil tanks, as well as uncontrolled releases of chemicals and detergents, have had serious consequences for life on the river. The situation improved when the industrial plants along the river closed and the remaining plants installed sewage treatment plants.
The amount of phosphorus and nitrogen discharged into the fjord from Akerselva was greatly reduced in the first half of the 1980s. The water quality improved significantly, the salmon returned and it was possible to swim again in the upper part of the river.
In 1987, all direct discharges into the city's rivers and streams were eliminated. The main source of pollution in Akerselva was now overflows and leaks in the municipality's sewers. Around. 20% of the sewage disappeared on the way to the sewage treatment plants, and during heavy rains, large amounts of sewage still went straight into the river along with the rainwater. The situation improved somewhat after the sewage system was renovated in the first half of the 1990s.
The number of emergency discharges also decreased, but in 2011 we had a serious chlorine discharge from the Oset water treatment plant. A boost was achieved with Midgardsormen, which was completed in 2014 at a cost of NOK 1.3 billion. This treatment plant will reduce emergency discharges from the sewer system by 90% and will cover the stretch along the Akerselva from Cuba to the fjord and the Bjørvika area.
On the route from Maridalsoset to Sannerbrua there are still 91 municipal and many private stormwater drains to Akerselva. 21 of the municipal storm sewers can discharge wastewater into the river. Therefore, one should not bathe in the river immediately after heavy downpours. Otherwise, the bathing water quality in the upper part of the river is good. The water quality is the worst in the lower part of the river.