Norwegian 19 km Water Supply Tunnel
- Did you know that tunnel water is pH-adjusted with CO2?

Being a mountainous country, no wonder Norway is world leading in tunnel construction. However, such projects generate large amounts of polluted water. Did you know that the best way to pH-adjust tunnel water is by using carbon dioxide (CO2)?

At present, Norway’s capital Oslo has no reserve water supply, and the main source of water, Maridalsvannet, delivers 90 per cent of the drinking water to Oslo’s inhabitants. In 2022 Skanska started the construction of a 19 km raw water tunnel in what is Norway’s largest water supply project, going from Holsfjorden outside Oslo to an underground water treatment system in Oslo.

Construction projects are met with continuously stricter demands regarding environmental measures. Use of CO2 in water treatment is one important step in the right direction. When roads, railways and tunnels are constructed, a lot of contaminated wastewaters is produced. This water is mostly very alkaline and poisonous for fish and for life in adjacent seas and lakes. In order to lower the pH level before the water is released into nature, CO2 gas is injected and dissolved into the wastewater treatment system, resulting in an acceptable pH level.

Tanks with liquid carbon dioxide and evaporation system, where CO2 is dissolved in water and the pH-value satisfy environmental standards.

From acid to CO2

The reason why tunnel water has such a high pH-value is the concrete-based sealant and use of cement spray, resulting in a very alkaline wastewater, with pH as high as 11-12. Values above pH 9 can be very harmful to fish and biological diversity in the natural environment close to the construction site.

Earlier tunnel water was neutralized by using hydrochloric acid and sulfuric acid. Now the use of carbon dioxide (CO2) is becoming first choice in many construction projects. Carbon dioxide is both safer, easier to handle and better for the environment than former cleaning methods with acids. With varying water amounts, it is challenging to dosage correctly with acid, while much more accurate with CO2-gas.

“Hydrochloric and sulfuric acid is harder to dose accurately and makes it difficult to fulfil the emission demands. 96 per cent sulfuric acid and 40 % hydrochloric acid is risky, both regarding transportation and handling on site. The use of carbon dioxide (CO2) is safer in the workplace, in addition to being easier to dose within the right measurement levels. With CO2 it is impossible to overdose», says Lars Engen, leader of water treatment in Skanska Norway.

Skanska met the strict environmental demands from Oslo Municipality and the drilling of the new water tunnel commenced in the beginning of 2023. The construction sites are fossil free and the construction machines, conveyor belts and TBM - Tunnel Boring Machines are run by electrical power. The project is planned to be finalized in 2028. 

«Linde provides tanks with liquid carbon dioxide and evaporator, to make sure the CO2 is dissolved in the water and the pH-levels fulfil the environmental requirements before the water is released into surrounding nature. Linde makes sure that the tanks always are filled up with CO2 using automatic remote monitoring”, says Kjetil Veidel, application engineer in Linde.

Text: Ingalill Sandal
Photo: Skanska and Shutterstock

Full overview on screen. CO2 always available with automatic filling of tanks