Wastewater in Stockholm is currently treated over several stages using mechanical, biological and chemical methods, and this process requires a lot of space. At the same time, Stockholm is growing at a record-breaking pace and the amount of wastewater to be treated is increasing.
Environmental, technical and socio-economic factors indicate that shutting down the Bromma treatment plant would be the best thing, concentrating wastewater treatment at the treatment plant in Henriksdal instead. A new tunnel will be constructed in order to channel wastewater from Västerort to Henriksdal. This is also what is implied by the decision made by Stockholm Municipal Council in May 2014.
The Bromma treatment plant is nowhere near as good at nitrogen treatment as Henriksdal. Retaining the Bromma plant would require upgrading its capacity and expanding the premises. But the Bromma treatment plant is located above ground in a densely populated area, and there has to be a safe zone of at least 200 metres around the plant. So it would be necessary to shut down the plant within ten to fifteen years anyway, as the buildings surrounding it would prevent it being expanded still further.
The Henriksdal plant is located below ground, in Henriksdalsberget, where there is plenty of space. The final stage in the treatment process at the new treatment plant will be replaced with modern membrane technology that is able to treat more water over a smaller area. Moreover, the water will will be cleaner than it is today. We will surpass environmental requirements by some way by 2040, and there will be space inside the mountain to expand the plant still further after that.