IVL Swedish Environmental Research Institute

Käppala – Turning Wastewater into Fuel and Heat

Käppalaverket is one of the world’s most efficient wastewater treatment plants, treating wastewater from more than 500,000 inhabitants in Stockholm. Here, the purification proess is turning waste to new resources, in the shape of biogas fuel as well as heat.

Käppalaverket is an underground facility, situated in the north-eastern part of Stockholm. Annually, the plant purifies about 50 million cubic metres of wastewater from households, schools, industries, offices and hospitals.

The purification process at Käppalaverket consists of mechanical, chemical and biological technologies. That’s why the process also comprises several purification steps and takes one to two days. The water is then released into the Baltic Sea and returns to nature’s own cycle.

From treatment plant to resource extraction unit

The treatment plant is viewed as a resource extraction unit: from the raw material, the wastewater, a number of products are produced – purified wastewater, sludge, biogas and heat. Besides the benefit that comes with the possibility to reuse the purified wastewater, the by-products are also taken care of and utilised as valuable resources.

6.6 million cubic metres of biogas

At Käppala, the sludge that remains after the purification process is digested to produce biogas. The digested product is certified for use as fertiliser for agricultural fields, as it contains nutrients such as nitrogen and phosphorus. The biogas produced from the process at the plant amounts to an impressive total of 6.6 million cubic metres every year.

An evaluation for how to best use the biogas showed that the alternative to produce vehicle fuel was the most advantageous one, both from an economic and environmental perspective. Therefore, the biogas is used as environmentally friendly fuel in the public buses in Stockholm.

Recovering the heat to district heating

The purified wastewater has a comparatively high temperature, and Käppala has also taken advantage of this by recovering the heat from the wastewater, using heat pump technology, and then delivering the heat to the district heating network in the nearby areas.

These ways to turn waste into new resources, makes Käppala an inspiring example of how to enable not only a more circular use of water, but also make the best possible use of by-products and resources (in this case sludge, biogas and heat) that would otherwise contribute to more waste and more warming.


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