Käppalaverket is one of the world’s most efficient wastewater treatment plants, treating wastewater from more than 500,000 people in Stockholm. Biogas is extracted from the sludge that remains after the purification process, and used as vehicle fuel for public buses in Stockholm.
Käppalaverket is an underground facility, situated in the north-eastern part of Stockholm. Annually, about 50 million cubic meters of wastewater from households, schools, industries, offices and hospitals is purified at the plant.
The purification process at the Käppala plant consists of mechanical, chemical and biological purification. The process takes place in 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.
– Sludge is digested to produce biogas. The digested product is certified for use as fertilizer for agricultural fields, as it contains nutrients such as nitrogen and phosphorus.
– About 6.6 million cubic meters of biogas is produced at the plant annually. An evaluation for alternative uses of biogas showed that the alternative to produce vehicle fuel was the most advantageous alternative, both from an economic and environmental perspective. Today, the biogas is used as environmentally friendly fuel in the public buses in Stockholm.
– The purified wastewater has a comparatively high temperature. The heat from wastewater is recovered using heat pump technology and delivered to the district heating network in the nearby district.