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District energy

District heating

Energy recovery from wastewater


Cleaning wastewater with energy surplus

16. April 2015

Solution provider


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With the help of Danfoss drives, the Aarhus wastewater treatment plant in Denmark not only ensures clean water – it also produces more electricity and heating than it consumes.

Water and wastewater facilities account for the largest consumption of electricity in most municipalities, typically 25-40% of the total energy use. In the Danish city of Aarhus, the local water company has managed to transform the wastewater facility from being just a wastewater treatment plant to also functioning as a combined heat and power plant.

In 2014, the wastewater plant in Aarhus produced 140% electricity and 2.5 GWh excess heat, which is led into the district heating system in the city, thereby reducing the city's carbon footprint. The heat and electricity production combined equals energy production of 190-200%, which is 90% more than the facility consumes.

This is achieved through advanced process optimization and by using more than 120 AQUA variable speed drives from Danfoss as control handles on almost all rotating equipment. The drives enable the wastewater facility to adapt to the change in load during the 24-hour cycle and save energy. When carried out optimally, this also creates the maximum amount of sludge/carbon for the digester, creating gas which is used for both electricity and heat production.

The vision for the water utility Aarhus Vand is to extend energy generation even further to achieve so much surplus energy from its wastewater treatment plants that it can also meet the energy requirement of the city’s potable water supply. This will transform the single largest electricity consumer in the municipality into an energy-neutral party.