The energy consumed in the wastewater treatment plants (WWTP) of the European Union (EU) is between 1 and 3% of the total energy produced in the Union, which represents about 10,000 GWh/year and causes the emission of more than 27 million tons of CO₂ to the atmosphere. Furthermore, this electricity demand is going to increase approximately 20% in 15 years. In this context, and given the need to reduce greenhouse gas emissions through the production of renewable energy (Directive 2009/28/EC), the energy recovery of wastewater is crucial, since it is estimated that the energy contained on this is about 2.7 kWh/m³.

Apart from the energy that can be extracted from wastewater, extra energy can be obtained in WWTP through the co-digestion of substrates, such as agri-food waste or manure. In this sense, the production of agri-food waste is about 88 million tons per year in the EU, and, on the other hand, between 2010 and 2014 the production of manure was 283 million tons in the EU (which caused the emission of 46,387 million tons of CO₂-equivalent with their direct application as fertilizer).

Energy production through co-digestion of waste in WWTP has significant environmental advantages in terms of energy production, offering an attractive alternative to fossil fuels in the production of energy and heat. These advantages are aligned with the commitments reached in the Paris Agreement in terms of reducing CO₂ emissions. In addition, it enables to transform waste into raw materials, as recommended by Directive 2008/98/CE.