Waste-to-energy is the use of waste products for energy purposes. After the reduction, reuse and recycling of materials, non-recicable municipal solid waste can be reused to produce electricity or heat before the remaining waste is treated and disposed of. A common method of waste-to-energy is waste incineration in a complex system of combustion, heat transfer and exhaust gas filtering. Other energy recovery methods are pyrolysis, gasification, anaerobic digestion and landfill gas recovery. While large-scale waste incineration does not count as renewable energy, it comprises a significant part of the energy mix of several European countries such as Denmark and Sweden. Waste-to-energy is mainly a waste management option that reduces the volume of solid waste in landfills and offsets the need for the extraction of fossil energy carriers.