The waste hierarchy promotes reuse and recycling of waste material but substantial amounts of wastes cannot be reused or recycled. However, the energy content of this waste can be utilized. Waste is constantly produced in agriculture, municipalities and industries and part of that thus provides a secure and a local source of energy. Wastes and biomass fuels are usually viewed as sustainable energy sources (Kothari et al., 2010). Waste can consist of many different materials. Considering using waste as a biomass resource, the organic fractions containing material of biological origin is of interest, see the examples in Table 6.1. Packed foodwaste fromgrocery shops is one important source of organicwaste in industrialized
countries. In Finland, for example, grocery shops’ food wastage due to overdated products totaled 5.4× 107 kg in 2007. This corresponds, on average, to 4.15× 103 kg per M€. This includes products that have exceeded the “use-by date” such as fresh meat, mince, fresh fish, fresh cheese, uncooked ready-made food and other food easily contaminated microbiologically. It is prohibited to sell or even give away these after the use-by date. “Best before” marking is used for other groceries, such as bread, grain products, other dry products, frozen food and canned food. It is possible to decrease their wastage by selling them at a discount. Moreover, contaminated groceries and groceries with damaged packages are thrown away. In addition, frozen food that has melted or for which the cold chain has otherwise been broken, cause wastage of groceries (EkoleimaAy, 2008). In Table 6.1 possible conversion processes for examples of organic wastes are also given.
Anaerobic digestion and fermentation are microbiological conversion methods while combustion, pyrolysis and gasification are thermochemical conversion methods. The product of the conversion processes of organic waste can be used as fuel in further energy conversion. Ethanol from fermentation and up-graded gas from anaerobic digestion can be used as a fuel in transportation. The gas from anaerobic digestion can also be used directly in an oven, gas engine or gas turbine for heat and power production. The gas from gasification and pyrolysis can be used in a similar way but can also be further reformed for production of other types of transportation fuels for example dimethylether (DME) or Fischer-Tropsch fuel.