Biogas generation from agricultural wastes was first introduced in Denmark in the late 1970s during the first energy crisis. Although not as widespread as on farms in Germany, where many small farms have digesters, Denmark has three categories of digester: (1) 20 community (centralized) biogas plants ranging from 540 to 7500 m3 in volume and fed with animal and industrial wastes, which provide electricity to the grid and heat for houses (Figure 4.8); (2) eight large farm biogas plants with CHP, but without co-digestion; and (3) 18 farm biogas digesters operated at thermophilic conditions in the capacity range of 150-800 m3 with co­digestion and CHP (Al Seadi and Holm-Nielsen 2000; Sannaa 2004).

Nearly all community biogas plants are performing co-digestion, partly for increased biogas production, but also for “tipping fees” from suppliers of organic wastes. The average community digester obtains its income from three sources: electricity, heat, and tipping fees from industries. In general, such plants get -20% to over 50% of their wastes from nonagri­cultural suppliers (e. g., slaughter houses, fish processing, pharmaceutical industry, hospital kitchens, hotels). Drivers for anaerobic digesters in Denmark have been investment grants to help capital costs (-20%), long-term loans at low interest rates, a requirement for a 9-month storage for untreated manure slurry, favorable prices for biogas-produced electricity, oppor­tunities for district heating, and demonstration and research programs.