The concept of centralized biogas plants has been developed in Denmark. At present, 10 plants are in operation with capacities ranging from 50 to 500 tonnes of biomass per day. The biomass consists of approximately 80% manure co-digested with 20% organic waste from abattoirs and various food industries. The effluent is returned as nutritionally defined fertilizer, partly to the farms that supply fresh manure, and partly to other crop farmers. The plants are sited to ensure that the energy can be utilized, mainly for CHP generation. The heat is used for district heating of urban communities. Operational experience shows considerable increases in gas production primarily due to co-digestion of the manure with organic wastes. Possibilities of process regulation have been revealed, and stable thermophilic digestion has been achieved. A two-step process has turned out to be advantageous, the second step being lower temperature post digestion. Environmental and agricultural benefits include savings for farmers, improved fertilization efficiency, less greenhouse gas emission, and cheap, environmentally sound waste recycling. The working economy of the centralized biogas plants has improved, but there is still a need for further reductions in running costs if financially competitive plants are to be created without public investment grants.

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