Co-digestion has attracted increased attention in The Netherlands as a result of a subsidy for electricity that is produced environmentally safe (MEP and SDE subsidy). Production of bio-energy by means of co-digestion turned financially profitable due to this financial support. Co-digestion is the simultaneous digestion of manure and a co-substrate and its conversion into biogas. We report on a methodology to assess the sustainability of bio-energy from co-digestion with emphasis on energy and green house gases (GHG) (Zwart et al., 2006). We have analyzed three cases, i.e. animal manure only or maize only and a mixture of 50% manure and 50% maize. On the basis of the gross ‘green gas’ and electricity production we calculated efficiencies according to the suggestions cf. Commission Cramer (Anon., 2007). We report that digestion of manure only is most efficient in reducing emissions of greenhouse gases. Digestion of maize without manure produces most energy. In terms of efficiency cf. Commission Cramer a mixture of manure and maize is most efficient. This is more efficient than digestion of maize alone as here emissions of greenhouse gases during crop production limits the efficiency cf. Commission Cramer.
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