1 The Department of Environmental, Social and Spatial Change, Roskilde University2 Aarhus University3 Environmental Risk, Administration Department of Roskilde University, Roskilde University
Algae as a substrate for biogas is superior to other crops since it has a much higher yield of biomass per unit area and since algae grows in the seawater there will be no competition with food production on agricultural lands. So far, the progress in treating different groups of algae as a source of energy is promising. In this study 5 different algae types were tested for biogas potential and two algae were subsequent used for co-digestion with manure. Green seaweed, Ulva lactuca and brown seaweed Laminaria digitata was co-digested with cattle manure at mesophilic and thermophilic condition. The results show that the methane yield of Laminaria from mesophilic anaerobic digestion was fairly stable (average 138 L CH4/kgVSadded). Methane generation from thermophilic reactors both for Ulva and Laminaria, on the other hand, varied significantly, as the feeding rate varied. While the thermophilic treatment of Laminaria produced an average of 142 L CH4/kgVS, Ulva yielded around 122 L/kgVS. Overall, it was found that algae are promising substrates for co-digestion with cattle manure and besides producing energy algae can remove substantial amounts of nutrients from the water environment that subsequent can be used for fertilizer in organic farming. In the study the digested fertilizer product has been evaluated and it has a high quality in terms of nutrients.