Macroalgae Biogas for the Isle of Man: the Effect of Co-digestion

Abstract

Laboratory-scale trials on anaerobic digestion of Laminaria digitata were undertaken in batch mode. Sludge from a working digester was used as inoculum. Macroalgae was digested individually and co-digested with creamery waste or sewage sludge pellets. The co-digestates are wastes on the Isle of Man, which forms a case study in marine bioenergy. Macroalgae digested with inoculum produced 159.67 ± 6.69 ml biogas per g lyophilised mass added. Macroalgae significantly increased the total volume of gas, relative to controls. Relative to controls, co-digestion of macroalgae and sewage pellets (50/50, lyophilised mass basis) had no significant effect on total gas. However, this co-digestion significantly decreased total gas relative to macroalgae only, possibly due to a low C/N ratio. Co-digestion of macroalgae with a 50/50 mixture of creamery waste and inoculum produced significantly more than controls. The same co-digestion had a greater effect than digestion of macroalgae without creamery waste. A 50/50 mixture of creamery waste and inoculum produced significantly more gas in total than did a higher ratio of creamery waste to inoculum, when macroalgae was added. An increase from 1 g/L to 2 g/L lyophilised mass of macroalgae added to mixed creamery waste and inoculum had no significant effect on total gas. However, some results were affected by liquid backflow. The decline in gas yields between trials is thought to have been due to microbial changes in the inoculum. The biogas had low methane content and would require optimisation. Areas for further experimental work were identified. Additional considerations related to the potential for, and impact of, macroalgae bioenergy on the Isle of Man.