The potential of algae blooms to produce renewable gaseous fuel

• Ulva lactuca (also known as sea lettuce) arises in shallow eutrophic bays.
• It is detrimental to the amenity of the bay and requires removal, treatment or reuse.
• Dried Ulva generated 250 L CH4/kg VS or 100 m3 CH4 per tonne.
• The C:N ratio of Ulva is low and as such it should be co-digested to improve the C:N ratio.
• Co-digestion with slurry resulted in an increase of 17% in biomethane output.

Ulva lactuca (commonly known as sea letuce) is a green sea weed which dominates Green Tides or algae blooms. Green Tides are caused by excess nitrogen from agriculture and sewage outfalls resulting in eutrophication in shallow estuaries. Samples of U. lactuca were taken from the Argideen estuary in West Cork on two consecutive years. In year 1 a combination of three different processes/pretreatments were carried out on the Ulva. These include washing, wilting and drying. Biomethane potential (BMP) assays were carried out on the samples. Fresh Ulva has a biomethane yield of 183 L CH4/kg VS. For dried, washed and macerated Ulva a BMP of 250 L CH4/kg VS was achieved. The resource from the estuary in West Cork was shown to be sufficient to provide fuel to 264 cars on a year round basis. Mono-digestion of Ulva may be problematic; the C:N ratio is low and the sulphur content is high. In year 2 co-digestion trials with dairy slurry were carried out. These indicate a potential increase in biomethane output by 17% as compared to mono-digestion of Ulva and slurry.