The concept of using algae as a fuel was first proposed by Meier (1955) for the pro­duction of methane gas from the carbohydrate fraction of cells. This idea was further developed by Oswald and Golueke (1960), who introduced a conceptual technoeco­nomic engineering analysis of digesting microalgal biomass grown in large raceway ponds to produce methane gas. In the 1970s, as the cost of conventional fuels began rising rapidly, the possibility of using algae as a fuel source received renewed atten­tion. A more detailed design and engineering analysis of this concept was carried out by Benemann et al. (1978), who concluded that such systems could produce biogas competitively with projected fossil fuel prices.

Anaerobic digestion of biowastes occurs in the absence of air, and the resulting gas, known as biogas, is a mixture consisting mainly of methane and carbon dioxide. Biogas is a valuable fuel that is produced in digesters filled with feedstock like dung or sewage. The digestion is allowed to continue for a period of 10 d to a few weeks.

Algal biomass can be used for biogas production. In Poland, there are numer­ous active biogas installations, from large-scale ones to small ones fed with straw and green plant fuel that serve a few farms; so far, however, algae have not been used as a fuel. Some macroalgal species like Macrocystis pylifera and genera such as Sargassum, Laminaria, Ascophyllum, Ulva, Cladophora, Chaetomorpha, and Gracilaria have been explored as potential methane sources (Filipkowska et al. 2008). But in spite of the large seaweed biomass in various regions of the world, anaerobic digestion for biogas generation appears to be unsatisfactory and therefore uneconomical (Gunaseelan 1997; Caliceti et al. 2002).

Anaerobic digestion of algal waste produces carbon dioxide, methane, and am­monia. Leftover nitrogen and phosphorus compounds can be reused as fertilizer for the algal process. Using methane as an energy source can further enhance energy recovery from the process.

Researchers have highlighted some key issues to be addressed in microalgal pro­duction:

— Sodium (in salt) can inhibit the anaerobic digestion process when using marine algae, although researchers say that suitable bacteria (anaerobic digesters) can adapt.

— Digestion of algae can be enhanced and the methane yield increased by physical or chemical pretreatment to break down cell walls and make the organic matter in the cells more accessible.

— The nitrogen content of certain algae can be high, resulting in greater levels of ammonia, which can also inhibit the digestion process. One strategy to overcome this problem uses a “codigestion” process, whereby other organic waste, which is higher in carbon and lower in nitrogen, is added to the algal waste.