A summary of the methane yield and volumetric production rate during algal diges­tion in continuous reactors appears in Fig. 11. The curves drawn through the data points on each figure represent general trends for easy comparison of the nonuni­form data.

Based on these data, algae as a substrate for ADP can be classified into three groups. The first group consists of algae with the methane yield larger than 0.3 L/gVS and includes brown macroalga M. pyrifera with high mannitol content, cyanobac­terium Arthrospira. green microalgae Chlorella and Scenedesmus. The second group has methane yield about 0.2 L/g VS and includes brown macroalga Laminaria and green macroalgae Ulva, Cladophora. and Chaetomorpha. Lastly, the third group has methane yield lower than 0.15 L/g VS and includes brown macroalga Sargassum, red macroalga Gracilaria, and green macroalga Enteromorpha.

Another important conclusion is that AD of M. pyrifera is stable at values of OLR up to 10 gVS/L-day. High values of the methane volumetric production rate are achieved 2.7 L(CH.)/L(digester)-day, reducing the required volume and the capital costs of the digester.

Finally, several methods such as application of advanced digestion reactors, co­digestion of algae with other substrates, algal hydrolysis and extraction of cellular liquids, and digestion of alginate extraction residues, significantly enhance methane yield, production rate, and the overall process efficiency.