Production of biogas from seaweeds and microalgae by AD was a subject of research starting in the mid-sixties [109, 110]. The first detailed large-scale study of seaweed cultivation and AD was performed in the Institute of Gas Technology by Chynoweth et al. [79, 111]. Substrate chemical composition and conversion parameters deter­mine the amount and composition of biogas generated in the ADP. The theoretical yield of biogas can be estimated by the Bushwell equation [112].

CcHhOoNn + yH2O ^ xCH4 + nNH3 + (c — x)CO2

where:

x = (4c + h — 2o — 3n — 2s) / 8
y = (4c — h — 2o + 3n + 3s) / 4

The theoretical methane yield from different algae is presented in Table 15.

Lipids have the lowest oxidation state and largest theoretical methane yield, which is more than twice the methane yield from proteins, glycerol, and carbohydrates. The theoretical methane yield correlates with the average carbon oxidation state of the substrate (Fig. 2) . Macroalgae with high carbohydrate content and cyanobacteria with high protein content are theoretically poorer feedstock for methane produc­tion while microalgae with high lipid content have higher potential methane yield.

Fig. 2 (a) Theoretical methane yield in relation to the average carbon oxidation state. circles— pure compounds; crosses—microalgae; pluses—macroalgae. (b) Mean theoretical methane yield and (c) C/N ratio with maximum and minimum values (base on Table 15)

Based on this observation, increasing the lipid content in algae is a promising approach for enhancing the methane yield. Another observation is that all microalgae have C/N ratio lower than optimal range for AD and high potential level for ammonia release. In contrast, several macroalgae and oil rich microalgae have a C/N ratio that is too high and possibly require addition of nitrogen for optimal AD conditions.