Digestibility, or the amount of VS reduced (converted to biogas) during AD, is one of the most important characteristics of the feedstock. The amount of algal VS reduced varies in the range from 20 to 60% for most macro- and microalgae (Tables 16-19). Consequently, the conventional ADP is not able to convert all algal organic matter to biogas and a large fraction of energy is lost as low-value residues. Pretreatment of algal biomass is one of the strategies used for conditioning and increasing algal digest­ibility, methane yield, and degradation rate. Possible goals of pretreatment include:

• Disruption of cell wall

• Size reduction and increase of specific surface area of particulate biomass

• Crystallinity reduction of fiber materials (e. g., cellulose)

• Solubilization of recalcitrant and poorly biodegradable materials (e. g., hemicel — lulose, lignin)

• Partial hydrolysis of cell polymers

• Deactivation of toxic materials

The important requirements for pretreatment methods are to preserve the total organic matter content and to prevent the formation of inhibitory materials. Little is known about providing efficient solutions for increasing algal digestibility. A variety of pretreatment methods have been tested on waste-activated sludge (WAS), live­stock manure, pulp and paper residues, and lignocellulosic biomass [21, 189-193]. Similar methods can be potentially applied for algal biomass conditioning. Methods applied for biomass pretreatment can be classified into the following groups:

• Mechanical—grinding, milling, homogenization, ultrasonic treatment, liquid shear [194-205]

• Thermal—drying, steam pretreatment, hydrothermolysis [195, 206-209]

• Chemical—acid or alkali hydrolysis, ozonation, hydrogen peroxide treatment [198,208,210-213]

• Biological—temperature-phased AD enzymatic treatment [195, 198, 214, 215]

• Electrical—electro-Fenton [216,217]

• Irradiation—gamma-ray, electron-beam, microwave [218-222]

• Combination—thermochemical, wet oxidation [208, 211, 223-226]