Co-digestion of different substrates has been recognized recently as an attractive approach that has many economical, logistic, and sanitary benefits [404]. The main advantage of co-digestion is the improvement of the nutrient balance that leads to greater reduction of VS and methane yield [405] . The C:N:P ratio is an important factor for AD as discussed earlier in the chapter. Co-digestion of low nitrogen bio­mass (municipal solid waste, paper, sisal pulp, straw, grasses, wood wastes) with higher nitrogen wastes (sewage sludge, chicken or livestock manure, slaughter­house, meat or fish processing wastes) can increase the loading rate and methane yield up to 60-100% [399, 406-409].

The C to N ratio varies for macro — and microalgae due to significant differences in biochemical composition. Carbohydrates are the main components of macroalgal

biomass with concentrations between 50 and 70% of dry weight. Microalgae usu­ally have a carbohydrate content in the range 10-20% while the protein content can reach 30-50% in some species. These differences in biochemical composition influence the C:N ratios and the strategies needed for co-digestion processes with macro — or microalgae.