The C/N ratios for microalgae are in the range of 4-6. The addition of carbon rich cellulosic materials can balance the high nitrogen content. For example, addition of 25 and 50% of waste paper to a mixture of Scenedesmus spp. and Chlorella spp. resulted in a 1.59- and 2.05-fold increase in the methane yield (Fig. 17a) [410]. The optimal ratio between algal biomass (Scenedesmus spp. and Chlorella spp.) and waste paper was found to be 40% algae and 60% paper with corresponding C:N ratio equal to 22.6. The influence of the C/N ratio on the methane yield is shown in Fig. 17b. The authors also reported that paper addition stimulated cellulase activity in the anaerobic digester from 1.26 ± 0.14 mg/L-min (no paper added, C:N is equal to 6.7) to 3.02 ± 0.09 mg/L-min (50% paper, C:N is equal to 18).

Addition of A. maxima biomass to sewage sludge, peat extract, and spent sulfite liquor improved the VS reduction and methane yield (Fig. 18) [411]. Nutrient rich algal and cyanobacterium biomass can be added to nutrient limited waste products that cannot be digested as sole substrate.

Co-digestion of Macroalgae

The mix of Ulva and manure has a larger methane yield and production rate com­pared to pure Ulva biomass (Fig. 11c, d). Methane production from a mixture of alginate extraction residues and manure was lower compared to methane production from separate substrates [245]. Morand and coauthors speculated that co-digestion of different seaweeds can be problematic due to dissimilarity in digestion speeds [412]. But addition of Ulva to Sargassum tenerrimum (1-17 ratio) increased the methane yield and production rate [413].