Enzymatic pretreatment of Chlorella vulgaris for biogas production: Influence of urban wastewater as a sole nutrient source on macromolecular profile and biocatalyst efficiency

Two biocatalysts, namely carbohydrases and proteases, were assessed for organic matter solubilisation and methane yield enhancement of microalgae biomass. This study evidenced Chlorella vulgaris carbohydrate accumulation (40% on VSS basis) when grown in urban wastewater. Despite of the carbohydrate prevailing fraction, protease pretreatment showed higher organic matter hydrolysis efficiency (54%). Microscopic observation revealed that carbohydrases affected slightly the cell wall while protease was not selective to wall constituents. Raw and pretreated biomass was digested at 1.5 kg tCOD m−3 day−1 organic loading rate (OLR1) and 20 days hydraulic retention time (HRT). The highest methane yield (137 mL CH4 g COD in−1) was achieved in the reactor fed with protease pretreated C. vulgaris. Additionally, anaerobic digestion was conducted at OLR2 (3 kg tCOD m−3 day−1) and HRT (15 days). When compared to raw biomass, methane yield increased 5- and 6.3-fold at OLR1 and OLR2, respectively. No inhibitors were detected during the anaerobic digestion.