Pretreatment of Scenedesmus sp. biomass as a potential anaerobic digestion substrate

Algal biomass can be a potential substrate for anaerobic digestion. However, raw algae cells show a resistance to biological degradation, resulting in a slower methane production rate. Varying thermal and chemical pretreatments of algal biomass were investigated in an attempt to increase soluble organic matter (SOM) yield, which would result in enhanced methane production during subsequent anaerobic digestion. Scenedesmus sp. was harvested using three different procedures: with flocculation, with flocculation and drying, and without flocculation or drying. For all pretreatments and algae types, fluorescence micrographs were obtained to visually confirm the degradation of the algal cell walls. A complete 2×3×4 factorial design was applied for the algal biomass pretreatment study, including two heating temperatures (50°C or 90°C), three heating durations (10, 30, or 60 min), and four NaOH concentrations (0%, 3%, 6%, or 12% g NaOH g^-1DW of algae). For algae cells with no flocculant addition, SOM yield increased by 15% with a moderate pretreatment of heating at 50°C in 3% NaOH for 60 min. For dried algae, the baseline SOM yield was higher than in the other algae, such that there was only a noticeable increase with the more severe pretreatments. For flocculated algae, the most severe pretreatment increased SOM yield by 17.2%, but overall the SOM increase was less than with fresh algae. Flocculation appears to inhibit cell wall disruption, but thermal chemical treatment can hydrolyze some flocculant polymers, which eases the flocculation and facilitates cell destruction.