Thermal pretreatment of algae for anaerobic digestion

The objective of this work was to determine the benefit of thermal pretreatment on biogas yield from microalgae-fed anaerobic digester mesocosms. Replicate Nanochloropsis oculata cultures were heated for 4 h at 30, 60, and 90 °C, as well as at a constant temperature of 90 °C for 1, 3.5, and 12 h. Net biogas production increased from 0.28 L biogas/g volatile solids added (VSa) for the control to 0.39 L biogas/g VSa (p < 0.01) when heated at 90 °C, but there was no improvement at 30 or 60 °C. Increased biogas production correlated with increased soluble chemical oxygen demand (COD). Net biogas production increased as a function of heating time, from 0.32 L biogas/g VSa for the control, to 0.41, 0.43, and 0.44 L biogas/g VSa (p < 0.05 for all combinations vs. control) when preheated at 90 °C for 1, 3.5, and 12 h, respectively. However, despite enhanced biogas production the energy balance is negative for thermal pretreatment.