Comparing pretreatment methods for improving microalgae anaerobic digestion: Thermal, hydrothermal, microwave and ultrasound

•The effect of thermal and mechanical pretreatments on microalgal biomass was compared.•The highest biomass solubilisation was attained for thermal pretreatment (95°C, 10h).•The highest methane yield increase (72%) was also attained for thermal pretreatment.•Biomass solubilisation and methane yield increase showed a positive correlation. The anaerobic digestion of microalgae is hindered by its complex cell wall structure and composition. Thus, several pretreatment methods have been used for increasing microalgae anaerobic biodegradability. Since the methane yield depends on biomass characteristics, pretreatments should be compared using the same microalgal biomass. In this study, physical pretreatments including thermal (95°C; 10h), hydrothermal (130°C; 15min), microwave irradiation (900W; 3min; 34.3MJ/kgTS) and ultrasonication (70W; 30min; 26.7MJ/kgTS) were evaluated in terms of microalgae solubilisation and methane yield increase in batch tests. Organic matter solubilisation was improved in all cases, with the highest increase on soluble proteins, followed by soluble carbohydrates and soluble lipids. This was attributed to the macromolecular and cell wall composition of the main microalgae species composing the biomass, i.e. Monoraphidium sp. and Stigeoclonium sp. Furthermore, the methane yield was increased by 72% for thermal, 28% for hydrothermal and 21% for microwave pretreatments, whereas no significant increase was found for ultrasonication as compared to control. Outstanding results of the thermal pretreatment should be validated in prospective pilot-scale studies in order to quantify the potential increase in biogas production upon continuous operation.