The effects of alternative pretreatment strategies on anaerobic digestion and methane production from different algal strains

•Anaerobic digestion of microalgae can be hampered by low biodegradability.•We examined the effect of pretreatment on algal solubilization and methane yields.•Chemical or thermal pretreatments were ineffective on methane yield.•Thermochemical pretreatment improved methane yield by 30–40% from specific species.•Algal type and the cell-wall nature strongly impacts on biogas and methane yields. The effect of various pretreatment strategies on methane yields following anaerobic digestion (AD) of five different microalgal strains was investigated. Pavlova_cf sp., Tetraselmis sp. and Thalassiosira weissflogii exhibited substantial methane yields of 0.4–0.5L/g volatile solids (VS) without pretreatment, providing up to 75–80% of theoretical values. In contrast, methane yields from Chlorella sp. and Nannochloropsis sp. were around 0.35L/g VS, or 55–60% of the theoretical values, respectively. Alkali treatment was not effective and thermal pretreatment only enhanced Nannochloropsis methane yields. Thermochemical pretreatment had the strongest impact on biomass solubilization with methane yields increasing by 30% and 40% for Chlorella and Nannochloropsis, respectively. The lipid content had a strong beneficial impact on the theoretical and observed methane yields as compared to protein and carbohydrate content. Other features such as cell-wall composition are also likely to be important factors dictating algal biodegradability and methane yields addressed in part by thermochemical pretreatment.