Effects of thermal pretreatment on anaerobic digestion of Nannochloropsis salina biomass

•Cell wall composition restricts anaerobic digestion of Nannochloropsis salina.•Thermal pretreatment of Nannochloropsis salina is beneficial for algal digestibility.•Semi-continuous digestion causes volatile fatty acid accumulation.•Degradation process is stabilized by adaption to higher ammonium and salt levels.•Energy input for thermal pretreatment is negligible compared to the benefit. The marine microalga Nannochloropsis salina was investigated as feedstock for anaerobic digestion under batch and semi-continuous conditions for the first time. Biodegradability and methane yield were low under both digestion conditions. Thermal pretreatment prior to anaerobic digestion significantly increased the methane yield from 0.2 to 0.57m3kgVS−1 under batch conditions and from 0.13 to 0.27m3kgVS−1 in semi-continuous digestion. Still, the methane yield was limited with semi-continuous feeding due to volatile fatty acid (VFA) accumulation in the digester caused by high ammonium and salt concentrations in the feedstock. Despite VFA accumulation adaption of the microorganisms to the changing conditions and high buffer capacity resulted in steady methane production. A first energy balance considering the required heat for thermal pretreatment revealed significant benefit from the pretreatment. Conversely, the high energy demand for dewatering algal cultures is one major bottleneck for industrial-scale processing of microalgae.