Mesophilic and thermophilic anaerobic laboratory-scale digestion of Nannochloropsis microalga residues

•Methane production from Nannochloropsis microalga residue was studied.•Wet extracted alga had superior biomethane potential compared with dry extracted.•Anaerobic digestion at 55°C was more efficient compared with digestion at 35°C.•Thermophilic process was inhibited because of ammonia with low loading.•Salt from marine alga did not inhibit anaerobic digestion. This paper studies methane production using a marine microalga, Nannochloropsis sp. residue from biodiesel production. Residue cake from Nannochloropsis, oils wet-extracted, had a methane potential of 482LCH4kg−1 volatile solids (VS) in batch assays. However, when dry-extracted, the methane potential of residue cake was only 194LCH4kg−1 VS. In semi-continuous reactor trials with dry-extracted residue cake, a thermophilic reactor produced 48% higher methane yield (220LCH4kg−1VS) than a mesophilic reactor (149LCH4kg−1VS). The thermophilic reactor was apparently inhibited due to ammonia with organic loading rate (OLR) of 2kgVSm−3d−1 (hydraulic retention time (HRT) 46d), whereas the mesophilic reactor performed with OLR of 3kgVSm−3d−1 (HRT 30d). Algal salt content did not inhibit digestion. Additional methane (18–33% of primary digester yield) was produced during 100d post-digestion.