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

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 482 L CH4 kg (1) volatile solids (VS) in batch assays. However, when dry-extracted, the methane potential of residue cake was only 194 L CH4 kg (1) VS. In semi-continuous reactor trials with dry-extracted residue cake, a thermophilic reactor produced 48% higher methane yield (220 L CH4 kg (1) VS) than a mesophilic reactor (149 L CH4 kg (1) VS). The thermophilic reactor was apparently inhibited due to ammonia with organic loading rate (OLR) of 2 kg VS m (3) d (1) (hydraulic retention time (HRT) 46 d), whereas the mesophilic reactor performed with OLR of 3 kg VS m (3) d (1) (HRT 30 d). Algal salt content did not inhibit digestion. Additional methane (18-33% of primary digester yield) was produced during 100 d post-digestion. (C) 2014 Elsevier Ltd. All rights reserved.