High-rate biomethane production from microalgal biomass in a UASB reactor

•CH4 production from microalgae was achieved in an Upflow Anaerobic Sludge Bed (UASB) reactor.•Full factorial design experiment was used to identify the effect of key operating parameters on biogas production.•UASB reactor featured an nine fold increase in volumetric rate of CH4 production as compared to CSTR.

Biomethane production from the microalga Scenedesmus sp. AMDD was demonstrated in an upflow anaerobic sludge bed (UASB) reactor. A full factorial design experiment was used to identify the effect of organic loading rate (OLR) of algal biomass and hydraulic retention time (HRT) on the volumetric rate of methane production. At an HRT of 4 days and an OLR of 3.23 gTVS LR− 1 d− 1 corresponding to an influent microalgae concentration of 12 g total volatile solids (TVS) L− 1, the volumetric rate of CH4 production reached 0.6 LSTP L− 1R d− 1. A methane yield of 0.18–0.2 L per gTVS of fed microalgae was estimated. A stable performance was observed throughout 3 months of UASB reactor operation. Due to the short HRT and the good performance of UASB reactor, operated at influent microalgae concentrations in a range of 4–12 gTVS L− 1, this reactor type is suitable for coupling with photobioreactors equipped with gravitational settlers.