Anaerobic digestibility of marine microalgae Phaeodactylum tricornutum in a lab-scale anaerobic membrane bioreactor

The biomass of industrially grown Phaeodactylum tricornutum was subjected in a novel way to bio-methanation at 33°C, i.e., in an anaerobic membrane bioreactor (AnMBR) at a hydraulic retention time of 2.5 days, at solid retention times of 20 to 10 days and at loading rates in the range of 2.6–5.9 g biomass-COD L⁻¹ day⁻¹ with membrane fluxes ranging from 1 to 0.8 L m⁻² h⁻¹. The total COD recovered as biogas was in the order of 52%. The input suspension was converted to a clear effluent rich in total ammonium nitrogen (546 mg TAN L⁻¹) and phosphate (141 mg PO₄-P L⁻¹) usable as liquid fertilizer. The microbial community richness, dynamics, and organization in the reactor were interpreted using the microbial resource management approach. The AnMBR communities were found to be moderate in species richness and low in dynamics and community organization relative to UASB and conventional CSTR sludges. Quantitative polymerase chain reaction analysis revealed that Methanosaeta sp. was the dominant acetoclastic methanogen species followed by Methanosarcina sp. This work demonstrated that the use of AnMBR for the digestion of algal biomass is possible. The fact that some 50% of the organic matter is not liquefied means that the algal particulates in the digestate constitute a considerable fraction which should be valorized properly, for instance as slow release organic fertilizer. Overall, 1 kg of algae dry matter (DM) could be valorized in the form of biogas (€2.07), N and P in the effluent (€0.02) and N and P in the digestate (€0.04), thus totaling about €2.13 per kilogram algae DM.