Fate of palmitic, palmitoleic and eicosapentaenoic acids during anaerobic digestion of Phaeodactylum tricornutum at varying lipid concentration

Abstract

The literature is showing an increasing trend in the use of microalgal biomass for the production of biofuel, with a recent keen interest for the conversion of microalgae into methane. The methane potential from Phaeodactylum tricornutum was similar for the whole algae, 362±5L/kg total volatile solid (TVS), compared with a sample from which the lipids were extracted, at 358±6L/kg TVS. The anaerobic digestion of long chain fatty acids (LCFAs) representative of the dominant acids found in P. tricornutum during 22weeks of incubation demonstrated that the usual incubation time of typical anaerobic digesters (3–6weeks) is insufficient to hydrolyze the lipids and maximize the energy yield contained in the microalgae. The results from methane potential assays targeting the LCFAs that were found in significant concentrations in P. tricornutum, namely palmitic, palmitoleic and eicosapentaenoic (EPA) acids), at concentration of 2.5 and 1.25g/L, showed a slow but high yield with palmitic acid (802±16L/kg), inhibition from palmitoleic acid and fair degradation of EPA (442±67L/kg). There was a 27% increase in the methane yield from a reconstituted P. tricornutum containing 25% of LCFA compared to 10%, at 456±1 vs 363±5mLSTP/g TVS. Increasing the free fatty acid fraction to 50% in the microalgal biomass resulted in the inhibition of the digestion after production of methane during the first 11days of incubation.