Sequential valorisation of microalgae biomass grown in pig manure treatment photobioreactors

Abstract

Sequential valorisation of microalgae biomass grown in pig manure was evaluated to harness the major accumulated components. First, supercritical carbon dioxide extraction (100, 200, and 300 bar; 40 and 60 °C) was applied to solubilise the lipid components. The maximum lipid extraction (75%) was achieved at 300 bar and 60 °C. The extraction kinetic increased with the temperature at all the pressures tested and decreased with pressure only for experiments at 60 °C.

After supercritical CO2 extraction, the exhausted solid fraction was assessed using subcritical water extraction (100, 130, 160, and 190 °C for 10 min) where the carbohydrate components were selectively solubilised. The raffinate solid fractions after subcritical water extraction retained 30 to 46% carbohydrates and 67 to 73% proteins. A 60% monosaccharide recovery yield was achieved, with a maximum carbohydrate degradation rate of 10%.

Subcritical water extraction was also evaluated using the initial raw biomass. Therefore, the solubilisation of carbohydrates and their recovery was lower than in the experiments using the exhausted solid fractions after supercritical CO2 extraction. As a result, supercritical extraction seems to be a promising method for the sequential valorisation, as was confirmed in the scanning electron microscopy analysis. Finally, the composition of raffinate solid fractions after subcritical water extraction was analysed to check their potential use as bio-fertiliser, applying a bio-refinery concept.