Carotenoids and biogas recovery from microalgae treating wastewater

This study evaluated the potential of microalgal biomass grown in wastewater for carotenoid and biogas recovery in a biorefinery approach. Microalgal biomass was harvested from two pilot high rate algal ponds treating urban wastewater with hydraulic retention times (HRTs) of 8 and 6 days. The maximum carotenoid content in harvested biomass was 4 mg·g total suspended solids (TSS)^-1 with a HRT of 8 days, while it reached around 1.5-2 mg·gTSS^-1 with a HRT of 6 days. Biochemical methane potential tests showed no significant differences between the methane yield of raw (159 NmLCH₄·gVS^-1) and carotenoid-extracted biomass (136 NmLCH₄·gVS^-1). Moreover, co-digestion of raw and carotenoid-extracted biomass with primary sludge increased the methane yield by 58 % and 86 %, respectively (75:25 ratio), and by 28 % and 44 %, respectively (50:50 ratio). These results demonstrate that wastewater-grown microalgae biorefineries enable the simultaneous recovery of value-added carotenoids and bioenergy, in the framework of a circular bioeconomy.