Integrated approach for enhanced crude bio-oil yield from microalgae cultivated on the aqueous phase of hydrothermal co-liquefaction with agar-free seaweed residues

Abstract

The present study evaluated the efficiency of microalgae cultivation on the aqueous phase (Aq-LP) produced from (Co-)hydrothermal liquefaction (HTL) of agar-free seaweed waste. Optimization of bio-oil production from Co-HTL of seaweed residue with the studied microalgal biomass grown in Bold's Basal Medium (BBM) was initially carried out using Box-Behnken model. The highest bio-oil yield of 42.21% was recorded by HTL of microalgal biomass at 250 °C for 60 min, which showed insignificant difference with that from Co-HTL. Comparatively, Aq-LP from microalgal biomass showed the highest organic load with total organic carbon of 11.69 g L−1, while that from seaweed residue showed the lowest value of 7.47 g L−1. However, mineral ions in the Aq-LP from seaweed residue showed higher values compared to microalgal biomass. Therefore, Co-HTL enriched the Aq-LP with nutrients from both feedstocks which resulted in 10.3% higher biomass yield than the control. The experimental bio-oil yield was 15.3% higher than the calculated value, which confirms a synergistic action during Co-HTL. In addition, the quality of the bio-oil obtained via HTL was improved after Co-HTL of both feedstocks. Results estimated that the higher biomass yield of microalgae due to cultivation in Co-HTL Aq-LP could promote the final bio-oil volumetric yield to 891 kg m−3, comparing to 819 kg m−3 using the synthetic medium.