Valorisation of harmful algae bloom (Enteromorpha prolifera) for polysaccharide and crude bio-oil production

[Display omitted] •Detecting curvature effect by 2 k factorial design with replicated center points.•High polysaccharide yield (12.1 wt%) and satisfactory antioxidant activities.•Re-utilizing the residual Enteromorpha prolifera (EP) for biocrude production.•The biocrude yield of residual EP (26.6 wt%, daf) was comparable with raw EP.•Hydrothermal co-liquefaction of residual EP and Chlorella sp. was feasible. The excessive growth of Enteromorpha prolifera (EP) could lead to the harmful algae bloom, making the disposal of EP (millions of tons) an inevitable problem. This study aimed to extract water-soluble polysaccharide from EP, followed by utilizing an emerging thermochemical technique (hydrothermal liquefaction, HTL) to generate crude bio-oil from residual EP. 2 k factorial design with replicated center points were employed to evaluate the necessity of using response surface methodology (RSM) to optimize polysaccharide extraction conditions. The curvature effect was identified to be temperature*temperature in this study, and the interaction effects (solid/water ratio*time & temperature*time) were also crucial for EP polysaccharide extraction. The maximal polysaccharide yield of 12.1 wt% was achieved under the optimal conditions (1:55 g/mL solid/water ratio, 70 °C, 60 min). The polysaccharide solution acquired under the optimal conditions had a DPPH radical scavenging rate of 24.3% and reducing power of 0.28. The residual EP after polysaccharide extraction was commonly discarded as waste, which was used as HTL feedstock for biocrude production in this study. HTL of residual EP resulted in a biocrude yield of 26.6 wt%, dry-ash free (daf) and a hydro-char yield of 22.4 wt%, daf, which were comparable with that of raw EP, suggesting the viability of obtaining value-added polysaccharide and biocrude simultaneously. We also co-liquified residual EP with microalgae (Chlorella sp.) under various mixing ratios, and satisfactory biocrude yield, chemical composition, higher heating value, energy recovery, and carbon recovery were achieved. The valorisation route developed in this study is of importance for full utilization of EP, thereby improving the economic viability and sustainability for EP biorefinery.