Upgradation and valorization of hydrothermally liquified wastewater algae to recover multiple products

Abstract

Algae has long been studied as a source of fuel, but till date its commercialization does not seem feasible. The only way forward is developing an algal biorefinery that can generate revenues from recovering multiple products via different routes. Integration of wastewater bioremediation using algae and a biomass processing technique like hydrothermal liquefaction (HTL) to valorize the biomass shows potential of developing a sustainable biorefinery. In the current study, algal biomass cultivated in municipal wastewater was processed using HTL to produce 43 % biocrude and multiple bioproducts of high value. Efforts were made to reduce the biocrude's nitrogen content to 1.2 % (of TS) and oxygen to 22.3 % (of TS) using ZSM-5, produce methyl esters (33–53 %) using alcoholic solvents (methanol or butanol) and increase hydrocarbons (68 %) in the biocrude using CO₂ as the reaction gas. On the other hand, two bioactives: minoxidil-18.96 ± 0.24 mg kg⁻¹ (dried algal biomass basis) and ethosuximide-41.07 ± 0.42 mg kg⁻¹ (dried algal biomass basis) were synthesized in the aqueous fraction. Further, the same aqueous fraction which was also rich in calcium and phosphorus was hydrothermally mineralized to produce hydroxyapatite (8.5 %, dry biomass basis), a bone mineral. The results are encouraging to adopt HTL as a process to valorize algal biomass and extract multiple high value products. Using hydrothermal upgradation and mineralization strategies for the HTL biocrude and aqueous fraction, a sustainable biorefinery can be developed.