Harnessing microalgae from wastewater for biocrude via hydrothermal Liquefaction: A sustainable pathway to biofuels and biochemicals

The search for renewable alternatives has turned attention to microalgae, which grow rapidly, achieve high biomass productivity, and accumulate substantial lipids. Unlike terrestrial feedstocks such as corn or sugarcane, microalgae can be cultivated on non-arable land using saline or wastewater streams, avoiding competition with food supplies. Wastewater cultivation is particularly compelling because municipal and agricultural effluents supply nitrogen, phosphorus, and organic carbon, while algal growth removes excess nutrients that would otherwise drive eutrophication; reported removal efficiencies often exceed 90 % under optimized conditions. Algal–bacterial consortia further enhance performance by generating oxygen through photosynthesis and capturing CO2, thereby lowering aeration costs. The major challenge lies not in cultivation but in conversion. Drying biomass is energy-intensive, whereas hydrothermal liquefaction (HTL) transforms wet algal slurries directly into biocrude at 250–350 °C and 10–20 MPa. Typical yields range from 30 to 50 % of dry weight, with an energy density of 38–41 MJ/kg, along with nutrient-rich aqueous fractions, gases, and char that can be recycled. Key barriers remain in lowering oxygen and nitrogen content, scaling reactors, and reducing upgrading costs. This review evaluates the integration of wastewater-based algal cultivation with HTL for renewable fuel production, examining wastewater characteristics, cultivation strategies, biomass yields, HTL fundamentals and process advances, product upgrading, and techno-economic and environmental aspects. By linking wastewater remediation with biofuel production, the review highlights opportunities for nutrient recycling, greenhouse gas mitigation, and circular-economy applications, while identifying the technical gaps that must be addressed for practical deployment.