Integrating advanced oxidation processes into algal hydrothermal liquefaction biorefineries for aqueous phase management and nutrient recovery: A review

Microalgae-derived biofuels produced via hydrothermal liquefaction (HTL) offer a promising pathway toward sustainable transportation energy, yet the commercial viability of algal HTL biorefineries is constrained by management of the HTL aqueous phase (HTL-AP). This high-strength stream—COD typically exceeding 100,000 mg/L and dominated by recalcitrant Maillard-derived nitrogenous heterocycles, phenolics, and volatile fatty acids—precludes direct nutrient recycling and poses severe environmental risks upon discharge. The present review provides a mechanistic synthesis of current knowledge (2020–2026) on the molecular-level toxicity of HTL-AP and on the application of advanced oxidation processes (AOPs)—specifically wet air oxidation (WAO) and electrochemical oxidation (EO)—for targeted detoxification. WAO achieves up to 97.6% COD removal while preserving ammonium nitrogen, and EO attains 57–99.7% COD removal with concurrent cathodic hydrogen generation, enabling downstream nutrient recovery through struvite precipitation and ammonia stripping toward zero liquid discharge (ZLD). Techno-economic and life-cycle assessments confirm that AP-treatment strategy is the single most influential variable on minimum fuel selling price and on greenhouse-gas reduction. Notably, this review shows that virtually all reported AOP investigations employ transferable, non-algal HTL-AP (sludge-, food-waste-, or lignocellulose-derived), leaving dedicated microalgal studies conspicuously absent; together with the lack of integrated MCDA and pilot-scale AOP-to-ZLD validation, closing this translation gap is identified as an urgent priority for commercially competitive, circular algal biorefineries. An illustrative TOPSIS analysis with Monte Carlo robustness verification identifies, at a screening level, EO with boron-doped diamond (BDD) electrodes as the most robust AOP candidate under the present performance matrix, ranking first in 55.6% of 10,000 Dirichlet-sampled weight combinations.