Post-treatment of Anaerobically Digested Hydrothermal Liquefaction Wastewater Using UV Photodegradation

Biocrude oil generation from microalgae and cyanobacteria through hydrothermal liquefaction generates an aqueous by-product (post-hydrothermal liquefaction wastewater (PHWW)) rich in organic matter and aromatic compounds. Although anaerobic digestion has been used with promising results, the process is insufficient to reduce the remaining recalcitrant compounds. Photodegradation using UV/TiO₂ was investigated as a post-treatment system of anaerobically digested-PHWW (AD-PHWW). The effects of initial pH and addition of H₂O₂ on the photodegradation efficiency in terms of chemical oxygen demand (COD), total phenolic content (TPh), and color removal were studied using a face-centered central composite design. Initial pH was highly influential for COD (p= 0.045) and TPh (p= 0.049) removal, whereas the addition of H₂O₂ had a higher impact on color removal (p= 0.000). Optimum conditions at pH of 9.6 and H₂O₂ concentration of 3.55 g L⁻¹ reached values of 50% for COD, 83% for TPh, and 95% for color after 240 min of irradiation. AD-PHWW photodegradation followed a pseudo-first-order kinetic. Energy recovery, including the biocrude oil from Spirulina, resulted in 41%. Results of ecotoxicity assays with Daphania similis and Eruca sativa Mill indicated that PHWW-treated samples were not negatively influenced by TiO₂/UV treatment.