Co-liquefaction of microalgae and industrial wastewater: The impact of temperature on product yield and quality

This study explored the impact of hydrothermal liquefaction (HTL) temperature (120–240 °C) on the distribution and characteristics of products in a microalgae industrial wastewater system, highlighting its role in regulating hydrolysis, pyrolysis, and polycondensation processes. While the liquid phase yield remained dominant (96.07–97.48 %), it decreased by 1.36 % with increasing temperature, and the solid phase yield significantly increased at 240 °C due to cellulose dehydration and protein cross-linking. Aqueous phase analysis revealed that chemical oxygen demand (COD) peaked at around 10,000 mg/L at 240 °C, attributed to the enhanced dissolution of organic matter through lipid decarboxylation and the Maillard reaction. Ammonium nitrogen (NH₄⁺-N) increased notably above 210 °C, linked to protein deamination, while phosphate (PO₄³⁻) remained low due to inorganic phosphorus precipitation. Structural analysis of the hydrochar showed that elevated temperatures promoted carbon aromatization, with C-C bonds reaching their peak at 73.31 % at 180 °C and Graphitic-N content increasing to 38.19 % at 240 °C. Bio-crude oil composition analysis indicated that medium-chain alkanes (C6-C12) and oxygenated compounds were most abundant at 150–180 °C, while long-chain alkanes (C12-C20) dominated at 240 °C, and nitrogen compounds decreased due to ammonia volatilization. These findings suggest that 150–180 °C is optimal for bio-oil production, while 240 °C favors functional carbon material synthesis.