Advanced treatment of hydrothermal liquefaction wastewater with nanofiltration to recover carboxylic acids

This study aimed to evaluate the innovative application of nanofiltration technology for the concentration and recovery of carboxylic acids from HTL wastewater. Four synthetic HTL wastewater solutions were processed by nanofiltration under both acidic (initial pH of ∼2.6) and alkaline (pH 8–10) environments. At alkaline pH, the permeate flux was much lower. A high DOC rejection of 82.5% was observed at pH 8 compared to a low DOC rejection of 17.3% at the initial pH of ∼2.6. Isobutyric acid was rejected to a higher degree relative to other carboxylic acids, exhibiting up to 95% rejection at pH 8. Phenol, butanone, and pyridine exhibited low rejection under the alkaline environment indicating that they can be separated from carboxylic acids. Nanofiltration technology presents a potential pathway to both concentrate and subsequently recover carboxylic acids from HTL wastewater.