Harnessing Activated Sludge for the Treatment of Hydrothermal Liquefaction Wastewater: A Proof-of-Concept Study

Although hydrothermal liquefaction (HTL) is the leading technology in converting wet biomass into bioenergy, the treatment of its toxic-laden aqueous phase wastewater presents a major challenge on its path toward commercial viability. This study presents the first-ever assessment of sewage sludge-fed HTL wastewater (SS-HTLWW) treatment and toxic compound removal using municipal activated sludge (AS) by optimizing its cultivation condition. It was found that AS with optimized pretreatment can remove up to 91.2% of the soluble chemical oxygen demand (sCOD) in SS-HTLWW, of which up to 82% can be attributed to biological mineralization and adsorption of sCOD by AS. Conventional bioprocess optimization techniques, including overliming, elevated temperatures, and nutrient supplementation, were found to raise the maximum rate of sCOD utilization (R_m) of AS treatment by 44%, 67%, and 45%, respectively. The variation in the maximum degradation potential (D_max) after 23 days of treatment across all groups was negligible. Adjusting the SS-HTLWW dilution factor from 20× to 10× resulted in no significant difference in R_m or D_max values due to the counteracting effects of high substrate and inhibitor concentrations. Additionally, AS was able to eliminate almost all N-heterocycles, phenolic compounds, and organic acids found in SS-HTLWW. This suggested that AS can both survive in and mitigate the high level of toxicity associated with SS-HTLWW; however, the high levels of recalcitrant COD after treatment may require further attention before it can be adequately discharged. The insights gained from this study are poised to interest engineers and treatment plant operators in search of efficient strategies for SS-HTLWW management and the broader application of HTL.