Aerobic biological treatment of hydrothermal liquefaction process water of sewage sludge: Nitrification inhibition and removal of hazardous pollutants

Integrating hydrothermal liquefaction (HTL) in wastewater treatment is promising for converting sewage sludge into biofuels and fertilizers. However, challenges arise due to the ecotoxicity and nitrification-inhibiting properties of HTL process water. This study investigated the activated sludge treatment of HTL process water in continuous lab-scale reactors, focusing on the adaptive capacity of microbial communities and degradation of HTL-derived pollutants. Continuous activated sludge reactors were operated with process water up to expected levels of 145 mgCOD⋅L^-1. Results showed that prolonged exposure did not adversely affect the removal of organic matter, organic nitrogen and ammonium nitrogen. Activated sludge treatment with a hydraulic retention time of 15 h was able to reduce the nitrification inhibition potential in the effluent by 63-69 % compared to the influent. Furthermore, nitrification inhibition assays showed a 2.6 times reduced sensitivity in adapted versus non-adapted biomass. The nitrifying community composition was unaltered after several months of exposure, suggesting that heterotrophic degradation of inhibitory compounds protected the nitrifying community from the cytotoxic effects. Chemical analysis identified 40 pollutants in the process water, of which activated sludge effectively degraded amines, linear and cyclic amides, cyclic ketones, and hydroxy aromatics with removal efficiencies above 90 %. However, nitrogen-containing heterocyclic compounds, particularly alkylated pyrazines and methylquinoline, were more recalcitrant to treatment, with removal rates ranging from 10 to 80 %. Effluent concentrations for some of these compounds, ranging from 1 to 50 µg⋅L^-1, were close to or surpassed the predicted no-effect concentrations, raising concerns about potential environmental impacts of integrating hydrothermal liquefaction into conventional wastewater treatment plants.