Evaluation of on-site biological treatment options for hydrothermal liquefaction aqueous phase derived from sludge in municipal wastewater treatment plants.

• HTL aqueous from mixed sludge cake at ≤ 340 °C was readily biodegradable aerobically. • HTL aqueous was 50 % and 32 % biodegradable under mesophilic and thermophilic AD. • HTL aqueous obtained at 360 °C was highly inhibitory for mesophilic/thermophilic AD. • HTL aqueous from mixed sludge cake was found unsuitable for dark fermentation. • HTL aqueous aerobic treatment and mesophilic AD were identified as feasible options. Aerobic treatment, mesophilic anaerobic digestion, thermophilic anaerobic digestion, and dark fermentation were evaluated for on-site biological treatment of municipal sludge derived HTL aqueous. For all four described batch test scenarios, municipal sludge-derived HTL aqueous samples obtained under 290–360 °C and 0–30 min retention time were used. In the aerobic respirometric tests, HTL aqueous samples resulted in a five-day biochemical oxygen demand range of 40.75 g/L (350 °C-25.6 min) to 54 g/L (325 °C-0 min). The calculated aerobic biodegradability index showed that approximately 50 % of the organics in HTL aqueous were easily biodegradable. Mesophilic and thermophilic biochemical methane potential tests resulted in specific yields of 151–179 mL CH 4 /g chemical oxygen demand (COD) and 103–122 mL CH 4 /g COD, respectively. HTL aqueous obtained under 360 °C-15 min condition caused total inhibition in both mesophilic and thermophilic anaerobic digestion. Possible causes for this inhibition were pyridine, pyrrolidinone, piperidinone, pyridinol, and phenolic compounds, which were higher in abundance in the 360 °C-15 min sample. HTL aqueous was found unfit for hydrogen production in dark fermentation due to inhibitory composition. In summary, on-site biological treatment of HTL aqueous was found to be most suitable under aerobic and mesophilic anaerobic conditions. [Display omitted] [ABSTRACT FROM AUTHOR]