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

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 CH4/g chemical oxygen demand (COD) and 103-122 mL CH4/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.

Biochar amendment rapidly shifts microbial community structure with enhanced thermophilic digestion activity.

Effects of powdered (<0.075 mm) biochar on thermophilic anaerobic digestion were investigated with biochemical methane potential (BMP) assays. The assays had substrate to inoculum ratios (SIR) of 2.2 and 4.4 g-volatile solids (VS)/g-VS and biochar dosing of 6 g/g-total solids (TS)inoculum. Compared to control, biochar amendment enhanced methane production rates by 94%, 75%, and 20% in assays utilizing substrates of acidified sludge at 70 °C, 55 °C and non-acidified mixed sludge, respectively. All controls experienced acute inhibition with lag phases from 12 - 52 days at SIR of 4.4 g-VS/g-VS, while assays with biochar generated methane from day 4. Biochar addition resulted in a rapid shift in microbial community structure associated with an increase in Methanothermobacteraeae (hydrogenotrophic) and Methanosarcinaceae archaea, as well as various volatile fatty acid (VFA)-degrading and hydrogen-producing bacteria. Biochar presents great potential to tackle VFA accumulation, abbreviate lag phase and increase methane rate, particularly at high organic loadings.