Abstract:
Hydrothermal liquefaction (HTL) converts wet organic feedstocks (e.g., lignocellulosic biomass, food waste, sewage sludge, or algae) into bio-crude oil under subcritical water conditions (250 – 374°C, 5 – 20 MPa, 5 – 60 min reaction time), simultaneously addressing waste valorization and renewable energy production. However, the resulting HTL process water retains over 30% of feedstock carbon and contains complex organic compounds (e.g., phenols and nitrogen-containing species), posing environmental risks if untreated. Anaerobic digestion (AD) offers a subsequent solution by recovering energy through methane production while mitigating pollutant release. This dissertation systematically investigates the toxicity, biodegradability, and microaeration potential for enhancing the AD of HTL process water.
In the first part, a critical review reveals that a higher HTL severity (≥ 300°C) generates more recalcitrant and inhibitory organic compounds in HTL process water, with feedstock composition dictating relative concentrations of organic compounds. Comparative batch tests were conducted using HTL process water that was derived from two typical feedstocks at 350°C for 15 minutes – a food-waste proxy (i.e., dog food, rich in proteins) and wheat straw (rich in lignocellulose). Results demonstrated feedstock-dependent inhibition patterns: wheat-straw process water (WSPW) exhibited higher toxicity due to condensed aromatic compounds but achieved greater methane yields (54% of theoretical potential) compared to food-waste process water (FWPW) (37%). Notably, microaeration acclimation significantly enhanced methane yield from FWPW by 27% compared to strict anaerobic conditions, yet showed negligible effects on WSPW.
The second part evaluates continuous bioreactor performance under anaerobic versus microaerobic conditions. Three representative organics exhibited distinct degradation behaviors: 2-pyrrolidinone was fully degraded regardless of oxygen, while methyl-pyrazine persisted in both systems. Microaeration accelerated 4-hydroxyacetophenone removal initially, but fully anaerobic systems caught up after microbial acclimation. Under incremental FWPW loading from 0.92 to 2.76 g COD·L⁻¹, the microaerobic bioreactor exhibited progressive methane production rate enhancements of 3.9% (0.92 g COD·L⁻¹), 7.8% (1.84 g COD·L⁻¹), and 13% (2.76 g COD·L⁻¹), contrasting with the limited 8.3% increase exclusively at the maximum load (2.76 g COD·L⁻¹) in the anaerobic digester.
These findings demonstrate that microaeration selectively enhances anaerobic treatment efficiency for protein-rich HTL process water while preferentially degrading structurally susceptible organics. This feedstock- and compound-specificity highlights the importance of tailored strategies to optimize waste-to-energy conversion in HTL-AD systems.
Hydrothermal liquefaction