Exploring the chemical behaviors of dissolved organic matter to thermal hydrolysis temperature at the molecular level and its fate in anaerobic membrane bioreactor

Thermal hydrolysis pretreatment (THP) coupled with anaerobic membrane bioreactor (AnMBR) to enhance biomass bioconversion and methane production is a promising biotechnology. Herein, we shed light on the effects of THP temperature on molecular structure changes of dissolved organic matter of sewage sludge and food waste and its underlying mechanisms on hydrolysis, and methane bioconversion. The optimal THP condition was 160 °C, with a 1.87-times increase in soluble chemical oxygen demand (6.35 ± 0.09 g/L). FT-ICR MS indicated most of the compounds were biodegradable after 160 °C THP treatment, which had low aromatic or polarity, corresponding to protein/amino sugars and unsaturated hydrocarbon regions. Side reactions, like Maillard reaction and caramelization, induced the production of recalcitrant formulas with high hydrophobic and aromatic structure content (lower O/C and H/C values). These recalcitrant formulas attributed to carboxylic-rich alicyclic molecules (CRAM) exhibited poor biodegradability. For homologous DOMs sharing the same Kendrick mass defect (KMD), compounds exhibiting lower nominal oxidation state of carbon (NOSC), higher H/C ratios, and lower O/C ratios tend to exhibit greater biodegradability. Microbial analysis revealed that samples after THP pretreatment showed enhanced enrichment of both organic matter-degrading bacteria (e.g., Prolixibacteraceae, Anaerolineae and SJA-15) and methanogenic archaea (e.g., Methanosaeta, Methanobacterium, and Candidatus Methanofastidiosum) during the AD process. leading to a synergistic effect among microorganisms (such as Anaerolineae and Methanosaeta). Our findings highlight the interactive mechanism among molecular-level DOMs composition, microbial community succession, and AnMBR's performance, which provides a basis for an in-depth understanding of the THP strategy on anaerobic digestion.