Thermal Hydrolysis Pretreatment-Anaerobic Digestion Promotes Plant-Growth Biostimulants Production from Sewage Sludge by Upregulating Aromatic Amino Acids Transformation and Quinones Supply

Micromolecular plant-growth biostimulants (micro-PBs) production from sewage sludge is attracting increasing interest, as it is expected to enhance the fertilizing effect of sludge for land application. This study attempted to promote effective micro-PBs production from sewage sludge through thermal hydrolysis pretreatment-anaerobic digestion (THP-AD) and explore the underpinning regulation mechanisms. Results showed that the highest effective micro-PB production in digested sludge was achieved in THP­(160 °C)-AD by day 12, with 80.73 mg/kg volatile solid (VS) of phytohormones and 417.75 mg/kg VS of allelochemicals, and these effective micro-PBs all originated from aromatic amino acids (AAAs). The metabolomic and metagenomic results revealed that, as compared with THP­(120 °C)-AD and AD without THP, THP­(160°C)-AD uniquely upregulated AAAs biosynthesis and consequently improved AAAs metabolism toward effective micro-PBs production. Further exploration of related microbial pathways and metabolites suggested that the upregulated AAAs biosynthesis in THP­(160 °C)-AD in the early stage was partially attributed to the enhanced carbohydrate release. More importantly, the results showed that the amount of quinones, which probably facilitate energy generation via acting as electron-transfer mediators, was significantly positively correlated with the abundance of AAAs biosynthesis genes (R² = 0.93). Hence, the improved initial release and biosynthesis of quinones are critical in enhancing the AAAs biosynthesis in THP­(160 °C)-AD. Moreover, the enhanced quinones supply and the consequent active AAAs transformation in THP­(160 °C)-AD reinforced the humification process, highly supporting effective micro-PBs stabilization. The important roles of quinones in effective micro-PBs production and stabilization in sludge anaerobic digestion should be considered in technology development for micro-PBs recovery.