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 degrees 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 degrees C)-AD and AD without THP, THP(160 degrees 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 degrees 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 (R-2 = 0.93). Hence, the improved initial release and biosynthesis of quinones are critical in enhancing the AAAs biosynthesis in THP(160 degrees C)-AD. Moreover, the enhanced quinones supply and the consequent active AAAs transformation in THP(160 degrees 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.

Automated summary

This study focuses on the production of micromolecular plant-growth biostimulants (micro-PBs) from sewage sludge and explores the regulatory mechanisms involved. The results showed that the highest production of effective micro-PBs was achieved through thermal hydrolysis pretreatment-anaerobic digestion at a temperature of 160 degrees C. The effective micro-PBs originated from aromatic amino acids (AAAs), and the upregulation of AAAs biosynthesis was found to be crucial for their production. Furthermore, the study revealed the important role of quinones in enhancing AAAs biosynthesis and stabilization of micro-PBs during sludge anaerobic digestion.