Hyperthermophilic pretreatment composting to produce high quality sludge compost with superior humification degree and nitrogen retention

Abstract

Composting of sewage sludge is widely used for stabilization treatment to realize the reduction of moisture and pathogen as well as the formation of humic substances. However, the major limitators were protein hydrolysis and functional bacteria activity. This study aimed to verify the potential of hyperthermophilic pretreatment composting (HPC) with HP at 120 °C for 30 min to improve sludge stabilization and to explore the key mechanism for enhancing sludge humification and improving organic nitrogen (N) retention. This study conducted the lab-scale facility of HPC in contrast to the control composting (CC) without thermal pretreatment. Surprisingly, HPC was more efficient at sludge degradation and phytotoxicity removal, further, the end sludge compost of HPC achieved 123.3% increase of the humification degree (HAC/FAC) and 14.2 % growth of organic N retention. Chemical analyses of extracellular organic substance indicated HPC had the more drastic protein transformation and HA polymerization. Solid-state 13C NMR and electrochemical measurements of HA indicated HPC enhanced HA’s aromatic structural development with greater quinones regeneration and condensation. Further mechanism analyses highlighted the essential effects of HP on stimulating HA’s precursors formation and functional bacterial community evolution. HP destroyed sludge EPSs allowing a 37.5% increase of proteins solubilization and a 112.6% enhance of HA release into SMPs; during which the quinones in HA exposed and activated thus improving redox properties to accelerate protein consumption towards new quinones regeneration, aromatic amino acids biosynthesis and metabolism. HP helped to inactivate Pseudomonas but accumulate thermophilic bacteria that enhanced proteins utilization to produce proteinaceous HA’s precursors with less N mineralization, facilitating the final enrichment of Actinobacteria with positive effects on N retention and humic polymerization.