Ammonia-assisted thermal hydrolysis of sewage sludge: Solid and liquid phases characterization

Abstract

This work investigated the effect of ammonia solution on hydrothermal carbonization (HTC) of sewage sludge (SS). Ammonia addition promoted hydrolysis of the SS, and the hydrochar (HC) yield increased with increasing temperature but was still relatively lower compared to the reaction in water alone. The total organic carbon (TOC) of the aqueous phase was significantly improved at all temperatures relative to those observed in water alone but decreased with increasing temperature. N-heterocyclics were promoted as the main compounds of the aqueous phase from ammonia addition, while O-containing compounds were predominant from HTC in water alone. Pyrazine derivatives were the most represented in the aqueous phase, followed by pyrrole derivatives. The formed amines and the ammonia would promote reverse aldol condensation reaction products that would further promote various pyrazine derivatives. A fraction of these N-compounds would undergo polymerization to increase the N content in the HC with increasing temperature. The basic environment created by ammonia addition promoted insoluble phosphorus in the HC. The improved P content and reduced C/N ratio make this HC a potential material for soil amendment, and the high TOC and N-heterocyclic content in the aqueous phase make it suitable as an N precursor for value-added material synthesis.