Effective nitrogen removal and recovery from dewatered sewage sludge using a novel integrated system of accelerated hydrothermal deamination and air stripping

In order to reduce considerable emissions of N-containing pollutants from combustion of sewage sludge derived solid fuel, an integrated system of hydrothermal deamination and air stripping was developed to effectively remove and recover nitrogen from dewatered sewage sludge (DSS). Three characteristic hydrothermal regimes contributing to deamination were identified. Initial hydrolysis of inorganic-N and labile protein-N was responsible for ammonium (NH4₊-N) released below 300 °C/9.3 MPa, whereas deamination of pyridine-N dominated when being raised to 340 °C/15.5 MPa. At 380 °C and 22.0 MPa, remarkable deamination of stable protein-N occurred, which was accompanied by formation of more heterocyclic-N compounds and resulted in 76.9% N removal from DSS and 7980 mg/L NH4₊-N solution. As a result of catalytic hydrolysis and cracking, calcium oxide additive not only accelerated deamination of stable protein-N, pyrrole-N, and pyridine-N, but also favored transformations of protein-N and quaternary-N to nitrile-N and pyridine-N, respectively, leading to 86.4% total N removal efficiency. The nitrogen transformation reactions and conversion pathways during hydrothermal deamination were proposed and elaborated in detail. Moreover, an efficient air stripping process was coupled to remove and recover ammonia from liquid fraction via ammonium sulfate. Consequently, this system achieved an overall N recovery rate of 62%.