Hydrothermal carbonization as an efficient tool for sewage sludge valorization and phosphorous recovery

Hydrothermal carbonization (HTC) of sewage sludge has been investigated in this study to demonstrate its capability to segregate phosphorous element and produce a solid energy dense material, i.e. hydrochar, which could find application as solid bio-fuel and/or soil amendment. In this study, centrifuged anaerobically digested sewage sludge (total solid content: 21 wt.%) from Trento North (Italy) waste water treatment plant was hydrothermally carbonized at 190 and 210 °C at 1 and 3 h of residence time. Hydrochars, recovered via filtration, were leached using 4 M HCl solution at room temperature to transfer phosphorous (P) and inorganic elements into the aqueous solution. Recovery of P was achieved via phosphate salt precipitation by alkalinisation, up to pH 9, of the acidic leachate using a 5 M NaOH solution. Sewage sludge and the corresponding hydrochars, before and after acid leaching, were characterized in terms of energy properties (HHV) by calorimetric analysis, P and inorganic elements content via ICP-OES. Results showed that HTC promotes segregation of P element in the hydrochar (up to 90 wt.% of the initial content on a dry basis at 190 °C, 1 h). The study demonstrated that P can be effectively recovered via precipitation via acidic leaching and subsequent alkalization with total recovery yields higher than 70 wt.%. The hydrochar residues, after leaching, showed inorganic elements content and energy properties compatible with their possible use as soil amendment in agriculture and/or as solid biofuel.