Comparison of Nutrient-Release Strategies in Hydrothermally Treated Digested Sewage Sludge

Abstract

This work studies the fate of nutrients (N, P, and K) during the hydrothermal treatment of anaerobically digested sewage sludge to raise their concentrations in the liquid phase and facilitate their recovery as solid minerals by chemical precipitation. The hydrothermal process has been optimized by evaluating the temperature (170–230 °C) and reaction time (5–60 min) in an acid-free medium or with the addition of HCl (0.1–0.5 M). In the acid-free hydrothermal reactions, nutrients were mainly concentrated in the hydrochar, which were extracted with 0.5 M HCl (10% w/v). Following this route, 6.9 g N/kg, 13.8 g P/kg, and 8.8 g K/kg contained in the feedstock were extracted from the hydrochar produced at 230 °C, which, considering direct nutrient solubilization to process water by acid-free hydrothermal treatments, accounts for 82, 83, and 78% N, P, and K release, respectively. In the HCl-assisted hydrothermal treatment, the release of nutrients directly into the process water was improved and depended mainly on the acid concentration used and to a lesser extent on the reaction temperature. Operating at 230 °C and 0.5 M HCl, the release of 98% N (more than 45% as NH4–N), 87% P (as PO4–P), and 70% K contained in the feedstock was achieved in the process water. Chemical precipitation of phosphorus and nitrogen from the process water allowed the recovery of a solid identified as crystalline struvite, with high contents in P, Mg, and NH4–N and negligible heavy-metal content. The estimated cost of digested sewage sludge treatment could reach 13.7 euros per tonne, considering the energy inputs required in the hydrothermal treatment.