Combined filtration and oxalic acid leaching for recovering phosphorus from hydrothermally carbonized sewage sludge

Hydrothermal carbonization is a promising pretreatment for the energetic utilization of municipal sewage sludge. However, the carbonized slurry must be dewatered and the valuable compound phosphate recovered. This article reveals a new approach of combined slurry filtration and subsequent phosphate leaching using oxalic acid (18-72 g l-1) in a single apparatus for the first time. A slurry of hydrothermally carbonized sludge was filtered, washed, leached twice with oxalic acid and washed again. A thorough characterization of the filtration performance and the process products was conducted. The suspension is difficult to filter, but the permeability of the filter cake increases during acidic leaching. The specific filter cake resistance of (4.5 ± 2.2) · 1015 m-2 indicates poor filterability of the hydrochar slurry. 41-83% of phosphate and sodium are leached from the hydrochar, while all other investigated inorganics are leached to a lower extent. 12-31% of iron, calcium and manganese are removed from the hydrochar. The relative contents of carbon, hydrogen and oxygen increase in the residual hydrochar after leaching. After leaching, oxalic acid accounts 10.5-16.9% of the dry hydrochar due to adsorption and poor cake washing. Co-leached calcium, bivalent iron and magnesium precipitate from the leachate as oxalate salts. In conclusion, combined filtration and phosphate leaching enables efficient phosphate removal and generates an ash-reduced hydrochar for energetic utilization. However, to enable its technical application, the filtration performance and system integration need to undergo further development.