Acid leaching of hydrothermally carbonized sewage sludge: phosphorus recovery and hydrochar characteristics

The options for managing sewage sludge and its utilization as fertilizer are becoming progressively limited as a result of stringent environmental regulations imposed by the European Union over the past 10 years. The limitation of moisture present in sewage sludge that affects conventional treatment options like incineration can be obviated by using the hydrothermal carbonization (HTC) process. This research investigates the acid leaching of hydrochar produced by hydrothermally carbonizing sewage sludge. The objective is to investigate and compare the effects of formic acid (HCOOH), sulfuric acid (H 2 SO 4 ), and acetic acid (CH 3 COOH) at varying pH levels on total phosphorus (TP) mobilization and hydrochar properties. The impact of independent parameters such as carbonization temperature of hydrochar, acid type, acid concentration, and acid leaching retention time on the TP mobilization, chemical oxygen demand (COD) of the leachate, and the fuel characteristic of the hydrochar was explored. A quadratic and cubic model was proposed to correlate the effects of independent parameters on TP and ortho-P mobilization, acid need, COD of leachate, and fuel characteristics of hydrochar using Design of Experiments and Response Surface Modeling. This approach was chosen in order to maximize the amount of data from a constrained number of experimental trials. The outcome of the study indicated a fractional amount of H 2 SO 4 was enough to reach and maintain the lower pH in hydrochar slurry compared to HCOOH and CH 3 COOH. TP mobilization from solid to liquid is highly favorable in the presence of H 2 SO 4 at lower pH compared to HCOOH and CH 3 COOH under similar reaction conditions. In addition, it was discovered that lowering the pH using HCOOH and H 2 SO 4 to acid-leach the hydrochar boosted the caloric value of the hydrochar. However, CH 3 COOH has a contradictory effect.