Migration and conversion of phosphorus in hydrothermal carbonization of municipal sludge with hydrochloric acid

Abstract

Purpose: Phosphate ore is a non-renewable resource, so finding a replacement is necessary. Municipal sludge has significant recycling potential because of its high phosphorus content and large discharge characteristics. Methods: The migration and transformation of phosphorus in sewage sludge treated with different concentrations of HCl were studied using the standards, measurements, and testing phosphorus extraction protocol from two aspects: phosphorus complexation and mineral form. Results: The results showed that more than 95% of phosphorus was concentrated in the solid products (hydrochar) after hydrothermal carbonization (HTC) without HCl, and the main form of phosphorus was organophosphorus (OP). With 0.5% to 2.5% HCl added, the phosphorus concentration of the liquid product (process water, PW) ranged from 13.14 to 219.41 mg/L, and the concentration of ammonia nitrogen (NH4+-N) increased by 0.32- to 1.88-fold. With the addition of 2.5% HCl, the phosphorus content in the hydrochar was 90% that of the original sludge, with a concentration of 64.17 mg/g, and the proportion of non-apatite inorganic phosphorus was approximately 94%. Conclusion: This study provides technical support for the recovery of phosphorus resources from municipal sludge.