Directional Sewage Sludge Transformation Using Acidic Hydrothermal Carbonization for Enhanced Alkaline Extraction of Phosphorus

Abstract

This study presents a two-stage process to recover phosphorus (P) from sewage sludge (SS) hydrochar, aimed at reducing trace element (TE) contamination. SS, mixed with Al-rich alum sludge (AS), underwent acidic HTC to convert Ca-P to Al-P. Alkaline extraction of hydrochar produced a P-rich, TE-deficient extract, as Al-P dissolves at high pH (> 12), leaving TEs insoluble. Optimal conditions for maximum P recovery were high temperature (~240°C), Al/P molar ratio (APMR) of ~4, and feedstock pH of ~3 – 4. This design process achieved 82% alkaline P recovery, 34% higher than the reference process. Overall P recovery ranged from 59 – 75% in the design process, compared to 30 – 37% in the reference process. Solid-state NMR revealed the Al-P association in hydrochar through surface complexation. TEs were mainly concentrated in the hydrochar, not the alkaline extract. This method offers a potentially cost-effective, and energy-efficient solution for P recovery.

Keywords: Phosphorus recovery, heavy metal, hydrothermal treatment, solid-state NMR, sequential extraction, biochar