Acidic hydrothermal carbonization of sewage sludge for enhanced alkaline extraction of phosphorus and reduced co-extraction of trace elements

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. Thus, this method offers co-treatment of two waste streams with simultaneous resource recovery.