Efficient phosphorus recycling and heavy metal removal from wastewater sludge by a novel hydrothermal humification-technique

This publication presents a simple and low-cost hydrothermal humification (HTH) treatment of sewage sludge (SS) together with alkali ash (AA) and biomass for simultaneous implementation of heavy metal removal, nutrient recovery (P) and ash refining. The H/C and O/C atomic ratio plots obtained from elemental analysis demonstrate that dehydration and decarboxylation under hydrothermal conditions are elemental reactions leading to sludge/biomass decomposition and artificial humic matter formation. Introduction of plant biomass into sludge-derived samples and adjustment of KOH or alkali ash (AA) mass effectively improve the recovery of P element, realizing high contents of dissolved phosphorus (DP) (from 7045 to 10075 mg/L) at appropriate pH values (6.5 to 7.7). ICP-AES results indicate the drop of Cr and Cd content below detection limit together with a sharp decrease of the elements Cu (from 0.07 to 0.46 mg/g), Zn (from 0.15 to 0.98 mg/g) and Pb (from 0.067 to 0.142 mg/g) after HTH treatment in sludge-derived liquid products and the heavy metal elements enriched in sludge-derived solids can be recovered into industrial salts by subsequent treatment. Pot planting experiments are conducted to investigate the P-availability in both sludge-derived liquids and solids (after treatment of heavy metal recovery) for promotion of plant growth. A higher proportion of shoot-to-root weight (62.1% versus 46.2%) and preserved moisture contents (84.7% versus 83.7%) when compared to the control groups demonstrate the effect of the presence of more nutrients after addition of sludge-derived liquid products. This work could provide a smart, energy utilization and sustainable fertilization route for planting growth.