Occurrence forms and leachability of inorganic species in ash residues from self-sustaining smouldering combustion of sewage sludge

Abstract This contribution reports the occurrence forms and leachability of major inorganic elements and heavy metals in the solid residue from smouldering combustion of sewage sludge (SS). Under the experimental conditions (moisture content: 50 wt%; sand-to-SS (wet) mass ratio of 4:1; and Darcy air flux: 3.5 cm/s), self-sustaining is achieved, destructing ∼98% of carbon in the SS and yielding a solid ash residue rich in Si, Al, P, Fe, K and Ca. The results from BCR (European Community Bureau of Reference) sequential extraction suggest that, majority of Na, K, Al, and Fe in the smouldering residue are presented as stable residue form, while Mg and Ca exist predominantly as acid-soluble. P in the residue is contributed by both reducible and residue fractions. All the heavy metals (As, Cr, Cd, Pb, Ni and Cu) in the residue are dominated by residue form, except Zn, which exists mainly as acid-soluble and reducible forms. Leachability tests demonstrate that considerable amounts of Ca (15.23—18.82%), Mg (11.92—16.74%), and K (5.91—7.04%) are readily leached out from the smouldering residue under pH 4.2—12.0. The concentrations of all the heavy metals in leachate meet the relevant regulatory values as determined by Toxicity Characteristic Leaching Procedure (TCLP). The smouldering residue generated here has potential to recover nutrients and is safe to be disposed via landfilling, addressing the environmental concerns that hinder the further development of smouldering combustion as a SS treatment technology.