Preservation of nitrogen and sulfur and passivation of heavy metals during sewage sludge composting with KH2PO4 and FeSO4

Composting is an effective method for treating sewage sludge. The aim of this work was to study preservation of nitrogen and sulfur and passivation of heavy metals during sewage sludge composting with KH2PO4 and FeSO4. The results show the loss rate of N decreased by 27.5% while that of S was increased by 32.1% compared with the control treatment during composting when KH2PO4 and FeSO4 were added. X-ray absorption near-edge structure spectra show that S was converted to a highly oxidizable state during sewage sludge composting with added KH2PO4. The mobility factors of Cu, Zn, and Pb after composting were found to decrease by 13.6%, 21.6%, and 3.8%, respectively, compared with those before composting when KH2PO4 was added. Adding these two materials to Cu and Zn inhibits Zn-3(PO4) (H2O)(4) and Cu-5(PO4)(2)(OH)(4) from transforming into more mobile forms, while adding these materials to Pb promotes Pb-3(PO4)(2) formation.