Removal of heavy metals from sewage sludge by sequential acidification with HCl–H₃PO₄ and precipitation with Na₂S–Ca(OH)₂

Considering the feasibility of using sulfide generated during anaerobic digestion (AD) of sewage sludge as a heavy metal (HM) precipitant, the removal of Cr, Ni, Cu, Zn, As, Cd, Hg, and Pb, from sludge via sequential acidification by nine acids and precipitation as sulfides and hydroxides was investigated. The results showed that HMs were dissociated with the highest efficiency at pH 1.0 using mixed HCl–H₃PO₄ and a 6 h reaction time. The eight HMs were then efficiently removed from the acidic centrifugate after the addition of the desired dosages of Na₂S and Ca(OH)₂, both of which acted as precipitants and raised the pH of the centrifugate to 12.47, satisfying the requirements for further AD treatment of the mixture of acidic centrifuged residual sludge (CRS) and HM-removed supernatant. The increase in soluble organic matter after acidification shortened the duration of further AD from 25 to 10 days. Compared with conventional AD, the proposed HM elimination pretreatment effectively reduced the residual content of eight HMs by 24.04 %∼96.53 % and improved the speciation distribution of HMs in the final dewatered sludge. Moreover, the removal efficiencies of soluble chemical oxygen demand (SCOD), total nitrogen (TN), and total phosphorus (TP) increased from 38.5 % to 73.7 %, 27.4 % to 36.2 %, and 21.8 % to 26.1 %, respectively, and the cumulative biogas production during AD increased from 41.12 to 70.38 L/(kg dry sludge). The treatment protocol proposed in this study provides an alternative strategy to address the challenge posed by HMs in the land disposal of massive amounts of sewage sludge.