Bioleaching and Chemical Fractionation of Heavy Metals from Municipal and Industrial Dewatered Sewage Sludge

With global sewage sludge production projected to surge to 68 million tons by 2050 from the current 45 million tons of dry solids, the need for effective management is more pressing than ever. The complex composition of sewage sludge, particularly the presence of heavy metals (HMs), poses significant challenges for sustainable management. The improper handling of this metal-laden sludge can lead to HMs entering various trophic levels of the ecosystem, including humans and animals. Therefore, the removal of HMs from the sludge is a critical step before direct land application. The current study evaluated the potential of removing HMs (Cu, Fe, and Zn) from two different types of sludge samples. The municipal and industrial sludge samples were collected from the Nesapakkam Sewage Treatment Plant (NSTP) and Common Effluent Treatment Plant (CETP), Ranipet, Tamil Nadu, India, respectively. Bioleaching studies were carried out at a reactor working volume of 150 mL, and dry sludge (2 % (w/v)). The initial concentration of Cu, Fe, and Zn in the MSTP and CETP raw sludge were 304.91 mg/kg and 184.21 mg/kg; 2211.86 mg/kg and 1127.84 mg/kg; and 709.47 mg/kg and 476.71 mg/kg, respectively. The removal efficiencies of Cu, Fe, and Zn were 72.12 % and 67 %, 79.70 % and 77.98 %, and 89.01 % and 86.34 % for NSTP and CETP sludge, respectively. After bioleaching, the concentration of Cu (85.02 and 60.79 mg/kg), Fe (449.07 mg/kg and 248.4 mg/kg), and Zn (79.58 mg/kg and 65.10 mg/kg) in the treated sludge were within the permissible limit of Indian Standards for both types of sludge, making it safer for further disposal. Furthermore, chemical fractionation using the Sequential Extraction Procedure could be carried out to find the bioavailability of all three metals and the potential reasons behind different removal of the metals for both types of sludge samples. This study reveals that the proposed bioleaching process using sulphur-oxidizing bacteria effectively removes HMs, and produces sludge with reduced metal toxicity, thereby helping in sustainable sludge management.