Investigating the humification degree and heavy metal risk as influenced by anaerobic co-digestion of sewage sludge with fresh leachate

This study aimed at investigating the influence of anaerobic digestion (AD) on the humification of sludge–leachate matrix and the risk of heavy metals (HMs). The characteristics of humic acid (HA) and HM in different digestate (D1–D4) were examined after AD of sewage sludge (SS) with fresh leachate (FL) at varying ratios (i.e., 0, 5%, 15%, 30%). The results showed that C and O constituted the major elements of HA, with the content of 51.73–52.35% and 28.34–28.73%, respectively. Further analysis of the ultraviolet–visible and Fourier transform infrared spectra indicated that AD of co-substrates containing 5% and 15% of FL presented increased humification degree compared to the mono-digestion, whereas co-digestion with 30% of FL showed the opposite results. Moreover, the HM risk assessment suggested that co-digestion decreased the risk level of Cr, Cu, Zn, and Mn in D2 and D3 but increased that of Cr, Ni, Cu, and Mn in D4. From these findings, AD of SS with FL at relatively low proportions (i.e., 5%, 15%) was conducive to enhancing the humification degree and reducing the HM risk.