Vermicomposting Technology as a Process Able to Reduce the Content of Potentially Toxic Elements in Sewage Sludge

Sewage sludge (SS) contains potential toxic elements (PTEs) that are harmful to the environment, and their bioaccumulation in the food chain is a major environmental health concern. Vermicomposting has been shown to reduce PTEs during composting of sewage sludge. However, the extent of PTE’s assimilation into the earthworm tissues during composting is largely unknown. The objectives of this study were to evaluate the potential of vermicomposting to decrease PTEs (As, Cd, Cr, Cu, Pb, and Zn) during composting of SS and whether the bioaccumulation of PTEs in earthworm tissue depends on feed quality. The initial SS was mixed in triplicate with varying proportions of pelletized wheat straw (PWS) (0%, 25%, 50%, and 75% (w/w)) along with a control (100% SS, no earthworms), and the variants were named VC1, VC2, VC3, VC4, and C0 (control), respectively. The experiment was conducted for 120 days using Eisenia andrei. In comparison to the control, mixing SS with PWS reduced Arsenic content by 14–67%, Cadmium content by 4–39%, Chromium contents by 24–77%, Copper content by 20–68%, Lead content by 39–75%, and Zinc content by 16–65%. The bioaccumulation factor’s (BCF) ranges were 20–80% for Arsenic, 20–60% for Cadmium, 6–16% for Chromium, 32–80% for Copper, and 37–115% for Zinc, demonstrating that the accumulation of PTEs in the earthworm tissues explains the low content of PTEs in the vermicompost. In terms of removal rate, the sludge mixtures with bulking agent can be arranged in the following order: VC4 > VC3 > VC2 > VC1. The total carbon loss showed a significant relationship with BCFAₛ (r = 0.989, p < 0.011), BCFCd (r = 0.996, p < 0.004), BCFCᵣ (r = 0.977, p < 0.023), BCFCᵤ (r = 0.999, p < 0.000), and BCFZₙ (r = 0.994, p < 0.006). The variant containing 75% PWS (VC4) appeared to be a suitable SS mixture to reduce PTEs. Hence, it is suggested that vermicomposting reduces the content of PTEs in SS.