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 BCF_As (r = 0.989, p < 0.011), BCF_Cd (r = 0.996, p < 0.004), BCF_Cr (r = 0.977, p < 0.023), BCF_Cu (r = 0.999, p < 0.000), and BCF_Zn (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.

Owing to rapid population growth, sewage sludge poses a serious environmental threat across the world. Composting and vermicomposting are biological technologies commonly used to stabilize sewage sludge. The objective of this study was to assess the carbon dioxide (CO₂) and methane (CH₄) emissions from sewage sludge composting and vermicomposting under the influence of different proportions of straw pellets. Four treatments were designed, by mixing the initial sewage sludge with varying ratio of pelletized wheat straw (0, 25%, 50%, and 75% (w/w)). The experiment was conducted for 60 days, and Eisenia andrei was used for vermicomposting. The results revealed that the mixing ratio influenced CO₂ (F = 36.1, p = 0.000) and CH₄ (F= 73.9, p = 0.000) emissions during composting and CO₂ (F= 13.8, p = 0.000) and CH₄ (F= 4.5, p= 0.004) vermicomposting. Vermicomposting significantly reduced CH₄ emissions by 18–38%, while increasing CO₂ emissions by 64–89%. The mixing agent (pelletized wheat straw) decreased CO₂ emission by 60–70% and CH₄ emission by 30–80% compared to control (0%). The mass balance indicated that 5.5–10.4% of carbon was loss during composting, while methane release accounted for 0.34–1.69%, and CO₂ release accounted for 2.3–8.65%. However, vermicomposting lost 8.98–13.7% of its carbon, with a methane release of 0.1–0.6% and CO₂ release of 5.0–11.6% of carbon. The carbon loss was 3.3–3.5% more under vermicomposting than composting. This study demonstrated that depending on the target gas to be reduced, composting and vermicomposting, as well as a mixing agent (pelletized wheat straw), could be an option for reducing greenhouse gas emissions (i.e. CH₄, CO₂).

The aim of this study was to evaluate the carbon dioxide (CO₂) and methane (CH₄) emissions during the composting and vermicomposting of sewage sludge under the effect of different proportions of straw pellets. Four treatments, including a control with three replicates, were designed to mix the initial sewage sludge with varying rates of pelletized wheat straw (0, 25%, 50%, and 75% (w/w)). Over a 60-day period, vermicomposting with Eisenia andrei treatments and composting were carried out. The results indicated that both composting and vermicomposting produce a significant (p < 0.001) amount of CO₂ and CH₄ emissions from all treatments. Vermicomposting significantly reduced CH₄ emissions by 18%, 34%, and 38% and increased CO₂ emissions by 75%, 64%, and 89% for the treatments containing 25%, 50%, and 75% straw pellets, respectively, compared to composting. However, CO₂ emissions decreased and CH₄ emissions increased during composting compared to vermicomposting. As a result of this finding, both composting and vermicomposting processes are recommended as an additive of pelletized wheat straw, depending on the target gas to be reduced.