Carbon Dioxide and Methane Emissions during the Composting and Vermicomposting of Sewage Sludge under the Effect of Different Proportions of Straw Pellets

Abstract

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).

Keywords

• thermophilic
• earthworms
• biosolids
• sewage sludge
• composting