Transformation and availability of nutrients and heavy metals during integrated composting–vermicomposting of sewage sludges

Transformation and availability of nutrients and some heavy metals were assessed during the integrated composting–vermicomposting of both primary sewage sludge (PSS) and waste activated sewage sludge (WAS) using matured vermicompost as indigenous bulking material and employing Eisenia fetida as earthworm species. Vermicomposting resulted in significant increase in total N (TN) (PSS: 41.7–64.6%, F=11.6, P<0.05; WAS: 36.4–58.6%, F=6.4, P<0.05), water soluble N (WSN) (PSS: 37.1–50.5%, F=30.1, P<0.05; WAS: 40.1–53.0%, F=27.6, P<0.05), total P (TP) (PSS: 39.9–69.8%, F=27.1, P<0.05; WAS: 32.2–56.6%, F=21.4, P<0.05) and water soluble P (WSP) (PSS: 25.2–34.3%, F=163.9, P<0.05; WAS: 24.1–34.2%, F=144.3, P<0.05) as compared to the initial compost material depending on different experimental conditions. The study demonstrated that the vermicomposting significantly improved the availability of nutrients in sewage sludges. In addition, vermicomposting considerably reduced the availability of heavy metals except Fe and Mn, presumably by forming organic-bound complexes in spite of several fold increase in their total content. The environmental conditions (i.e., temperature and relative humidity), in general, showed significant effect on the transformation and availability of nutrients and heavy metals. There was no effect of earthworm density on the transformation and availability of heavy metals and nutrients except N and P, possibly due to prior exposure during acclimation period in sewage sludge.

► Integrated composting–vermicomposting system for better treatment of sewage sludge.
► Vermicomposting significantly improved the availability of nutrients.
► Vermicomposting considerably reduced the availability of heavy metals except Fe and Mn.
► Significant effect of temperature and humidity on transfer and availability of nutrients and heavy metals.
► No effect of earthworm density on transfer and availability of heavy metals and nutrients except N and P.