Four-Pronged Preprocessing Approaches for Maximum Utilization of Raw Tannery Sludge by Earthworms

Abstract

The tanning industry is one of the most pollution-intensive and export-oriented industries. Transformation of raw hides into finished leather requires large volume of freshwater and consequently generates wastes in the form of tannery effluents and tannery sludge. The major challenges for safe disposal of tannery sludge being generated from the biological treatment of tannery effluent are linked to its large volume and its characteristics such as high water content, organic matter, chloride and heavy metals content. Vermicomposting employing earthworms has emerged as suitable technique for processing various organic wastes to produce valuable end product, i.e., vermicompost for agronomic applications. Preprocessing is warranted before utilization of raw tannery sludge by earthworms during vermicomposting due to toxicity owing to its characteristics. In this context, four-pronged aerobic preprocessing approaches based on (i) dry weight, (ii) wet weight and (iii) earthworm stress factor such as salinity of raw tannery sludge as well as (iv) water holding capacity of partially dried cow dung as bulking material have been attempted in this study to achieve its maximum utilization by Eisenia fetida. Results showed that a 13-day aerobic preprocessing period is the minimum requirement for the maximum utilization of raw tannery sludge by earthworms without mortality at 3:1 ratio (i.e., 75% tannery sludge and 25% partially dried cow dung) on wet weight basis. The maximum amount of raw tannery sludge that can be utilized by E. fetida was about 292.2 L with 9.22% solids content per 100 kg of partially dried cow dung with 42.13% moisture content.