Contents lists available at ScienceDirect Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop Recycling of leather industrial sludge through vermitechnology for a cleaner environment—A review Ananthanarayanan Yuvaraj b , Natchimuthu Karmegam c , Balasubramani Ravindran d, *, Soon Woong Chang d , Mukesh Kumar Awasthi a, *, Soundarapandian Kannan e , Ramasundaram Thangaraj b, * a College of Natural Resources and Environment, Northwest A&F University, Taicheng Road 3#, Yangling, Shaanxi, 712100, China b Vermitechnology and Ecotoxicology Laboratory, Department of Zoology, School of Life Sciences, Periyar University, Salem, 636 011, Tamil Nadu, India c Department of Botany, Government Arts College (Autonomous), Salem, 636 007, Tamil Nadu, India d Department of Environmental Energy and Engineering, Kyonggi University, Youngtong – Gu, Suwon, 16227, South Korea e Proteomics and Molecular Cell Physiology Laboratory, Department of Zoology, Periyar University, Salem, 636 011, Tamil Nadu, India ARTICLE INFO Keywords: Leather industry Hazardous materials Earthworms Solid waste management Vermitechnology ABSTRACT In the last few decades, an enormous number of industrial sectors have been developed particularly, leather industries. The leather industries consume a large quantity of freshwater and various chemical substances during the leather making process and eject different solid waste materials, hides, buffing dust materials, and waste- water sludge. Besides, leather industrial wastes contain a significant amount of hazardous substances like heavy metals (e.g. cadmium, chromium, lead, nickel, and cobalt), aluminum sulfate, and magnesium oxide. Insecure disposal of wastewater sludge creates serious environmental problems in soil and groundwater. This situation warrants the easiest way to rationalize such conflicts through urgent environment-friendly disposal technologies. The main objective of the present review is to highlight the remediation of leather industry wastewater sludge through eco-friendly vermiremediation employing earthworms. Additionally, earthworms and microorganisms can eliminate toxic chemical substances. The present review also addresses earthworm species employed in a polluted environment, factors (i.e., pH, humidity, and temperature) and amendment materials essential for the stimulation of earthworm activity. In the hope of moving past this state, the earthworms are used for in-situ and ex-situ remediation of leather waste materials. The present review points to the real significances of vermi- treated leather wastes as plant growth-promoting medium that is useful in the field of sustainable agriculture. Furthermore, the fastidiously gathered evidence proves that vermi-treatment would be a potential alternative strategy to our surroundings with improved agroecosystems. 1. Introduction Leather industry plays a pivotal role in the world economy and produces different footwear, bags as well as garments. In the last dec- ades, about 1.7 billion square meters of leather products were produced globally with a value of around 40 billion dollars (Fratzl, 2008). Ap- proximately, 6.6 million tons of hides from domestic cattle and 0.8 million tons of skins from sheep are transformed into valuable leather materials every year (FAO, 2015). According to FAO (2015), Asian countries produce a large number of leather products (Fig. 1). About 3000 tannery sectors are found in India (Patel et al., 2016). To support the entrepreneur, the Government of India is providing research training through central leather institutes for the public and promotes leather industries in India. Moreover, leather industries dump a huge quantity of waste materials through red meat industries and convert them into leather products by complex processes (Hao et al., 2020). The leather manufacturing process is one of the tedious processes, especially tanning at the time of the leather products (Fig. 2). In general, the leather industries are having several options for tanning, such as ve- getable tanning (Madhan et al., 2001), synthetic tanning (Dasgupta, 1980), aldehyde tanning (Wojdasiewicz et al., 1992), and chrome tanning (Chagne et al., 1996). As illustrated in Tables 1 and 2, a wide spectrum of deleterious chemical substances is released at the end of the tanning process that pushes the ecosystem into a tight corner. Most of https://doi.org/10.1016/j.indcrop.2020.112791 Received 28 May 2020; Received in revised form 10 July 2020; Accepted 14 July 2020 ⁎ Corresponding authors. E-mail addresses: balasubramani.ravindran@tdtu.edu.vn, kalamravi@gmail.com (B. Ravindran), mukesh_awasthi45@yahoo.com (M.K. Awasthi), thangaraj@periyaruniversity.ac.in (R. Thangaraj). Industrial Crops & Products 155 (2020) 112791 0926-6690/ © 2020 Elsevier B.V. All rights reserved. T