By E. Dring
The footwear industry produces billions of shoes each year. Most of these shoes end up in landfill (Jadhav and Jadhav, 2020). Compostability could be a sustainable method of disposing of leather products in the future. Though, for the compost to meet the UK PAS 100 standards, several properties need to be controlled and implemented during the process.
What elements should be considered when making leather?
If leather was produced with compostability in mind, for its end-of-life process, when disposing of leather products, they could be sent to a composting company. Industrial composting could take place at the product's origin; however, this may involve overseas transportation which would be expensive and increase the product’s carbon footprint. Therefore, a third-party company would be beneficial.
To make the compost solely from leather; some green waste; inoculum; and water, a variety of leather products would need to contain the correct proportion of nutrients (i.e., a suitable C: N ratio and pH). Nitrogen could be added during the tanning process. However, microorganisms would still need to be introduced to begin the composting of leather. Alternatively, the process could be done if the leather were composted with other waste products, to produce high-quality compost. Potential pollutants in the leather would be diluted in this scenario.
Earthworms can be introduced during mesophilic composting to reduce the concentration of some heavy metals and improve the carbon: nitrogen ratio (Ravindran et al., 2014). However, it is important to consider the ecotoxicity of leather at the start of the process. Ecotoxicity refers to the polluting effects of substances on ecosystems. The chemicals used in the tanning process could be hazardous and not suitable for composting. For example, high concentrations of heavy metals and petrochemicals in leather could increase the concentration of toxic substances within the compost. Polluting compounds in soil, air and water can lead to phytotoxicity. When leather compost is used as a growing medium, the plants may experience abnormal growth from phytotoxins. Effects can include, failure to germinate, surface rooting, the abnormal colouring of leaves, stunting of plant height, and other deformations. These effects should be considered before tanning. Alternatively, depending on the chemistry of the leather, some plants may experience enhancing effects.
With the leather products currently in circulation, the chemistry of the leather would be difficult to identify. The composition of the leather product should be recognisable when entering a compost treatment plant, to avoid harmful chemicals entering the compost. This information could be recorded on a chip tag attached to the product, like RFID technology.
Leather products must be designed with their end of life in mind. Take a shoe, for example, will the entire shoe be biodegradable, or will components need to be removed before the composting process (Figure 1)? Metals are often magnetic so they can be identified and separated. Laces can be cut off and removed.
What will improve the compostability of leather?
The pre-treatment of leather will improve its compostability. The de-tanning of vegetable and chrome leather has been observed to increase the biodegradation of organics (Dhayalan et al., 2007). Sulphuric acid and thermolysis are methods used to break the linkage between chromium and collagen protein (Manavalan et al., 2015).
Hydrolysis is a reaction where hydrogen ions break apart chemical peptides (Kit and Thomason, 2006). Steam exposure is a fast and efficient method of extracting gelatine (Scopel et al., 2019). Metals that remain in the protein mixture can be washed out with water, then neutralised with hydroxyl groups and precipitated. The precipitated metals (e.g., chromium) can then be safely disposed of or recycled.
If hydrolysis is applied before composting, this would reduce the metal concentrations of the final compost product. Bacteria and fungi easily break down proteins. Unfortunately, this process does produce unpleasant smells from the release of amines and ammonia, and can often smell fishy, rancid, and sour. Protease and lipase enzymes could further increase the disintegration of the leather (Stefan et al., 2012).
Alternatively, remediation methods could be applied to the compost to remove hazardous compounds. Phytoremediation uses plants to improve the fertility of the land (Christopher et al., 2016) However, it would be more efficient to reduce phytotoxicity risk during the tanning process by avoiding certain chemistry. The substitution of contaminates to biodegradable non-hazardous materials can make a product more suitable for composting (Staikos et al., 2006).
Summary
It is important to consider the composition of leather during production, especially if it is to be composted at the end of its life. The leather should be non-toxic to microorganisms and could be supplemented with nutrients for plant enrichment. The product should be made with a tag, stating components suitable for compostability. Before composting, machines need to be designed to remove non-biodegradable components. Chemical processes could separate metal pollutants before composting the leather. The idea of industrial composting could improve the sustainability of leather.
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References
Christopher, J.G., Kumar, G., Tesema, A.F., Thi, N.B.D., Kobayashi, T. and Xu, K., 2016. Bioremediation for tanning industry: a future perspective for zero emission. Management of hazardous wastes. INTECH, pp.91-102.
Dhayalan, K., Fathima, N.N., Gnanamani, A., Rao, J.R., Nair, B.U. and Ramasami, T., 2007. Biodegradability of leathers through anaerobic pathway. Waste management, 27(6), pp.760-767.
Jadhav, N.C. and Jadhav, A.C., 2020. Waste and 3R’s in Footwear and Leather Sectors. Leather and Footwear Sustainability, pp.261-293
Kite, M. and Thomson, R. eds., 2006. Conservation of leather and related materials. Routledge.
Manavalan, F.D.M., Chellappa, M. and Chellan, R., 2015. Detanning of Chrome-ladden Collagenous Matrix for Protein Recovery from Tannery Solid Waste. Current Journal of Applied Science and Technology, pp.254-266
Ravindran, B., Contreras-Ramos, S.M., Wong, J.W.C., Selvam, A. and Sekaran, G., 2014. Nutrient and enzymatic changes of hydrolysed tannery solid waste treated with epigeic earthworm Eudrilus eugeniae and phytotoxicity assessment on selected commercial crops. Environmental Science and Pollution Research, 21(1), pp.641-651
Scopel, B.S., Restelatto, D., Baldasso, C., Dettmer, A. and Santana, R.M., 2019. Steam explosion as pretreatment to increase gelatin extraction yield from chromium tanned leather wastes. Environmental Progress & Sustainable Energy, 38(2), pp.367-373
Staikos, T., Heath, R., Haworth, B. and Rahimifard, S., 2006, May. End-of-life management of shoes and the role of biodegradable materials. In Proceedings of 13th CIRP International Conference on Life Cycle Engineering. pp. 497-502
Stefan, D.S., Dima, R., Pantazi, M., Ferdes, M. and Meghea, A., 2012. Identifying microorganisms able to perform biodegradation of leather industry waste. Molecular Crystals and Liquid Crystals, 556(1), pp.301-308
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