Life cycle assessment of Indian silk
Introduction
The environmental impact of agriculture and textile production is of growing interest for legislators and consumers that demand environmental credentials of products and services. In order to quantify environmental impact and understand best ways to improve production processes, life cycle assessment (LCA), is often used as a decision support tool to compare production systems (e.g. conventional vs. organic farming), and analyse tradeoffs between them. LCA has been extensively used to understand the impact of fibre production and analyse advantages and disadvantages of manmade vs. natural fibres (Van der Velden et al., 2013). Despite its long history and unique properties as a biomaterial, what literature is available on the environmental impact of current silk production has to our knowledge never been synthesised using a common methodological framework such as LCA.
Silk is a natural fibre consisting of the protein fibroin, and used in textiles for at least 5000 years. Over 90% of commercially produced silk is extrusion spun by the domesticated silkworm Bombyx mori, a monophagous insect whose diet is restricted to the leaves of the mulberry tree. In regions such as South India, mulberry plants are harvested five to six times per year and used to feed silkworms in specialised rearing facilities. The silkworms go through 5 instars before spinning their cocoons, a process taking about 28 days. In India, once spinning is complete but before the moth emerges, cocoons are sold to reelers at regulated markets. In temperate regions storage of cocoons prior to further processing is necessary as generally only two crops a year are harvested. Cocoons are dried with hot air, killing the pupae and preventing eclosion of the moth. Importantly, correct drying increases both silk yield and quality, and is recommended practice even in tropical areas with year-round availability (Yong-woo, 1999). Reeling requires immersing the cocoons in hot water, in order to soften the sericin protein which binds the fibres together to form the tough cocoon shell. Softening enables brushes to find and pull the end of the silk filament. The free silk ends of several cocoons are attached to a reeling machine and unravelled onto spools. Finally, the silk is dried and re-reeled onto standardised spools. The resulting consolidated fibre is ‘raw silk’. Raw silk is an internationally traded commodity (United Nations, 2013) and further processing steps are largely similar to those of other textiles. Co-products generated in reeling are unreelable silk, sericin and pupae.
Detailed information on silk production methods is available from sericulture manuals (Dandin et al., 2003, Ganga, 2003a, Ganga, 2003b) and guidance to farmers provided by public sericulture extension agencies. In India this function is performed by the Central Silk Board and associated agencies, as well as state-level sericulture departments. Several surveys of sericulture are available, analysing productivity, profitability and yield gaps. Mulberry yields are usually assessed (Mattigatti et al., 2009; Reddy et al., 2008), input use is, however, rarely reported. Balasaraswathi et al. (2006) conducted a survey of 100 bivoltine sericulture farmers in Tamil Nadu, reporting fertilizer use, leaf yields and cocoon yields, one of the more complete snapshots of farm practices.
Few studies analyse the direct or indirect environmental impact of silk production. Akter et al. (1998) documented health and safety issues in small-scale silk production in Bangladesh. Fabiani et al. (1996) and Dai-gang (2013) documented emissions to water from the reeling and degumming processes, where the sericin ‘binder’ is removed, leaving only the silk fibre. Mande et al. (2000) and Shenoy et al. (2010) reported on energy and water use in the silk reeling industry in Karnataka, India. Vollrath et al. (2013) compiled a first life cycle inventory LCI of farm practices based on a pilot survey in Karnataka, focussing on energy use. Results showed that energy requirements for silk were above other natural fibres and farm practices diverged significantly from guideline values. A full LCA incorporating co-products has to our knowledge not been performed previously on either the agricultural or reeling aspects of silk production. Sara and Tarantini, 2004, Sara and Tarantini, 2003 performed a pilot life cycle assessment of silk yarn and fabric production.
Section snippets
Methods
The methodology employed in this analysis was LCA according to ISO 14040/44. We constructed models of mulberry cultivation, silkworm rearing, and silk reeling. The resulting life cycle inventory was parameterised using literature data from peer-reviewed publications and government reports and guidelines. Two sets of results are presented, for silk production according to published guidelines and for observed farm practices. Mulberry production is assumed to take place under irrigated
Impact of silk production following recommended practices
Table 3 summarizes the results of the impact assessment models for silk and other fibres from Ecoinvent that are used as a basis for comparison. The majority of environmental impact of sericulture is related to cocoon production (Fig. 2). Embodied solar energy of mulberry leaves and the wood used during cocoon processing account for the majority of R CED. Electricity for cocoon drying and irrigation are the main sources of non-renewable CED. Most GWP100 is related to fertilization and emissions
Impact of silk production
LCA results from different studies should be compared with caution, as not all follow the same methodology or have similar scope. It should also be noted that silk is a much higher value textile material than other natural fibres, with very different mechanical properties (Porter and Vollrath, 2009). Nevertheless, Ecoinvent datasets are constructed using similar methods, and compared with Chinese cotton, Nylon 6,6, and wool, our results show silk has a larger environmental impact across most
Conclusion
Even under recommended practices, on a mass basis the environmental impact of silk is higher than that of other fibres in most impact categories. Farm practices in Dharmapuri district diverge from recommended practices and environmental burden are correspondingly higher. A number of environmental issues identified, such as composting practices and cocoon drying, can be addressed relatively easily. Field emissions from fertilization in particular are more challenging, and scope for reducing
Acknowledgements
We would like to thank G.K. Rajesh for his assistance in answering numerous questions on the Indian silk industry. Thanks also to the European Research Council ERC-2012-PoC (324607) and SP-GA-2008 (233409) for generous funding.
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