Indigo is a cornerstone of denim production, yet its environmental impact remains shrouded in mystery—even within the industry itself. A new report from the Transformers Foundation called “A Reference for Water Consumption During Indigo Dyeing,” challenges outdated data on indigo dyeing, bringing clarity to the conversation.
The objective of the study was to define, establish and validate a reference standard for conventional or standard dyeing of denim warps, Transformers stated, adding that without a benchmark, mills’ best-intentioned efforts can be perceived as greenwashing. Transformers added that the report arrives at a critical moment as the EU Green Claims Directive, the UK Green Claims Code, and multiple lawsuits in the U.S. targeting misleading environmental claims are forcing stakeholders across the supply chain to support sustainability statements with measurable and transparent data.
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In the study, the foundation found that water consumption for the pre-dyeing is nearly consistent across the mills. However, Transformers began to see variability in the water used in the post-dyeing boxes for rinsing and washing.
Though different types of machinery make it impossible to estimate the exact difference between slasher dyeing and rope dyeing, Transformer said experts believe the latter uses roughly 30 percent more water due to the need to clean the ropes thoroughly. Manual adjustments by operators as well as the level of automation and control systems can also lead to variability in water usage data.
The study found the average quantity of water used for washing and rinsing ranged from 15 to 20 L/kg for rope dyeing and 10 to 15 L/kg for slasher dyeing. Implementing best practices may decrease this amount to 3.85 L/kg for rope dyeing and 3.40 L/kg for slasher dyeing, the report stated.
Indigo efficiency—or the percentage of fed indigo remaining on the yarn at the end of the dyeing process—for rope dyeing should be between 93-95 percent and 85-94 percent for slasher dyeing.
Transformers noted that the purpose of the study is to not champion one process over the other, highlighting the benefits of both. Rope dyeing’s high productivity and lower wastage makes it suitable for mass production, while slasher is generally lower cost, more efficient and delivers a greater variety of colors.
Seven mills from four countries participated in the two-phase study: Naveena Denim, Crescent Bahuman, Soorty and Diamond Denim from Pakistan; Orta Anadolu from Turkey; Candiani Denim from Italy; and Advance Denim from China. Machinery support was provided by Morrison Textile Machinery and Karl Mayer, with additional technical oversight from Bluesign and DyStar.
Fixed parameters were applied for a like-for-like evaluation of water usage across mills. Constants included rope and slasher dyeing, 100 percent cotton warp, ring spun NE 9 yarn, 100 percent pure indigo, pre-reduced liquid indigo, dark indigo with 4 percent indigo ratio, no additional chemicals and dry and wet crocking of the yarn.
