Dyeing can take place at multiple stages within the supply chain, from dyeing fibres and yarns through to fabrics and garments. The environmental impact of dyeing is related to the type of dye, type of material being dyed, application method used, stage applied and overall desired effect. In general, dyeing requires significant amounts of water and energy due to the use of heated dye baths and rinsing baths. Many of the chemicals used in dyeing also present a concern.
Water is the biggest concern. The Global Leadership Award in Sustainable Apparel has reported that the global apparel industry uses over 5 trillion litres of water as a whole and 20 % of freshwater pollution comes from textile treatment and dyeing.
In a dyeing plant, a substantial amount of energy is required during production. A significant share of thermal energy can also be lost through wastewater, heat released from equipment, exhaust gas loss, idling, evaporation from liquid surfaces, unrecovered condensate, as well as loss during condensate recovery, and during product drying (e.g. by over-drying). These losses can be reduced by implementing different energy efficiency measures. Savings vary depending on various plant and process-specific factors, the type of fibre, yarn, or fabric used, the quality of raw materials, and the specifications of the product.
As factories can have very different set-ups, machinery and processes, the best practices also vary considerably and cannot be presented in full here. The following recommendations provide guidance which can be implemented in any size and type of dye house:
- Select quality dyestuffs and chemicals: It is a false economy to save small amounts of money on dyestuffs and chemical procurement that can then lead to huge reprocessing costs.
- Strive for accuracy: Weigh and measure all dyes and chemicals accurately and monitor closely water consumption and wastewater treatment.
- Analyse process performance: This improves quality and reduces unnecessary steps and excessive use of dyestuffs and chemicals (machine performance can be rationalised by adding a controller unit to monitor processes and operations around the dye house).
- Review suppliers: Request technical and process reviews from your dyestuff and chemical suppliers. The Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers (ETAD) is a good source for further information. Members are obliged to adhere to ETAD’s ethics code, which is based on the principles of responsible care. They must also comply with all national and international chemical regulations.
- Maintain equipment: To maintain quality standards and productivity, perform regular equipment checks, audit, calibrate and optimise all critical parts such as electrical drives and pumps, and review machine safety.
- Consult specialists: Seek guidance from suppliers and experts on the latest developments in dyeing, printing and finishing equipment, attend fairs and expos to pick up tips and ideas, and consider cost-effective upgrading or refitting options instead of investing in totally new equipment.
Different fabrics, different factors
More efficient dyeing techniques allow for both resource- and costs savings, while bringing environmental credentials to textiles processed in this way. Different techniques should always be evaluated according to the type of fibres and fabrics processed. For cotton, one of the most commonly used fibres in clothing and textiles, dyeing processes are often complex and use large amounts of water, energy, and chemicals. Reactive dyeing, traditionally used for cotton and other cellulosic fibres, requires vast amounts of salt, water, and energy. The process is quite inefficient, leading to low fixation rates (around 75 %). Repeat rinses are needed to remove the unfixed dye, increasing both water, and energy use as rinse water is often heated. Wastewater produced from reactive dyeing contains high levels of both salt and dye, which is difficult to treat. Reducing the amount of salt used in reactive dyeing not only reduces costs and environmental impacts, but also makes removal of unfixed dyes much easier.
There are also less impactful families of dyes for cotton-like high fixation reactive dyes (HFRD), which can achieve ‘fixing’ rates of between 85 and 90 % using half the amount of salt compared to conventional reactive dyes, according to reports. By increasing the fixation rates, the water usage decreases as fewer rinses are required. This also leads to energy and time savings. Using these dyes with a lower impact dyeing method, such as Cold Pad Batch, allows for further reductions in energy and water use.
Going even further, some breakthrough technologies like supercritical CO2 dyeing for polyester fabrics eradicate the need for water in the dyeing process. CO2 dyeing technology uses a totally water-free carbon dioxide process for dyeing polyester fabrics. This new technique puts CO2 under high pressure to make it ‘supercritical’. In addition to zero water use, as the CO2 cools, the dyed fabrics dry quickly without the need of additional heat. The CO2 is returned to its gaseous state with 95 % being recycled for subsequent use. It should be noted that these innovations often require potentially expensive new machinery. Savings can be achieved during production through reduced consumables and process steps.
To better understand the quantitative impact of dyeing processes, view MADE-BY’s online wet processing benchmark, which explains the range of water- and energy impacts of most common wet processing techniques. It can also be used as a tool to better understand their efficiency compared to industry norms.
Reference: https://www.resourceefficient.eu/en/measure/eco-efficient-textile-dyeing
