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Rethinking Textile Dyeing: The Chemistry Behind Colour and Change

Published on 
Author: TANVI MUNJAL

Tanvi Munjal

Textile dyeing is a water- and chemical-intensive stage of textile wet processing, with consumption varying significantly by fibre, process route, shade depth and equipment. A 2023 peer-reviewed field study of 18 textile wet-processing factories in Bangladesh reported average groundwater consumption of 164 L/kg of textile material, dyehouse water consumption of 136 L/kg and wastewater generation of 119 L/kg.

Yet the same chemistry that creates vivid hues is also driving a steady shift in industrial practice: new dye classes, bio-based colourants and waterless technologies are reshaping how the industry approaches colour.

The Chemistry Behind the Colour

Dyeing depends on how effectively colour molecules interact with and attach to textile fibres. Reactive dyes, the dominant dye class for cotton and other cellulosics, form covalent bonds with fibre hydroxyl groups under alkaline conditions.  This chemistry delivers excellent wash fastness but comes with a catch: a significant fraction of the dye hydrolyses before it can bond, especially at typical dyehouse temperatures and pH levels.  In exhaust dyeing, fixation rates of around 60–80% of the applied reactive dye are realistic, meaning a substantial proportion can remain unfixed or hydrolysed and enter subsequent wash-off streams.

To drive fixation, conventional dyeing relies on large quantities of electrolytes (often sodium chloride or sodium sulphate at concentrations of up to about 50 g/L) and alkalis.  

These auxiliaries help the dye move from the dye bath onto the negatively charged fibre surface, improving dye exhaustion and fixation. However, reactive dyeing can also generate high-TDS effluent because of the salts and other dissolved chemicals used in the process.

Environmental Footprint: What the Numbers Say

Water intensity varies significantly by fibre, dye class and equipment. EU Best Available Techniques (BAT) data also show substantial variation by process, with indicative ranges of 10–150 L/kg for batch fabric dyeing, 3–140 L/kg for batch yarn dyeing, 13–60 L/kg for loose-fibre dyeing and 2–16 L/kg for continuous dyeing. In the Bangladesh field study, specific water consumption ranged from 28 L/kg for denim dyeing to 285 L/kg at a knit composite factory, illustrating the effect of process configuration and operating conditions.

Regulatory Pressure and ZLD

India’s textile effluent standards do not impose a blanket ZLD requirement on every textile dyeing unit. The notified standards prescribe effluent limits for standalone large-scale textile units, while CPCB or SPCBs, with CPCB approval, may mandate ZLD for large-scale units in environmentally sensitive or critical areas.

The notified textile effluent standards specify an inorganic TDS limit of 2,100 mg/L for treated effluent. The permitted TDS level can be higher when the incoming water contains more than 1,100 mg/L of dissolved solids, provided the conditions set out in the applicable standards are met.

ZLD is designed to prevent routine liquid discharge by treating and recovering water from process effluent, with residual concentrate managed through further treatment and, where applicable, evaporation and crystallisation. A zero liquid discharge (ZLD) plant may use a combination of biological treatment, chemical or physical separation, membrane filtration and reverse osmosis. Evaporation and crystallisation may also be needed to manage the concentrated residual stream, depending on the wastewater and the system configuration.

Beyond the Vat: Emerging Technologies

Supercritical CO₂ dyeing. Dutch firm DyeCoo Textile Systems has commercialised a waterless dyeing process that uses supercritical carbon dioxide (scCO₂) as the dyeing medium for polyester.  In a closed-loop system, reclaimed CO₂ is pressurised to ~250 bar and heated to ~120°C, dissolving powdered disperse dyes and penetrating polyester fibres without process water or conventional aqueous dyeing auxiliaries.  DyeCoo states that its CO₂ dyeing process is available at industrial scale, uses reclaimed CO₂ in a closed loop, achieves more than 98% dye uptake, and requires no process water for the dyeing stage.

A key limitation is fibre scope: current commercial systems are optimised primarily for hydrophobic fibres such as polyester.

Microbial colourants. UK-based Colorifix uses engineered microbes to produce and fix pigments directly onto textiles. A third-party life-cycle assessment commissioned by Colorifix reported 77% lower water consumption, 80% lower chemical use and 53% lower electricity consumption compared with the conventional process tested.

Natural and bio-based dyes. Natural dyes remain a niche option in industrial textile colouration, while synthetic dyes continue to dominate large-scale production. Natural colourants can offer advantages in biodegradability and reduced reliance on some synthetic chemicals, but their environmental performance depends on factors such as extraction, mordants, auxiliaries, energy use and wastewater treatment. Barriers to wider adoption include variability in yield, colour fastness, shade reproducibility and production cost.

Low-liquor dyeing and digital printing. Low-liquor-ratio dyeing machines can reduce water consumption by lowering the volume of dye liquor required per unit of textile, although actual consumption depends on machine design, fibre, dye class, shade depth, liquor ratio and rinsing requirements.  Digital inkjet textile printing applies colour through controlled ink deposition rather than through an immersion dye bath during the printing step. It can substantially reduce water use compared with conventional screen printing, although the savings depend on the ink technology, substrate, pretreatment, fixation and washing requirements.

What Buyers and Brands Should Verify

Sustainability claims in dyeing are increasingly common, making verification important. When a manufacturer advertises “eco-friendly” or “low-impact” dyeing, ask for:

  • Water intensity metrics (L/kg fabric) benchmarked against relevant BAT values, together with chemical-use data and the defined process boundary.
  • Wastewater performance: Ask for recent third-party laboratory results for TDS, COD, BOD, colour and relevant restricted substances, together with the sampling date, sampling point and applicable discharge standard.
  • Regulatory documentation: Ask for the unit’s applicable Consent to Establish/Consent to Operate, consent conditions, recent regulatory compliance records and, where ZLD is specifically mandated, documentation demonstrating compliance with the applicable ZLD requirement.
  • Technology provenance: For waterless or low-water claims, confirm the technology being used, its supplier or developer, and whether it is operating at commercial scale.
  • Chemical conformance and certification: Ask chemical suppliers for ZDHC MRSL conformance documentation or relevant ChemCheck information, and for OEKO-TEX ECO PASSPORT certification where applicable.

The Path Forward

Colour will always be central to textiles, but the chemistry that delivers it is evolving. From supercritical fluids to microbial pigments, the next decade of dyeing will be shaped by processes that use less water, fewer salts and tighter recovery loops. For mills, brands and buyers, the priority is clear: demand data, verify claims and invest in technologies that deliver measurable environmental improvements rather than relying on marketing language.

Sources and further reading

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