Rinsing should be designed as mass transfer, not a water quota

7.2 Unfixed dye and residual electrolytes define the separation load

Textile wet processing uses water in two distinct physical roles: first as a reaction and transport medium during preparation and dyeing, and second as a separation solvent during washing and rinsing.[S01][S02][S03] Across the literature reviewed by VTT Technical Research Centre of Finland, wet processes account for 60 to 90 percent of total water used in textile manufacturing, and washing follows almost every chemical step across desizing, scouring, bleaching, mercerising, dyeing, printing and garment finishing.[S01][S03] When a factory tracks only total groundwater extraction, which averaged 164 L/kg of fabric (with 136 L/kg in the dyehouse and 119 L/kg of wastewater generated) across surveyed Bangladeshi mills in one study and spanned 99.15 to 198.20 L/kg (averaging 138.26 L/kg) across long-term mill baselines in another, it conceals how much water is consumed not by the colouration reaction, but by multi-stage wash-off sequences designed to strip unfixed chemicals from the fibre.[S04][S05][S0702]

The magnitude of the post-dyeing separation duty is set by the fixation efficiency of the preceding dyebath.[S31][S36][S0702] In conventional exhaustion dyeing of cotton knit fabrics with reactive dyes, covalent bond formation with cellulose hydroxyl groups competes with alkaline hydrolysis in the bath.[S31][S36][S0702] Even when inorganic electrolyte (sodium chloride or sodium sulphate at 10 to 80 g/L depending on shade depth) and alkali (sodium carbonate or sodium hydroxide) are dosed to overcome cotton's negative surface potential and drive exhaustion, reactive dye fixation on conventional cotton remains between 50 and 70 percent.[S31][S0702] Consequently, 30 to 50 percent of the applied dye remains in the fibre pores and residual liquor as unfixed or hydrolysed dye alongside the entire mass of added salt and alkali.[S0702] Even in low-liquor-ratio batch dyeing at a 1:6 liquor ratio, structured reactive yellow dyes (MRY-1, MRY-2 and MRY-3) recorded fixation rates of 84.9, 71.1 and 75.0 percent respectively, leaving 15.1 to 28.9 percent of the dye load to be removed during wash-off.[S36]

If hydrolysed reactive dye or loose indigo pigment remains trapped in inter-yarn capillaries or adsorbed weakly on the fibre surface when fabric advances to drying, it migrates during laundering, perspiration contact or rubbing.[S31][S37][S0702] Rinsing therefore performs two coupled mass-transfer tasks: first, diluting and displacing the concentrated electrolyte and alkali in the retained liquor so that internal ionic strength and pH drop rapidly, reversing the substantivity that holds hydrolysed dye against the cellulose wall; and second, supplying thermal kinetic energy and a concentration gradient at the fibre-liquor interface so hydrolysed dye diffuses out of the amorphous regions of cotton into the bulk wash water.[S02][S31][S0702]

The stage-by-stage measurements in Exhibit 28 show how solute extraction progresses across a dark-shade reactive wash-off sequence on 160 g/m2 cotton knit fabric.[S0702] In the regular black dyeing route (5 percent on weight of fabric reactive dye with 80 g/L sodium chloride and 20 g/L sodium carbonate), the exhausted dyebath recorded pH 11.8, total dissolved solids (TDS) of 9,780 mg/L, electrical conductivity (EC) of 13,820 uS/cm, chemical oxygen demand (COD) of 1,842 mg/L, five-day biochemical oxygen demand (BOD5) of 870 mg/L and dissolved oxygen (DO) of 2.69 mg/L.[S0702] After bath drain (pH 10.7, TDS 4,740 mg/L, EC 8,540 uS/cm, COD 1,354 mg/L, BOD5 590 mg/L), a 10-minute hot wash at 60 degrees Celsius lowered EC to 4,120 uS/cm and TDS to 3,620 mg/L (COD 1,017 mg/L, pH 8.5), the first 10-minute boiling soaping at 90 degrees Celsius reduced EC to 1,760 uS/cm and TDS to 1,910 mg/L (COD 823 mg/L, pH 7.6), and the second 10-minute boiling soaping at 90 degrees Celsius brought the final bath to EC 930 uS/cm, TDS 1,150 mg/L, COD 497 mg/L, BOD5 210 mg/L and pH 7.3.[S0702]

7.3 Endpoints must be specified by product and chemical route rather than bath count

Exhibit 28 shows why a fixed rule of three hot washes and two soapings for every batch is an accounting failure disguised as standard procedure.[S02][S0702] In the same study, when cotton knit was pre-cationised with 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (EPTMAC) to eliminate salt addition during dyeing, the dark-shade dyebath entered wash-off at pH 10.9 with TDS 3,650 mg/L, EC 6,300 uS/cm, COD 1,186 mg/L and BOD5 480 mg/L.[S0702] Following drain (TDS 1,940 mg/L, EC 3,140 uS/cm, COD 923 mg/L) and one hot wash (TDS 1,060 mg/L, EC 1,120 uS/cm, COD 527 mg/L, pH 8.23), a single soaping step at 75 degrees Celsius reached a cleaner final bath (TDS 540 mg/L, EC 490 uS/cm, COD 319 mg/L, BOD5 140 mg/L, pH 7.3) than the conventional route achieved after two boiling soapings at 90 degrees Celsius, while maintaining wash and rubbing fastness ratings of 4 to 5 and cutting total water use across five shades from 53 to 89 L/kg down to 41 to 57 L/kg (a 20 to 37 percent reduction).[S0702]

Similarly, when salt-free and alkali-free exhaustion dyeing of cotton with C.I. Reactive Yellow 145 (RY 145) and C.I. Reactive Red 195 (RR 195) used tetra-sodium-1,3,6-naphthalenetrisulfonate (TNA) instead of inorganic salt and soda ash, followed by cold rinsing, hot rinsing and boiling soaping with 2 g/L non-ionic detergent at a 1:50 wash liquor ratio for 15 minutes, dyebath effluent COD, BOD and TDS fell by 99, 97 and 97 percent for RY 145 (from 2,212 mg/L COD, 410 mg/L BOD and 41,220 mg/L TDS down to 29 mg/L COD, 11 mg/L BOD and 1,120 mg/L TDS) and by 99, 99 and 97 percent for RR 195 (from 2,175 mg/L COD, 365 mg/L BOD and 42,630 mg/L TDS down to 31 mg/L COD, 5 mg/L BOD and 1,089 mg/L TDS), while achieving ISO 105-C06 wash fastness and ISO 105-X12 dry and wet rubbing fastness of 4 to 5.[S31]

A rinse specification must therefore define the target state at the end of washing rather than prescribing a fixed water quota.[S02][S31][S0701] In GFT's engineering grammar, every washing step is specified by four boundary variables: 1. Entering contaminant mass load on the textile, set by preceding bath concentration, fixation efficiency (71.1 to 84.9 percent at 1:6 liquor ratio in S36, or 50 to 70 percent in S0702) and retained liquor volume per kilogram of wet cloth.[S01][S36][S0702] 2. Substrate release specification, comprising neutralised fabric extract pH, zero residual oxidant or reductant where carryover harms downstream steps, and verified wash fastness (ISO 105-C06), rubbing fastness (ISO 105-X12) and CIELAB colour difference (dE) within tolerance.[S31][S37][S0702] 3. Final rinse bath residual threshold, expressed as maximum electrical conductivity (EC), TDS, optical absorbance (or Pt-Co colour) and pH in the final compartment or drain before unloading.[S0701][S0702] 4. External discharge and reuse compliance boundary, matching the segregated or composite stream against Zero Discharge of Hazardous Chemicals (ZDHC) Wastewater Guidelines (V2.2) or national limits such as Tunisian standards NT 106-002 and NT 106-003 (pH 6.5 to 9, TSS 400 mg/L, COD 1,000 mg/L, EC 7,000 uS/cm, TDS 5,000 mg/L).[S33][S0701]