Textile mills are discovering that the cleanest fabric comes from controlling two simple levers: temperature and pH. Specialty detergents, peroxide neutralizers, and disciplined rinse sequences are cutting water use 10–30% while protecting whiteness and strength.
Scouring (alkaline cleaning that removes waxes, oils, pectins) and bleaching (oxidation that removes pigments) leave more than whiteness behind: residues from caustic soda, surfactants, hydrogen peroxide, stabilizers, and salts can cling to cotton and sabotage dyeing or even weaken cellulose if they’re not washed out thoroughly. Mills that neutralize leftover H₂O₂ and alkalinity reliably avoid uneven dye uptake and strength loss.
Regulatory pressure adds a hard boundary. Post‑bleach rinse water needs to fall within environmental pH norms of around 5–9 (www.mdpi.com), with many jurisdictions — including Indonesia — enforcing effluent limits of roughly pH 6–9 and tight BOD/COD (oxygen‑demanding load) controls. Inside the mill, cloth is typically brought to near‑neutral pH ≈6–7 before dyeing to prevent carryover of alkali or oxidizer (www.mdpi.com).
The practical upshot: wash baths must be tuned. Contemporary studies find temperature and pH drive removal efficiency, while specialized rinsing agents and neutralizers target what’s left behind. Final rinse pH near 6–7 is a common “bleach cleanup” target on the floor.
Specialty wash agents and neutralizers
Detergents and surfactants — low‑foaming anionic or nonionic types such as alkyl sulfates and ethoxylates — help dissolve and suspend the oils, greases, and waxes saponified during scouring. Wetting agents improve penetration in textured goods, while softening/leveling agents can reduce water spotting. In hard water, chelating/sequestering agents like sodium tripolyphosphate, EDTA, or phosphonates bind calcium and magnesium, preventing soap scum and aiding soil removal.
Where hardness management is designed upstream rather than only in‑bath, facilities may pair chelants with equipment options (see softener) to keep Ca/Mg load in check. The paper’s emphasis, however, is squarely on in‑bath chelation during rinsing.
After bleaching, the oxidizer must be “killed.” For alkaline hydrogen‑peroxide bleaching, mills use peroxide decomposers such as sodium bisulfite or proprietary agents like Rucorit INPK, an inorganic peroxide‑quenching auxiliary designed to convert H₂O₂ to water and O₂ under alkaline conditions, avoiding further oxidation of dyes or cellulose (www.racl.co.in), (www.racl.co.in). Reducing agents such as sodium dithionite or hydrogen sulfite can also neutralize trace oxidizer.
Enzymes are gaining share in “green” mills. Catalase, applied in a post‑bleach bath or even directly in the dye bath, decomposes residual H₂O₂. In one study, catalase‑treated bleaching allowed one wash cycle to be eliminated without altering final color (www.scielo.br), (www.scielo.br). The same work found that adding 0.4 g/L catalase to the dye bath yielded dye fixation comparable to multiple rinses and markedly reduced water use (www.scielo.br), (www.scielo.br).
To neutralize leftover alkalinity from scouring/bleaching, mills finish with a mild acid — often acetic acid or dilute sulfuric — titrating gently to avoid acid hydrolysis of cotton. Targets are slightly acidic to neutral, around pH 6–7; for instance, after an alkaline bleach at about pH ~11, the bath is cooled and acetic acid dosed to about pH 7 (pmc.ncbi.nlm.nih.gov). Facility effluent typically needs to land between about pH 5.5 and 9 for legal discharge (www.mdpi.com).
Temperature programming in wash baths
Temperature is the quiet workhorse. Scouring is run hot — often 80–100 °C — to fully saponify waxes and pectins; its rinse steps stay warm (60–80 °C) so fats remain soluble. Conventional hydrogen‑peroxide bleaching operates near boiling,≈95–100 °C, to maximize oxidation rates. Raising temperature accelerates reactions and lowers viscosity, aiding penetration.
Recent research quantifies the gains. In a trial with 0.9% glycerol triacetate + 1.0% peracetic acid activators, increasing peroxide bleach temperature from 75 °C to 80 °C significantly improved cotton whiteness (pmc.ncbi.nlm.nih.gov). An 80 °C/30 min bleach achieved a whiteness index of 61.09 (CIE Berger), about 97% of a 62.94 target normally obtained at 100 °C (pmc.ncbi.nlm.nih.gov). Put differently, dropping from 100 °C to 80 °C still delivered near‑full bleaching when activators were used (pmc.ncbi.nlm.nih.gov), (pmc.ncbi.nlm.nih.gov).
Operating windows are fairly tight in practice: scouring at 80–95 °C and pH≈11–12; bleaching around ≈95 °C and pH≈10.5–11 (pmc.ncbi.nlm.nih.gov). Rinsing follows suit: initial rinses after scouring/bleaching stay warm (40–60 °C) to keep surfactants dissolved and avoid starch gelling; cold final rinses (20–30 °C) may save energy but, if too cold, can precipitate soaps and slow neutralization. Many mills therefore insert an intermediate ~50 °C rinse before cooling. If the final rinse pH drifts above about ~7.5–8, additional acid is added to bring it back.
Multi‑stage rinse sequence and metrics
Effective rinsing is staged. A representative sequence: (1) hot rinse (60–80 °C) with detergent at pH ~9–10 to flush soaps and loose peroxides; (2) catalyst or scavenger bath — for example, ferrous sulfate or enzyme — at ~50 °C, pH 8–9 to decompose oxidizer; (3) water rinse (40–50 °C) to remove decomposition products; (4) acid rinse (30–40 °C) to neutralize to about pH ~6–7; and (5) one or two cold rinses (20–30 °C) for final washout. Catalytic H₂O₂ destruction by Mn or Fe salts is typically run near pH ~9 and 50–60 °C for speed and safety.
Quality control hinges on numbers. Properly scoured/bleached cotton approaches ~60–63 on the CIE whiteness scale (Berger). Using the optimized 80 °C bleach with activators above, finished whiteness reached 61.1 under D65 illuminant — essentially indistinguishable from a full‑boiler 100 °C bleach (pmc.ncbi.nlm.nih.gov). Tensile strength is monitored: incomplete removal of residual NaOH or stabilizers can cause up to 3–5% extra strength loss; proper neutralization keeps loss within the normal ~3–5% for bleaching.
Water efficiency and counter‑current rinsing
Rinse design is a water‑use lever. Counter‑current washing — feeding the last rinse water back to the previous stage — is repeatedly cited as an “inexpensive” way to cut fresh water in multi‑stage washing, including bleaching, yielding 10–30% savings (www.fibre2fashion.com), (www.fibre2fashion.com). Some facilities even reuse the spent alkaline bleach liquor: the hot residual bath — still rich in NaOH and heat — is held and later refreshed with peroxide for the next batch (www.fibre2fashion.com). Limits arise as impurities accumulate (metals like Fe), but case work suggests reusing bleach liquor can reduce heating costs and soda consumption by roughly 20–30% in large operations.
Sustainability metrics back these shifts. Mills report that optimized neutralization and rinse scheduling can drop effluent COD significantly; internal reports point to 50–70% reductions when advanced neutralization agents and tuned rinsing steps are deployed.
Controllers, sensors, and dosing
Real‑time control closes the loop. Mills employ online pH and temperature sensors and, in many cases, real‑time titration or sensor‑based dosing. Post‑bleach pH is often locked at 6.5–7.0 with titration controllers that feed acetic acid on demand. Hydrogen peroxide residuals are checked by iodometric tests; a standard target is “no detectable peroxide,” effectively less than 0.1 g/L before dyeing.
These routines depend on repeatable dosing hardware; titration control strategies typically meter acid via accurate chemical dosing systems such as a dosing pump. The instrumentation supports the core mandate: keep bleach near pH ~11 (pmc.ncbi.nlm.nih.gov), land final rinses near neutral, and ensure residual oxidizer and surfactants are minimal by the time fabric enters dyeing.
Trends, limits, and compliance
The trajectory is toward efficient, lower‑temperature, lower‑water rinsing without giving up quality. Enzyme aids such as catalase and chemical activators that enable lower‑temperature bleaching have demonstrated energy and water savings while maintaining whiteness and hand feel (pmc.ncbi.nlm.nih.gov), (pmc.ncbi.nlm.nih.gov). Regulatory drivers — including Indonesia’s limits on effluent pH, BOD, and COD — are pushing rinse processes to be tighter and more measurable (www.mdpi.com).
The bottom line in the data: counter‑current rinsing and reuse strategies deliver 10–30% water reductions (www.fibre2fashion.com), while optimized bleaching at 80 °C with activators reaches ≥97% of conventional whiteness (61.09 vs 62.94 CIE Berger) (pmc.ncbi.nlm.nih.gov). Catalase can eliminate an entire wash cycle without altering color, cutting water and chemical footprints (www.scielo.br), (www.scielo.br). And keeping effluent within about pH 5.5–9 to 6–9 remains the compliance guardrail (www.mdpi.com).
