Textile Dyeing ETP Chemicals | Colour, COD, Process Stages

Textile effluent composition varies dramatically between process stages and production batches. Scouring and desizing effluent contains high BOD from starch, PVA and CMC sizing agents, along with waxes and natural impurities from raw cotton. Dyeing effluent carries the highest colour load — reactive dyes on cotton are particularly difficult to treat because the chromophore molecules resist conventional coagulation. Printing effluent adds binder chemicals, thickeners and pigment particles. Finishing effluent contains softeners, resins, anti-crease agents and silicones that create foam and emulsify oils.

The combination of high COD, intense colour, fluctuating pH (often alkaline from scouring at pH 11–12, then acidic from dyeing at pH 4–6) and seasonal production volume changes makes textile ETP operation one of the most demanding in industrial water treatment. A chemical program that works for reactive dye effluent in summer may fail when the mill switches to disperse dyes on polyester blends in winter. Regular jar testing — at least monthly, and whenever dye recipes change — is essential for maintaining compliance.

Gujarat's textile clusters face additional pressure from ZLD mandates in certain industrial estates, CETP membership requirements and GPCB enforcement actions. Mills targeting water reuse for process washing or cooling tower make-up need tertiary treatment chemicals (activated carbon, advanced coagulation) beyond standard primary clarification. Understanding the full process flow and chemical requirements at each stage is the foundation of an effective textile ETP program.

Stage 1 — Screening and Equalisation: Raw effluent passes through bar screens to remove fabric scraps, yarn and debris. Equalisation tank homogenises flow rate and pollutant load over 8–24 hours. No chemicals are dosed at this stage, but defoamer may be added if foam from surfactants accumulates on the equalisation tank surface.

Stage 2 — pH Adjustment: Effluent pH is corrected to 7.0–8.5 using caustic soda (for acidic dye bath discharge) or sulphuric acid (for alkaline scouring discharge). Automated pH controllers with metering pumps maintain consistent pH before coagulation. Incorrect pH at this stage is the most common cause of coagulation failure in textile ETPs.

Stage 3 — Coagulation (Primary Treatment): PAC powder or ferric chloride is dosed into a flash mixer at 80–300 mg/L depending on colour and TSS load. PAC is preferred for reactive dye effluent due to superior colour removal. Ferric chloride may be added as a secondary coagulant when single PAC dosing fails CPCB colour limits. Rapid mixing for 1–3 minutes destabilises dye molecules and suspended solids.

Stage 4 — Flocculation: Anionic polyelectrolyte (1–5 mg/L) is dosed in the flocculation chamber with slow stirring for 15–30 minutes. Flocs grow larger and denser, preparing for gravity settling. Oil and grease from scouring effluent respond well to anionic polyelectrolyte with DE OIL POLY-type formulations.

Stage 5 — Clarification and Sludge Handling: Flocs settle in a primary clarifier or dissolved air flotation (DAF) unit. Sludge is collected, thickened and dewatered — cationic polyelectrolyte (1–3 mg/L) is often dosed at the sludge dewatering stage to improve filter press or centrifuge performance.

Stage 6 — Biological Treatment: Clarifier overflow enters aeration tanks where activated sludge microbes degrade dissolved organic matter (BOD reduction). Bacterial culture may be added during startup or after toxic shock. Nutrients (urea, DAP) supplement C:N:P ratios. Defoamer controls aeration tank foam.

Stage 7 — Tertiary Treatment and Discharge: Secondary clarifier overflow may receive activated carbon dosing (100–500 mg/L) for residual colour polishing, sodium hypochlorite (5–15 mg/L) for disinfection, and final pH adjustment before discharge to CETP, surface water or reuse system.

Coagulants — PAC powder is the primary coagulant for textile ETPs in Gujarat and across India. Dose range 80–300 mg/L for dyeing effluent, lower for finishing effluent. Ferric chloride (50–200 mg/L) serves as secondary coagulant or primary coagulant for heavy colour loads. Ferric alum is a lower-cost alternative where colour limits are less stringent.

Flocculants — Anionic polyelectrolyte (Deoil-Poly type) at 1–5 mg/L for primary flocculation. Cationic polyelectrolyte (DWP Poly Electro type) at 1–3 mg/L for sludge dewatering. Polyelectrolyte selection depends on effluent charge characteristics — jar test to confirm.

pH Adjusters — Caustic soda lye or flakes for raising pH from acidic dye bath discharge. Sulphuric acid for lowering pH from alkaline scouring effluent. Hydrated lime as an economical alternative to caustic soda where calcium hardness aids coagulation.

Specialty Chemicals — Defoamer (10–50 mg/L) for equalisation tanks and aeration basins. Activated carbon (100–500 mg/L) for tertiary colour polishing. Sodium hypochlorite for disinfection. Bacterial culture powder for aeration tank seeding and recovery. These specialty chemicals represent smaller volumes but are critical for specific treatment stages.

CPCB standards for textile industry effluent discharge to inland surface water require pH 5.5–9.0, BOD ≤ 30 mg/L, COD ≤ 250 mg/L, TSS ≤ 100 mg/L and colour ≤ 100 Pt-Co. Gujarat CETPs often enforce tighter limits on member industries to protect the collective CETP discharge quality. ZLD mandates require all water recovered through RO and evaporator systems, driving demand for advanced coagulation and activated carbon treatment.

Chemical cost typically represents 40–60% of total ETP operating cost in textile mills, with coagulant (PAC) and polyelectrolyte being the largest line items. Optimisation strategies include: monthly jar testing to right-size coagulant dose; switching from ferric alum to PAC where colour compliance requires it; consolidating chemical procurement to reduce logistics cost; and implementing equalisation to reduce peak coagulant demand during high-colour production batches.

Drashti Chemicals conducted a program optimisation for a Gujarat textile processor that reduced ETP chemical costs by 30% while improving colour removal consistency — see our textile ETP case study. Similar savings are achievable for mills willing to invest in jar testing, dose automation and consolidated chemical supply.