Textile Industry Wastewater: The Battle Against Color and Chemicals
The textile industry is one of the largest water polluters. Learn about the complex ETPs needed to treat dye bath effluents, manage high salt loads, and achieve Zero Liquid Discharge (ZLD).
The textile industry consumes massive amounts of water and chemicals for dyeing, printing, and finishing fabrics. The breakdown of this effluent is notoriously difficult to treat.
The Composition of Textile Water
Textile effluent is a complex cocktail. It varies drastically between the "Dyeing" and "Sizing" stages.
| Stage | Pollutant Type | Impact |
|---|---|---|
| Sizing | Starch, PVA, Gums | Extremely High BOD (Biological load). |
| Scouring | Caustic soda, Wax, Fats | High pH and COD. |
| Bleaching | Peroxide, Chlorine | High Toxicity to bacteria. |
| Dyeing | Reactive/Vat Dyes, Salt | High Color and High TDS (Salinity). |
The Dye Problem: Reactive vs. Disperse
- Reactive Dyes: These are the worst for ETPs. They are highly soluble in water and resistant to light and biological attack. They require chemical breaking (Oxidation) to remove.
- Disperse Dyes: These are insoluble and can be removed more easily by physical settling or filtration.
The Treatment Process (ZLD Approach)
Because the water is so salty and colored, discharging it is often banned. Most modern textile clusters move toward Zero Liquid Discharge (ZLD).
Step 1: Physical & Chemical Pre-treatment
- Equalization: Large tanks to mix acidic and alkaline flows.
- Coagulation/Flocculation: Adding Alum or Poly-Aluminium Chloride (PAC). This "clumps" the dye particles and some of the COD into a sludge that settles in a clarifier.
- Oxidation: Using Ozone (O3) or Chlorine to "bleach" the remaining color that chemicals couldn't catch.
Step 2: Biological Treatment (The Aeration Tank)
High-efficiency bacteria consume the starches and soaps.
- Acclimatization: In textile ETPs, we must breed specialized bacteria that can survive the high salt levels (halophilic bacteria).
Step 3: Membrane Treatment & Recovery (The Heart of ZLD)
This is where the money is saved (and spent).
- Ultrafiltration (UF): Protects the next stage by removing all bacteria and micro-solids.
- Brine Recovery RO (Stage 1): Reclaims up to 80% of the water for reuse.
- High-Pressure RO (Stage 2): Pushes the remaining water to recover an additional 10-15%.
- Multiple Effect Evaporator (MEE): The remaining 5% of hyper-salty brine is boiled. The water is recovered as distilled water, and the salt is recovered as a solid.
Operational Costs (OpEx)
Running a textile ZLD plant is expensive:
- Chemical Treatment only: $₹20 - ₹40$ per $m^3$.
- Full ZLD (with Evaporation): $₹150 - ₹250$ per $m^3$. The high cost is mainly driven by the steam/electricity needed for the evaporator.
Challenges
- Cost: ZLD is incredibly expensive to run (especially the evaporator).
- Sludge: The chemical sludge produced in Step 1 contains metals and dyes, making it Hazardous Waste.
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