Textile effluent is one of the hardest wastewater streams in Pakistan to treat — and one of the most regulated. A mid-sized integrated dyeing and finishing unit in Faisalabad, Karachi, or Lahore discharges 200–800 m³ per tonne fabric, with COD 800–3000 mg/L, colour 1500–4000 Pt-Co, salinity 5000–15,000 mg/L, and a pH that swings between 4 and 12 across the production day.
Provincial environmental quality standards (Sindh, Punjab) require COD < 150 mg/L, BOD < 80 mg/L, and "no visible colour" at discharge. NEQS is tightening across the next regulatory cycle; some buyer brands now require Zero Liquid Discharge (ZLD) as a condition of orders. Here is the chemistry that gets those numbers.
What makes textile effluent hard
| Stream | COD (mg/L) | Colour | Salinity | Dominant contaminants |
|---|---|---|---|---|
| Sizing / desizing | 3000–8000 | Low | Low | PVA, starch, CMC |
| Scouring / bleaching | 1500–3000 | Yellow | Moderate | Surfactants, peroxide residual, NaOH |
| Reactive dyeing | 800–2000 | Very high | High (Na₂SO₄ at 50–80 g/L) | Hydrolysed reactive dye, salt |
| Disperse dyeing (polyester) | 500–1500 | High | Low | Disperse dye, dispersing agent |
| Finishing | 200–800 | Low | Low | Softeners, crosslinkers, formaldehyde |
Each stream wants different chemistry. The standard mill ETP receives all of them mixed in an equalisation tank, which averages the contaminants but raises an averaging penalty — the chemistry has to be designed for the worst-case stream, not the average.
The treatment train
- Screening + grit + oil removal — physical pre-treatment
- Equalisation — 8–24 hour HRT; smooths pH, temperature, and load
- pH adjustment — typically H₂SO₄ for high-pH scouring streams, NaOH for acid streams; target pH 6.5–7.5 for downstream coagulation
- Coagulation + flocculation — PAC at 100–250 ppm + APAM or NPAM at 1–3 ppm; primary mechanism for colour removal
- DAF or primary clarifier — removes coagulated colour bodies and dye-PAC complexes
- Biological treatment — extended aeration activated sludge, MBBR, or SBR; reduces BOD and residual COD
- Secondary clarifier
- Tertiary polishing — sand filtration + activated carbon (residual colour and COD) or ozonation
- RO / membrane bioreactor if ZLD is required — produces clean permeate for reuse, concentrated brine for evaporation
Colour removal — the chemistry that matters
Reactive dye (the dominant chemistry in Pakistani cotton mills) is the hardest colour to remove because the dye is water-soluble and chemically stable. Three colour-removal mechanisms work in series:
- Coagulation with PAC at pH 6.0–7.0 — neutralises the anionic dye sulfonate groups, forms PAC-dye complexes that settle. Removes 60–80% of visible colour when dose is tuned.
- Biological decolourisation under anoxic conditions — sulfate-reducing or denitrifying bacteria cleave azo dye chromophores. Requires anoxic basin upstream of aerobic stage; reduces residual colour by another 40–60% but produces aromatic amine intermediates that the aerobic stage then mineralises.
- Activated carbon adsorption — polishing step for the last 5–15% of colour that survives biology. Typical AC consumption 0.5–2 kg per m³ effluent depending on residual dye load.
Alternative oxidation routes — ozone, Fenton reagent, electrochemical — are technically effective but capital- and operating-cost intensive. They show up in mills with buyer-mandated colour limits below 50 Pt-Co at discharge, where conventional chemistry cannot reach the spec.
COD reduction strategy
COD reduction is a two-stage problem: easily biodegradable COD (sugars, alcohols, simple organics) reduces in primary biological treatment to ~150–250 mg/L; recalcitrant COD (dye chromophores, surfactant residues, sizing polymer fragments) requires polishing.
- Activated sludge with extended HRT (24–48 hours) — gets to COD 200–400 mg/L on typical textile mix
- MBBR (Moving Bed Biofilm Reactor) — better for recalcitrant COD due to biofilm community diversity; reaches 150–250 mg/L
- MBR (Membrane Bioreactor) — replaces secondary clarifier with UF membrane; produces effluent at 50–100 mg/L COD and 5–20 mg/L BOD, suitable for reuse or final discharge
- Advanced oxidation (ozone, Fenton) — required for COD <100 mg/L on dye-heavy streams; expensive but compliant
Zero Liquid Discharge — when buyers force it
ZLD means no liquid effluent leaves the plant. Practically, it means recovering 95–98% of effluent as clean water for reuse, evaporating the remaining 2–5% brine to dry salt, and disposing of that salt to landfill or selling it (rare). The architecture:
- Pre-treatment — full conventional ETP, must deliver permeate at COD < 100 mg/L and turbidity < 1 NTU to the RO inlet
- RO stage — 70–80% recovery on first-pass brackish RO, concentrating the salt load from 8000 mg/L feed to 30,000+ mg/L reject
- High-pressure RO or membrane brine concentrator — further concentrates reject to 70,000–100,000 mg/L
- Evaporator (MVR or multi-effect) — boils off remaining water; produces salt cake at 95%+ dry solids
- Crystalliser — final dry-salt production; salt may be reusable in dyeing if Na₂SO₄ purity is acceptable, otherwise hazardous-waste disposal
ZLD adds 40–80% to ETP capex and 60–120% to operating cost. It pays back where buyer brand standards or provincial regulations make the alternative — losing orders, paying penalties — more expensive. Many Punjab and Sindh mills are 18–36 months from being forced into ZLD; the plants that start preparing now will have a cost advantage over those that wait for the enforcement event.
For textile-effluent treatment design, retrofit, or ZLD pre-feasibility studies, contact us. See also PAC on a Faisalabad textile mill, AD-402 biocide on textile dyeing, and textile-mill industry overview.