Introduction
The textile industry is one of the largest consumers of water treatment chemicals worldwide. A typical textile mill uses 100-200 liters of water per kilogram of fabric processed, generating highly colored, high-COD wastewater that must meet increasingly strict discharge limits. PAC (Poly Aluminium Chloride) combined with PAM (Polyacrylamide) provides the most cost-effective primary treatment method for textile effluent. This guide covers dye classification, optimal treatment parameters, and troubleshooting common problems.
Textile Wastewater Characteristics
| Parameter | Typical Range | Discharge Limit (Typical) |
|---|---|---|
| pH | 6.0-12.0 (highly variable) | 6.0-9.0 |
| COD | 800-3,000 mg/L | 150-250 mg/L |
| BOD | 200-600 mg/L | 30-50 mg/L |
| TSS | 200-800 mg/L | 50-100 mg/L |
| Color (Pt-Co units) | 500-5,000 | Often “no visible color” |
| Temperature | 30-45°C | <40°C |
| TDS | 2,000-10,000 mg/L | 2,100 mg/L (varies) |
Dye Types and Their Coagulation Behavior
Not all dyes respond to PAC treatment equally. Understanding which dyes you’re dealing with is critical for setting treatment expectations.
| Dye Class | Solubility | PAC Coagulation Efficiency | Optimal PAC Dose (mg/L) | Notes |
|---|---|---|---|---|
| Reactive dyes | Highly water-soluble, anionic | Moderate to Good (60-85%) | 300-800 | Most common dye type. Hydrolyzed forms harder to remove. High basicity PAC (70-85%) works best. |
| Disperse dyes | Low solubility, suspended | Excellent (85-99%) | 100-300 | Easiest to remove — mostly suspended solids. Low basicity PAC sufficient. |
| Direct dyes | Water-soluble, anionic | Good (70-90%) | 200-500 | Linear molecule structure responds well to charge neutralization. |
| Acid dyes | Water-soluble, anionic | Moderate (50-75%) | 300-600 | Used for wool, silk, nylon. Small molecule size — harder to coagulate. |
| Vat dyes | Insoluble (applied as soluble leuco form) | Good to Excellent (80-95%) | 150-400 | Insoluble pigment particles after oxidation — behave like disperse dyes. |
| Sulfur dyes | Insoluble | Good (75-90%) | 200-500 | Contains sulfide — may need pre-oxidation before coagulation. |
| Basic (cationic) dyes | Water-soluble, cationic | Poor with PAC alone | — | Cationic dyes are not neutralized by cationic PAC. Use anionic PAM or activated carbon polishing. |
| Pigment (with binder) | Dispersed | Good (80-95%) | 150-400 | Binder chemistry affects removal — acrylic binders easier than polyurethane. |
Recommended Treatment Process
Stage 1: Pre-treatment
- Screening: 3-6mm mechanical screen to remove fibers, lint, fabric scraps
- Equalization tank: Minimum 8-hour retention for flow/load balancing. pH pre-adjustment to 7.0-8.0 with H2SO4 or NaOH as needed. Equalization is critical for textile effluent due to batch-to-batch variability.
- Cooling: If temperature >40°C, cooling tower or heat exchanger before chemical treatment. PAC performance degrades above 40°C.
Stage 2: Coagulation with PAC
- PAC type: High basicity (65-85%), spray dried, 28-30% Al2O3
- Dosage: 200-800 mg/L (determined by jar test on equalized sample)
- Rapid mix: 150-200 rpm, 1-2 minutes
- pH: Maintain 6.5-8.0 during coagulation
Stage 3: Flocculation with PAM
- PAM type: Anionic PAM, 12-18 million MW
- Dosage: 1-5 mg/L
- Slow mix: 40-60 rpm, 5-10 minutes
- Floc target: 2-5mm diameter, fast settling
Stage 4: Solid-Liquid Separation
- Primary clarifier or DAF unit. Surface loading rate 0.8-1.2 m3/m2/h for clarifier.
- Sludge: Typically 1-2% solids from clarifier. Dewater with filter press or belt press, condition with cationic PAM at 2-5 kg/ton dry solids.
Stage 5: Biological Treatment (Secondary)
- Activated sludge or MBBR for dissolved BOD/COD removal. Coagulation removes 30-50% of COD; the rest requires biological treatment.
- PAC-treated water has better biodegradability because toxic dyes are removed — biological stage performance improves.
Stage 6: Polishing (if required)
- For zero liquid discharge or reuse: UF + RO after biological treatment
- For strict color limits: Activated carbon column (granular, 8×30 mesh) or ozone oxidation
- PAC polishing dose (5-10 mg/L) before multimedia filter if residual color persists
Troubleshooting Textile Wastewater Treatment
| Problem | Possible Cause | Solution |
|---|---|---|
| Poor color removal | Reactive dyes with high hydrolysis; cationic dyes present | Increase PAC basicity (try 80%), add activated carbon polishing. Test for cationic dye stream segregation. |
| High residual COD after PAC | Dissolved organics (sizing agents, surfactants) not removed by coagulation | Normal — PAC removes 30-50% COD. Ensure biological stage is sized for remaining load. Consider powdered activated carbon addition. |
| Fine flocs, slow settling | Insufficient PAM or wrong PAM type | Increase anionic PAM dose. Test higher MW grade. Check mixing energy — over-mixing breaks flocs. |
| Sludge doesn’t dewater | Gelatinous Al(OH)3-dye flocs hold water | Add cationic PAM dewatering polymer. Consider adding fly ash or lime as filter aid for filter press. |
| pH too high after PAC (pH>9) | Highly alkaline dye bath carryover | Pre-adjust pH to 7-8 in equalization tank. Use acid (H2SO4) dosing with pH controller. |
| Re-stabilization (milky, won’t settle) | PAC overdose — particles re-charge positive | Reduce PAC dose. Run jar test to confirm optimal dose. Consider streaming current detector for real-time dose control. |
Case Example: Reactive Dye Treatment
Mill profile: Cotton knit fabric dyeing, reactive dyes (Remazol, Procion types), 800 m3/day, COD 1,500 mg/L, color 2,500 Pt-Co.
Treatment: PAC (75% basicity, spray dried) at 500 mg/L + anionic PAM (16M MW) at 3 mg/L.
Results: Color removal 88%, COD reduction 42% (from 1,500 to 870 mg/L), TSS from 450 to 45 mg/L. Sludge production ~0.6 kg dry solids per m3 treated.
After secondary biological treatment: Final effluent COD 160 mg/L, color <50 Pt-Co — meeting discharge limits.
HydroChemix supplies high-basicity PAC and anionic PAM specifically optimized for textile wastewater. Contact jingshuicc@gmail.com with your textile mill’s production processes and wastewater parameters for a customized treatment recommendation and free chemical sample for on-site testing.