Applications of PAC in Water Treatment — Potable Water, Wastewater, Swimming Pools, Textile Dyeing
5 Key Points to Remember:
- Potable water treatment: PAC dosage 15–40 mg/L, removes turbidity, organic matter, bacteria, heavy metals
- Wastewater treatment: dosage 50–300 mg/L, combined with anionic polymer to enhance flocculation efficiency
- Swimming pool water: low dosage 5–10 mg/L — used only as a coagulation aid, not a substitute for chlorine disinfection
- Textile dyeing: PAC + cationic polymer removes 90–95% of reactive dye color
- All dosages are reference values — jar tests with actual plant water sources are required
1. PAC in Potable Water Treatment (Drinking Water)
1.1. Role of PAC in Potable Water Treatment Process
PAC is the primary coagulant in the coagulation — flocculation — sedimentation — filtration process of potable water treatment plants. Objectives: remove suspended solids (TSS), turbidity, dissolved organic matter (DOC), bacteria, and partially remove heavy metals from raw water (river, lake, well).
1.2. Mechanism of Action
When PAC is dosed into raw water, the aluminum polymer chains neutralize the negative charge of colloidal particles (clay, humus, microorganisms), forming Al(OH)₃ flocs sized 0.5–2mm. These flocs settle to the bottom of the sedimentation tank within 60–120 minutes, removing ≥95% of suspended solids. The remaining solids are retained in the sand filter.
1.3. Dosage and Operating Conditions
| Parameter | Typical Value | Notes |
|---|---|---|
| PAC 31% Dosage | 15–40 mg/L | Depends on raw water turbidity |
| Raw Water Turbidity | <150 NTU → 15–25 mg/L; >500 NTU → 30–50 mg/L | Dosage increases with turbidity |
| Optimal pH | 6.5–7.5 | Adjust with NaOH or HCl if needed |
| Rapid Mixing Time | 1–3 minutes | 80–150 RPM |
| Slow Mixing Time | 15–30 minutes | 20–40 RPM |
| Sedimentation Time | 60–120 minutes | Surface loading rate 1–2 m³/m²/h |
| Residual Al after Treatment | <0.2 mg/L | Meets TCVN, significantly lower than alum |
1.4. Advantages Over Alum in Potable Water Treatment
- Wider pH operating range (5.0–9.0) — less pH adjustment required compared to alum
- 30–50% less sludge production → reduces sludge treatment costs
- Lower residual Al → safer for consumers, especially important for drinking water
- Stable performance during rainy seasons (high turbidity, low temperature) — a weakness of alum
2. PAC in Industrial Wastewater Treatment
2.1. Textile Dyeing Wastewater Treatment
Textile dyeing wastewater contains reactive dyes, disperse dyes, starch, surfactants — COD 800–2,000 mg/L, color 500–3,000 Pt-Co.
Typical Treatment Process:
- PAC 100–200 mg/L → color coagulation, rapid mixing for 2 minutes, slow mixing for 15 minutes
- NaOH → raise pH to 7.0–8.0 (if needed)
- Anionic polymer 0.5–1.0 mg/L → forms large flocs, increases settling speed
- Sedimentation for 60 minutes → clarified water achieves color <50 Pt-Co (85–90% reduction)
For wastewater containing vat dyes or sulfur dyes — harder to coagulate — combine PAC + FeCl₃ or PAC + cationic polymer to achieve >90% color removal efficiency.
2.2. Electroplating Wastewater Treatment
Electroplating wastewater contains heavy metals: Cr⁶⁺, Cr³⁺, Ni²⁺, Zn²⁺, Cu²⁺. PAC precipitates these ions as metal hydroxides at pH 8.0–9.5:
- Cr³⁺ → Cr(OH)₃ ↓ (pH 8.0–9.0)
- Ni²⁺ → Ni(OH)₂ ↓ (pH 9.0–9.5)
- Zn²⁺ → Zn(OH)₂ ↓ (pH 8.5–9.5)
- Cu²⁺ → Cu(OH)₂ ↓ (pH 8.0–9.0)
PAC dosage: 100–250 mg/L. Cr⁶⁺ must be reduced to Cr³⁺ using NaHSO₃ or Na₂S₂O₅ before PAC application.
2.3. Food Processing and Seafood Wastewater Treatment
Food processing wastewater characteristics: high COD (1,000–5,000 mg/L), high TSS, oily. PAC is used in the coagulation — flotation (DAF) unit to separate oil and TSS before biological treatment.
Typical dosage: 150–300 mg/L PAC 31%, pH 6.0–7.0. After coagulation + DAF: COD reduced by 40–60%, TSS reduced by 80–90%, reducing load on subsequent aerobic tanks.
3. PAC in Swimming Pool Water Treatment
Swimming pool water needs to be clear and clean, but standard sand filters only retain particles >20 microns. Superfine particles (1–5 microns) — dust, dead skin cells, cosmetics — pass through the sand layer and return to the pool, causing cloudiness.
PAC acts as a coagulation aid: binds superfine particles into flocs large enough for the sand filter to retain. Typical dosage: 5–10 mg/L, dosed directly into the circulation line before the filter. Perform 1–2 times/week or after heavy pool usage.
Important Note: PAC does not disinfect — maintain free chlorine at 1.0–3.0 mg/L separately. Use PAC after adjusting pH to 7.2–7.6.
4. PAC in Textile Dyeing Industry (Dye Coagulation)
In the textile dyeing process, PAC plays a dual role:
Role 1 — Mordant in the dye bath: Al³⁺ ions from PAC form complexes with dye molecules and cellulose/protein fibers, ensuring better and more even color fixation on fabrics. This is particularly important for natural dyes and reactive dyes.
Role 2 — Dye coagulation in wastewater treatment: PAC removes residual dyes and dyeing auxiliaries from wastewater. Mechanism: polymer bridging between dye molecules → forms colored flocs → sedimentation. Combine PAC 100–200 mg/L + cationic polymer 0.5–1.0 mg/L for up to 95% reactive dye removal efficiency.
5. PAC in Paper and Pulp Production
In paper machines, PAC is used as a retention aid and drainage aid:
- Retention Aid: Retains fine cellulose fibers and fillers (CaCO₃, kaolin, TiO₂) on the wire, preventing loss into white water. Dosage: 0.5–2.0 kg/ton of paper.
- Drainage Aid: Helps water drain faster through the wire → increases machine speed, reduces steam consumption.
- White Water Treatment: PAC 50–100 mg/L coagulates fine fibers and fillers in white water → recovery and recycling back into the process.
Advantages of PAC over alum in paper production: less impact on system pH, no need for acid addition, reduces equipment corrosion.
6. Frequently Asked Questions
What is the PAC dosage for potable water treatment?
Typical PAC 31% dosage: 15–40 mg/L. Low turbidity raw water (<50 NTU): 15–20 mg/L. Medium turbidity (50–150 NTU): 20–30 mg/L. Flood season (>500 NTU): 30–50 mg/L. Always conduct jar tests to determine the exact dosage for specific water sources.
Is it necessary to combine PAC with polymer?
In potable water treatment: usually not needed, PAC forms sufficiently large flocs on its own. In wastewater treatment: combine with anionic or cationic polymer (0.5–1.0 mg/L) to increase floc size, accelerate settling, and reduce PAC dosage by 20–30%.
Can PAC treat oil and grease in wastewater?
Yes, but most effective when combined with DAF flotation. PAC 150–300 mg/L breaks the oil/water emulsion, forms oil flocs — then DAF air bubbles lift the flocs to the surface for skimming. PAC + sedimentation alone is not recommended for oily wastewater.
Does PAC affect downstream biological treatment?
At normal dosages (50–300 mg/L), PAC is not toxic to aerobic microorganisms. However, residual Al³⁺ >5 mg/L may partially inhibit sludge activity. Control PAC dosage and pH entering the biological tank at 6.5–8.0.
Should the pool filter pump be stopped when using PAC?
Yes. When dosing PAC, run the filtration system normally. After 4–6 hours, stop the pump, allow PAC to coagulate fine particles, then backwash the filter to flush out retained flocs.
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