Last Updated: July 2026 | Reading Time: 10 minutes
Introduction
Poly Aluminium Chloride (PAC) and Poly Ferric Sulfate (PFS) are two of the most widely used inorganic polymer coagulants in water and wastewater treatment. Both offer significant advantages over traditional coagulants like alum and ferric chloride, but they have different strengths, optimal applications, and cost profiles. This comprehensive comparison helps you choose the right coagulant for your specific application.
What are PAC and PFS?
Poly Aluminium Chloride (PAC)
PAC is an inorganic polymer coagulant with the general formula [Al₂(OH)ₙCl₆₋ₙ]ₘ. It is produced by the reaction of bauxite or calcium aluminate powder with hydrochloric acid. PAC is available in both liquid and powder forms, with varying Al₂O₃ content and basicity levels.
Poly Ferric Sulfate (PFS)
PFS is an iron-based inorganic polymer coagulant with the general formula [Fe₂(OH)ₙ(SO₄)₃₋ₙ/₂]ₘ. It is typically produced from ferrous sulfate or iron ore through oxidation and polymerization. PFS is known for its strong flocculation ability and effectiveness in removing phosphorus and COD.
Side-by-Side Comparison
| Parameter | PAC (Poly Aluminium Chloride) | PFS (Poly Ferric Sulfate) |
|---|---|---|
| Active Ingredient | Aluminium oxide (Al₂O₃) | Iron (as Fe³⁺) |
| Typical Concentration | 28-30% Al₂O₃ (solid), 10-18% (liquid) | 21% Fe₂(SO₄)₃ (solid), 11-13% (liquid) |
| Appearance | Yellow/light brown powder or liquid | Reddish-brown powder or liquid |
| Optimal pH Range | 5.0 – 9.0 | 4.0 – 11.0 |
| Floc Formation Speed | Fast (1-3 minutes) | Fast (1-5 minutes) |
| Floc Density | Medium | High (denser, faster settling) |
| Sludge Volume | Low | Medium-High |
| Phosphorus Removal | Good | Excellent |
| COD Removal | Good (30-55%) | Better (40-65%) |
| Color Removal | Good | Excellent |
| SS Removal | Excellent (80-95%) | Excellent (85-95%) |
| Low Temperature Performance | Excellent | Good |
| Residual Metal in Effluent | Aluminum (can cause issues at high pH) | Iron (tends to cause yellow color) |
| Typical Dosage | 20 – 200 mg/L | 30 – 300 mg/L |
| Cost (USD/MT solid) | $280 – $420 | $200 – $320 |
Performance Comparison by Application
1. Drinking Water Treatment
| Factor | PAC | PFS |
|---|---|---|
| Residual metal concern | Aluminum regulated (0.2 mg/L) | Iron regulated (0.3 mg/L) |
| Taste/odor impact | Minimal | Can cause metallic taste at high dose |
| Turbidity removal | Excellent | Excellent |
| Color removal | Good | Excellent (but can add iron color) |
| Recommendation | Preferred for drinking water | Secondary option, use with caution |
Verdict: PAC is generally preferred for drinking water treatment due to lower residual concerns and better taste profile. PFS can be used but requires careful dosing to avoid iron-related discoloration and taste issues.
2. Municipal Wastewater Treatment
| Factor | PAC | PFS |
|---|---|---|
| SS removal | Excellent | Excellent |
| Phosphorus removal | Good (80-90%) | Excellent (90-98%) |
| COD removal | Good (30-50%) | Better (40-60%) |
| Sludge production | Lower | Higher |
| pH sensitivity | More sensitive | Less sensitive |
| Recommendation | Good for primary/tertiary | Better for P removal |
Verdict: Both work well for municipal wastewater. PFS is better where phosphorus removal is critical (eutrophication control). PAC is better where minimizing sludge volume is important. Many plants use a combination.
3. Industrial Wastewater Treatment
Textile Dyeing Wastewater
- Color removal: PFS > PAC (PFS achieves 85-95% vs PAC 60-80%)
- COD removal: PFS > PAC
- Recommendation: PFS preferred, often combined with PAC and decolorizing agent
Pulp & Paper Wastewater
- SS removal: Both excellent
- COD removal: PFS slightly better
- Sludge handling: PAC produces less sludge
- Recommendation: PAC for primary treatment, PFS for tertiary polishing
Mining Wastewater
- Heavy metal removal: PFS > PAC (iron-based better for co-precipitation)
- Suspended solids: Both excellent
- Low pH tolerance: PFS better
- Recommendation: PFS preferred for mining and metallurgical wastewater
Food & Beverage Wastewater
- High organic load: Both require biological pretreatment
- Phosphorus: PFS better
- Recommendation: Post-biological polishing with either; PFS for P control
Cost Comparison
Direct Cost per MT
PFS is typically 20-30% cheaper per metric ton than PAC on a solid basis. However, this doesn’t tell the whole story — you need to consider dosage requirements and treatment efficiency.
True Cost Comparison (per 1,000 m³ treated)
| Scenario | PAC Cost | PFS Cost | Winner |
|---|---|---|---|
| Drinking water (turbidity removal) | $15 – $25 | $18 – $30 | PAC |
| Municipal primary (SS + BOD) | $25 – $40 | $24 – $38 | Roughly equal |
| Phosphorus removal (to <0.5 mg/L) | $40 – $65 | $30 – $50 | PFS |
| Industrial COD removal | $50 – $100 | $45 – $90 | PFS (slight) |
| Color removal (textile) | $60 – $120 | $45 – $90 | PFS |
Note: Costs are approximate and vary by local pricing, wastewater characteristics, and treatment targets.
Hidden Costs to Consider
- Sludge disposal: PFS typically produces more sludge than PAC, increasing disposal costs
- pH adjustment: PFS may require more caustic for pH neutralization
- Equipment corrosion: PFS is more corrosive than PAC, potentially increasing maintenance
- Storage and handling: Both are similar; PFS can stain surfaces
PAC + PFS Blend: The Best of Both Worlds
Many treatment plants find that combining PAC and PFS produces better results than either alone. The blend leverages PAC’s fast floc formation with PFS’s strong COD and phosphorus removal capability.
Common Blend Ratios
- 70% PAC + 30% PFS: General purpose, good SS removal with enhanced COD
- 50% PAC + 50% PFS: Balanced performance for industrial wastewater
- 30% PAC + 70% PFS: Maximum COD/color/phosphorus removal
Benefits of Blending
- Synergistic effect: Combined floc structure is denser and settles faster
- Wider pH tolerance: Better performance across a broader pH range
- Lower overall dosage: Synergy reduces total chemical requirement
- Flexible optimization: Adjust ratio based on specific treatment goals
- Cost optimization: Replace some expensive PAC with cheaper PFS
When to Choose PAC
Choose PAC when:
- You treat drinking water or food-grade process water
- Low aluminum residual is required
- Minimizing sludge volume is a priority
- Operating at low water temperatures (<10°C)
- Treating relatively low-COD, high-SS water
- pH is in the 6.0-8.0 range (PAC’s sweet spot)
- You need rapid floc formation for DAF systems
When to Choose PFS
Choose PFS when:
- Phosphorus removal is a primary treatment goal
- You need high COD removal efficiency
- Color removal is critical (textile, dyeing wastewater)
- Wastewater pH is outside PAC’s optimal range
- Treating mining, metallurgical, or heavy metal-containing water
- You want to reduce chemical cost (lower per-MT price)
- Sulfate in effluent is not a concern
How to Decide: Step-by-Step
- Define your treatment targets: What are your key parameters (SS, COD, TP, color, turbidity)?
- Characterize your wastewater: pH, temperature, alkalinity, COD fractions, TSS
- Check regulatory limits: Are there specific limits for aluminum, iron, sulfate, or pH?
- Run jar tests: Test both PAC and PFS (and blends) side by side with your actual water
- Calculate total cost: Include chemical cost, sludge disposal, pH adjustment, and maintenance
- Pilot test: For large facilities, run a pilot before full-scale conversion
- Start with a blend: For most industrial applications, a PAC+PFS blend offers the best balance
Storage and Handling Comparison
| Factor | PAC | PFS |
|---|---|---|
| Shelf life (solid) | 24 months | 12-18 months |
| Shelf life (liquid) | 6-12 months | 3-6 months |
| Corrosivity | Low-Medium | Medium-High |
| Staining | Minimal | Can stain surfaces rust-colored |
| Hygroscopicity | Low | Medium-High |
| Storage material | PE, PP, FRP | PE, FRP, lined steel |
Conclusion
The choice between PAC and PFS depends on your specific water quality, treatment targets, and cost considerations. PAC is the go-to choice for drinking water and general turbidity removal, while PFS excels at phosphorus, COD, and color removal in industrial and municipal wastewater. For many applications, especially industrial wastewater treatment, a blend of PAC and PFS provides the optimal balance of performance and cost. Always start with jar testing to determine the best coagulant (or blend) and optimal dosage for your specific water.
Not sure which coagulant is right for your application? Contact our technical team for a free water analysis and personalized recommendation.
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