Choosing between Polyaluminum Chloride (PAC) and Aluminum Sulfate (Alum) is one of the most important decisions for water treatment facilities. While both are aluminum-based coagulants that remove turbidity and contaminants, they differ significantly in performance, cost-effectiveness, and operational complexity. This comprehensive guide provides data-driven comparisons, real-world cost analyses, and a practical decision framework to help you select the optimal coagulant for your application.
| Factor | PAC | Alum | Winner |
|---|---|---|---|
| Dosage Required | 30-50% less | Baseline | PAC ✅ |
| pH Range | 5.0-9.0 | 6.5-7.5 | PAC ✅ |
| Floc Formation | Fast, dense | Slower, looser | PAC ✅ |
| Sludge Volume | 30-50% less | Baseline | PAC ✅ |
| Cold Water Performance | Excellent | Poor | PAC ✅ |
| Residual Aluminum | 0.05-0.1 mg/L | 0.3-0.5 mg/L | PAC ✅ |
| Initial Cost | Higher | Lower | Alum ✅ |
| Total Operating Cost | Lower | Higher | PAC ✅ |
Verdict: PAC wins 8 out of 9 categories, making it the superior choice for most applications.
Chemical Name: Aluminum Sulfate [Al₂(SO₄)₃]
Form: White crystalline solid, easily soluble
How It Works:
1. Dissolves in water, releasing Al³⁺ ions
2. Hydrolyzes to form aluminum hydroxide [Al(OH)₃]
3. Forms flocs that trap suspended particles
Key Characteristics:
● Simple inorganic salt
● Traditional coagulant (used for 100+ years)
● Acidic (pH 3.0-3.5 in solution)
● Widely available globally
Chemical Name: Polyaluminum Chloride [Alₙ(OH)ₘCl₍₃ₙ₋ₘ₎]
Form: Yellow/white powder or liquid, various grades
How It Works:
1. Pre-hydrolyzed aluminum polymer
2. Already contains multi-core hydroxy-aluminum complexes
3. Rapid charge neutralization upon addition
4. Forms large, dense flocs through bridging
Key Characteristics:
● Advanced inorganic polymer
● Pre-polymerized structure
● Less acidic (pH 4.0-5.0)
● Higher charge density
Alum Typical Dosage:
● Drinking water: 20-50 mg/L
● Wastewater: 80-200 mg/L
● High turbidity: 100-300 mg/L
PAC Typical Dosage:
● Drinking water: 10-30 mg/L (50% less)
● Wastewater: 40-120 mg/L (50% less)
● High turbidity: 50-180 mg/L (40% less)
Why PAC Uses Less:
● Higher aluminum content (30% vs 8% in alum)
● Pre-polymerized structure more efficient
● Higher charge density
Example Calculation:
For 10,000 m³/day at 30 NTU turbidity:
Alum Required: 10,000 m³ × 40 mg/L = 400 kg/day PAC Required: 10,000 m³ × 20 mg/L = 200 kg/day Chemical Savings: 200 kg/day = 50%
Alum:
● Optimal pH: 6.5-7.5 (narrow window)
● Performance drops sharply outside this range
● Significantly lowers water pH (typically 0.5-1.5 units)
● Usually requires pH adjustment with lime/caustic
PAC:
● Optimal pH: 5.0-9.0 (wide window)
● Consistent performance across broader range
● Minimal pH impact (typically 0.1-0.3 units)
● Rarely requires pH adjustment
Cost Implications:
Water plant treating 5,000 m³/day:
Alum Scenario: - pH drops from 7.5 to 6.0 - Lime needed: 20 mg/L = 100 kg/day - Lime cost: 100 kg × $0.20 = $20/day - Annual lime cost: $7,300 PAC Scenario: - pH drops from 7.5 to 7.3 - No pH adjustment needed - Annual savings: $7,300
Floc Formation Speed:
| Stage | Alum | PAC |
|---|---|---|
| Rapid mix time | 1-2 min | 1-2 min |
| Slow mix time | 20-30 min | 15-20 min |
| Settling time | 3-4 hours | 2-3 hours |
| Total Treatment Time | ~4 hours | ~3 hours |
Floc Characteristics:
Alum Flocs:
● Smaller size (0.5-2 mm)
● Lighter, fluffier structure
● Settling velocity: 0.5-1.0 m/hr
● Easily disrupted by mixing
PAC Flocs:
● Larger size (2-5 mm)
● Dense, compact structure
● Settling velocity: 1.5-3.0 m/hr
● More resistant to shear
Impact on Plant Capacity:
For existing clarifier (surface area 100 m²):
With Alum: Overflow rate: 1.0 m/hr max Capacity: 100 m² × 1.0 m/hr × 24 hr = 2,400 m³/day With PAC: Overflow rate: 2.0 m/hr possible Capacity: 100 m² × 2.0 m/hr × 24 hr = 4,800 m³/day Capacity Increase: 100% with PAC
Volume Reduction:
Treating 10,000 m³/day with 100 NTU turbidity:
Alum: - Coagulant: 50 mg/L - pH adjustment: 25 mg/L lime - Total chemicals: 75 mg/L - Sludge: 750 kg dry solids/day PAC: - Coagulant: 25 mg/L - No pH adjustment - Total chemicals: 25 mg/L - Sludge: 400 kg dry solids/day Sludge Reduction: 47%
Sludge Disposal Cost Impact:
Annual Sludge (Alum): 274 tons Disposal cost: $100/ton = $27,400/year Annual Sludge (PAC): 146 tons Disposal cost: $100/ton = $14,600/year Annual Savings: $12,800
Performance at Different Temperatures:
| Temperature | Alum Efficiency | PAC Efficiency |
|---|---|---|
| >20°C | 100% (baseline) | 100% |
| 10-20°C | 70-85% | 95-100% |
| 5-10°C | 50-70% | 85-95% |
| <5°C | 30-50% | 70-85% |
Why This Matters:
Cold water plants often must:
● With Alum: Increase dose 50-100% in winter
● With PAC: Increase dose only 10-20% in winter
Example (Northern climate plant):
Summer (25°C): Alum: 30 mg/L, PAC: 15 mg/L Winter (5°C): Alum: 60 mg/L (100% increase) PAC: 18 mg/L (20% increase) Winter Chemical Cost Difference: Alum: Double the cost PAC: Nearly same cost
Typical Residual Levels:
| Coagulant | Treated Water Al Residual | WHO Guideline |
|---|---|---|
| Alum | 0.3-0.5 mg/L | <0.2 mg/L |
| PAC | 0.05-0.1 mg/L | <0.2 mg/L |
Compliance:
● Alum: Often exceeds guideline, requires optimization
● PAC: Consistently meets guideline
Health & Regulatory Implications:
● Lower residual aluminum reduces health concerns
● Easier regulatory compliance
● Better for sensitive populations
Alum:
Alum dose: 40 mg/L Price: $350/ton Daily cost: 10,000 × 40 / 1,000,000 × $350 = $140 Lime (pH adjustment): 25 mg/L Price: $200/ton Daily cost: 10,000 × 25 / 1,000,000 × $200 = $50 Total Chemical: $190/day = $69,350/year
PAC:
PAC dose: 20 mg/L Price: $550/ton Daily cost: 10,000 × 20 / 1,000,000 × $550 = $110 pH adjustment: None needed Total Chemical: $110/day = $40,150/year Chemical Savings: $29,200/year
Alum Sludge: 274 tons/year × $100 = $27,400 PAC Sludge: 146 tons/year × $100 = $14,600 Sludge Savings: $12,800/year
Alum: - Frequent pH monitoring/adjustment: +10 hr/week - Sludge handling: +5 hr/week - Total: 15 hr/week × 52 × $30/hr = $23,400/year PAC: - Minimal pH adjustment: +2 hr/week - Less sludge: +2 hr/week - Total: 4 hr/week × 52 × $30/hr = $6,240/year Labor Savings: $17,160/year
Alum: - Higher corrosion = $5,000/year maintenance - Longer treatment time = $3,000/year extra energy PAC: - Lower corrosion = $2,000/year maintenance - Shorter treatment time = standard energy Equipment Savings: $6,000/year
| Cost Category | Alum | PAC | Savings |
|---|---|---|---|
| Chemicals | $69,350 | $40,150 | $29,200 |
| Sludge Disposal | $27,400 | $14,600 | $12,800 |
| Labor | $23,400 | $6,240 | $17,160 |
| Equipment/Energy | $8,000 | $2,000 | $6,000 |
| TOTAL | $128,150 | $63,000 | $65,150 |
Annual Savings with PAC: $65,150 (51% reduction)
Payback Period:If switching requires $20,000 investment → Payback in 3.7 months
✅ Total operating cost is priority✅ Water pH varies significantly✅ Cold water treatment (winter operations)✅ Strict aluminum residual limits✅ Sludge disposal is expensive✅ Limited clarifier capacity (need higher overflow rates)✅ Automation/minimal operator attention desired✅ Long-term (5+ year) operation planned
✅ Initial capital budget extremely limited✅ Short-term/temporary operation✅ Very warm water year-round (>25°C)✅ Existing infrastructure optimized for alum✅ Alum supply chain already established✅ No access to quality PAC supplier
Phase 1: Testing (2-4 weeks)
1.Obtain PAC Samples
● Request 3-5 different PAC grades
● Specify your application (drinking water, wastewater, etc.)
2.Conduct Jar Tests
● Test current water with both alum and PAC
● Record optimal doses, floc formation time, clarity
● Measure residual aluminum
3.Pilot Testing (if possible)
● Run side-by-side for 1-2 weeks
● Compare actual plant performance
● Monitor all parameters
Phase 2: Preparation (1-2 weeks)
4.Equipment Assessment
● Check chemical storage compatibility
● PAC requires corrosion-resistant tanks (HDPE, PP, FRP)
● Verify metering pump compatibility
5.Staff Training
● Prepare new dosing calculations
● Update SOPs
● Train operators on different characteristics
6.Procurement
● Order initial PAC supply
● Negotiate pricing (bulk discounts)
● Arrange delivery schedule
Phase 3: Implementation (1 week)
7.Initial Switchover
● Start at 50% of jar test optimal dose
● Monitor closely for first 24 hours
● Adjust based on performance
8.Optimization
● Fine-tune dose over 3-7 days
● Eliminate pH adjustment if possible
● Document new optimal parameters
Phase 4: Monitoring (Ongoing)
9.Regular Testing
● Weekly jar tests initially
● Monthly after stabilization
● Track all cost savings
Before (Alum):
● Capacity: 15,000 m³/day
● Alum dose: 35 mg/L
● Annual chemical cost: $95,000
● Sludge disposal: $42,000/year
● Aluminum residual: 0.4 mg/L (above limit)
After (PAC):
● Same capacity
● PAC dose: 18 mg/L
● Annual chemical cost: $62,000
● Sludge disposal: $21,000/year
● Aluminum residual: 0.08 mg/L (compliant)
Results:
● Total savings: $54,000/year (36%)
● Payback period: 4.5 months
● Regulatory compliance achieved
● Operator workload reduced 40%
Before (Alum):
● Flow: 5,000 m³/day
● Alum: 80 mg/L + lime 40 mg/L
● Treatment time: 4 hours
● Annual cost: $78,000
After (PAC):
● Same flow
● PAC: 45 mg/L, no lime
● Treatment time: 2.5 hours
● Annual cost: $48,000
Results:
● Savings: $30,000/year (38%)
● Faster treatment = increased capacity
● Simpler operations
● Better cold-weather performance
❌ Wrong: "We used 40 mg/L alum, so we'll use 40 mg/L PAC"
✅ Right: Start with jar tests, typically need 40-60% less PAC
❌ Wrong: Keep adding lime as before
✅ Right: PAC often eliminates need for pH adjustment, test first
❌ Wrong: Buying cheapest PAC available
✅ Right: Match PAC grade (basicity, Al content) to application
❌ Wrong: Switch without operator training
✅ Right: Comprehensive training on different behavior
❌ Wrong: Switch based on sales pitch alone
✅ Right: Always conduct jar tests and pilot trials
Q: Is PAC safe for drinking water?
A: Yes. PAC is approved by WHO and NSF for potable water. Produces lower aluminum residuals than alum.
Q: Can I mix PAC and alum together?
A: Not recommended. Use one or the other. Switching should be complete transition.
Q: How long does PAC solution last?
A: Liquid PAC: 6-12 months. Powder PAC: 18-24 months. Prepared solutions: 24-48 hours.
Q: Will PAC work in my existing equipment?
A: Usually yes, but check material compatibility. PAC is less corrosive than alum.
Q: Is PAC environmentally better than alum?
A: Yes. Less sludge, lower residuals, no sulfate addition, better for aquatic life.
Q: What if PAC costs more per kg?
A: Focus on cost per m³ treated, not per kg. Total operating cost almost always favors PAC.
✅ PAC outperforms alum in 8 out of 9 categories
✅ Total operating cost 30-50% lower with PAC
✅ Faster treatment, less sludge, simpler operations
✅ Better cold-water performance
✅ Lower aluminum residuals for compliance
✅ Payback period typically 3-6 months
The Choice is Clear: For most applications, PAC is the superior coagulant delivering better performance and lower total cost despite higher unit price.
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