Introduction
Poly Aluminium Chloride (PAC) and Aluminum Sulfate (alum) are the two most widely used aluminum-based coagulants in water and wastewater treatment. While alum has been the traditional choice for over a century, PAC has rapidly gained market share due to superior performance. This guide provides a comprehensive comparison to help treatment plant operators and procurement managers make informed decisions.
Chemistry and Mechanism
Alum (Al2(SO4)3·14H2O) hydrolyzes in water to form Al(OH)3 flocs and consumes 6 moles of alkalinity per mole of Al3+. This means significant pH depression and often requires supplemental alkalinity addition (lime or soda ash).
PAC ([Al2(OH)nCl6-n]m) is pre-hydrolyzed during manufacturing. It contains pre-formed Al polymer species (Alb, Alc) that coagulate immediately upon dosing without consuming significant alkalinity. This results in faster floc formation, wider pH tolerance, and less pH adjustment.
Performance Comparison
| Parameter | PAC 30% (Spray Dried) | Aluminum Sulfate (Alum) |
|---|---|---|
| Effective pH range | 5.0-9.0 | 5.5-7.5 |
| Optimal pH range | 6.5-7.5 | 6.5-7.0 |
| Alkalinity consumption | Low (pre-hydrolyzed) | High (6 eq/mol Al) |
| Cold water (<5C) performance | Good to excellent | Poor (slow hydrolysis) |
| Relative dosage (vs alum) | 1.0 (reference) | 1.5-3.0x more |
| Sludge production | 30-50% less than alum | Higher volume |
| Floc formation speed | Fast (pre-formed polymers) | Slower (requires hydrolysis) |
| Floc strength | Stronger, larger | Weaker, smaller |
| Residual aluminum (treated water) | <0.05 mg/L (typically) | 0.05-0.20 mg/L |
| Sulfate addition to water | None | Adds SO4- (corrosion risk) |
| Chloride addition to water | Minimal | None |
| Chemical storage and handling | Powder — dry, easy to store | Often delivered as liquid (weight) |
| Unit price (per ton) | Higher (but lower dosage needed) | Lower (but higher dosage needed) |
Total Cost of Ownership Analysis
While PAC has a higher unit price per ton, the total treatment cost per cubic meter of water is often lower due to:
- 30-70% lower dosage — less chemical needed for equivalent performance
- 30-50% less sludge — reduced sludge handling and disposal costs
- No/less pH adjustment chemicals — eliminating lime or soda ash purchase
- Lower shipping cost per m3 treated — less chemical weight for same treatment volume
- Simplified operation — one chemical instead of coagulant + pH adjuster
When Alum May Still Be the Right Choice
- Existing plant optimized for alum with sunk infrastructure investment
- Local alum availability at very low prices
- Applications where sulfate addition is beneficial (rare)
- Very small systems where switching cost exceeds savings
Case Study: Municipal Drinking Water Plant
A 100,000 m3/day drinking water plant in Southeast Asia switched from alum (40 mg/L) to PAC 30% (15 mg/L), achieving:
- 62.5% reduction in chemical dosage (by weight)
- Elimination of 3,000 kg/day of lime for pH adjustment
- 40% reduction in sludge disposal volume
- Improved cold-season turbidity removal from 85% to 95%+
- Estimated annual savings: $120,000+
HydroChemix supplies premium spray dried PAC 30% with consistent quality backed by COA for every shipment. Contact jingshuicc@gmail.com for product specifications, a free 1-2 kg sample for bench testing, and a cost comparison tailored to your plant’s operating parameters.