Introduction
When purchasing Poly Aluminium Chloride (PAC), buyers often focus on Al2O3 content (e.g., 28%, 30%) but overlook an equally critical parameter: basicity. PAC with the same Al2O3 content but different basicity can perform completely differently in the same application. This guide explains what basicity is, how it affects coagulation, and how to select the right basicity for your water treatment needs.
What Is PAC Basicity?
Basicity (also called %OH or B value) is the ratio of hydroxyl (OH-) ions to aluminum (Al3+) ions in the PAC polymer structure, expressed as a percentage:
Basicity (%) = [OH-] / 3[Al3+] x 100
In simple terms: basicity measures how much the PAC has already been “pre-neutralized” during manufacturing. Higher basicity = more pre-formed polymer chains = less alkalinity consumed during coagulation.
Basicity Grades and Their Properties
| Basicity Range | Classification | Appearance | Key Properties |
|---|---|---|---|
| 10-40% | Low basicity | Light yellow powder | High positive charge density, strong charge neutralization, consumes more alkalinity, narrower pH range |
| 40-65% | Medium basicity | Yellow to light brown powder | Balanced charge neutralization and bridging, moderate alkalinity consumption, wider pH range |
| 65-85% | High basicity | Dark yellow to brown powder | Large pre-formed polymer species, excellent bridging flocculation, least alkalinity consumption, widest pH tolerance, fastest floc formation |
| 85%+ | Ultra-high basicity | Dark brown powder or liquid | Specialty products for low-alkalinity waters, maximum polymer content, potential stability issues in storage |
How Basicity Affects Coagulation Mechanism
Charge Neutralization vs Bridging
Low basicity PAC (10-40%): Contains mostly monomeric and low-polymer Al species (Ala). These have high positive charge density and work primarily through charge neutralization — neutralizing the negative charge on suspended particles so they can aggregate. Mechanism is similar to alum but faster due to pre-hydrolysis.
High basicity PAC (65-85%): Contains mostly polymeric Al species (Alb, Alc) with large molecular weight. These work primarily through adsorption and inter-particle bridging — polymer chains attach to multiple particles simultaneously, creating larger, stronger flocs. Requires less dosage for equivalent turbidity removal.
Effect on pH
Every Al3+ ion that hydrolyzes consumes alkalinity (HCO3-) and releases H+, lowering pH. Low basicity PAC has more un-hydrolyzed Al3+, so it consumes more alkalinity and depresses pH more. High basicity PAC is already partially neutralized, causing minimal pH shift.
Practical example: Treating 1000 m3 of water at 20 mg/L PAC dose:
- 40% basicity PAC: pH drops from 7.2 to 6.5, may need 5-10 mg/L soda ash supplement
- 70% basicity PAC: pH drops from 7.2 to 7.0, no pH adjustment needed
Selecting the Right Basicity by Application
| Application | Recommended Basicity | Reason |
|---|---|---|
| Drinking water (low turbidity, <10 NTU) | 40-60% | Charge neutralization dominates for low particle count; higher charge density more effective |
| Drinking water (high turbidity, >50 NTU) | 60-75% | Sweep flocculation needed for high solids; bridging mechanism more efficient |
| Textile dye wastewater | 70-85% | High organic load needs strong bridging; wide pH tolerance handles variable dye bath pH |
| Industrial wastewater (general) | 50-70% | Balanced performance across varied wastewater characteristics |
| Municipal wastewater primary treatment | 60-80% | High TSS benefits from bridging; faster settling reduces clarifier size |
| Mining tailings | 70-85% | High solids, variable pH — high basicity provides fastest settling and widest pH tolerance |
| Electroplating wastewater | 40-60% | Lower basicity provides stronger charge neutralization for dissolved metal ions |
| POME (Palm Oil Mill Effluent) | 65-80% | High organic + inorganic solids mix benefits from strong bridging |
| Food processing | 50-70% | Variable loads; medium basicity provides flexibility |
| Swimming pool water | 40-60% | Low dosage, low solids; charge neutralization dominant |
How to Test Basicity Suitability — Jar Test Protocol
- Obtain samples of different basicity PAC: Request 40%, 60%, and 80% basicity samples from your supplier (all at same Al2O3 content, e.g., 30%)
- Prepare 1% stock solutions: 10g PAC in 1000mL distilled water for each basicity grade
- Run parallel jar tests: Dose each basicity at 50, 100, 200, 300 mg/L on your actual wastewater
- Measure: Supernatant turbidity, residual aluminum, pH change, floc size, settling velocity
- Select the basicity that achieves treatment targets at the lowest dosage with acceptable pH impact
Common Misconceptions
Myth: “Higher basicity is always better.” Reality: For low-turbidity water with few particles to bridge, high-basicity PAC’s polymer chains can actually re-stabilize particles if over-dosed. Low basicity PAC is better for charge-neutralization-dominant applications.
Myth: “Basicity doesn’t matter — just look at Al2O3%.” Reality: Two PAC products both at 30% Al2O3 but 40% vs 80% basicity can differ in required dosage by 30-50% for the same application. Always specify basicity in procurement.
Myth: “All high-basicity PAC is spray-dried.” Reality: Both spray-dried and drum-dried processes can produce high-basicity PAC. The drying method affects dissolution rate and purity, not basicity range.
HydroChemix supplies PAC across the full basicity range (40-85%) with consistent quality backed by Certificate of Analysis for every shipment. Contact jingshuicc@gmail.com to discuss your application and request samples of different basicity grades for jar testing.