Polyaluminium chloride (PAC) is a highly effective inorganic polymer coagulant widely used in water treatment systems—both municipal and industrial. Its unique molecular structure and high charge density enable rapid destabilization of suspended solids, turbidity removal, and sludge volume reduction. This guide covers everything you need to know about PAC: what it is, how it works, its chemical properties, applications, dosing strategies, advantages, and best practices for optimal performance.
Polyaluminium chloride (often abbreviated PAC) is an inorganic polymer formed by partial neutralization of aluminum chloride with a base—typically aluminum hydroxide. It exists as a yellowish to light-brown powder or liquid and is highly soluble in water. PAC’s effectiveness as a coagulant stems from its high molecular weight and polymeric aluminum hydroxide species, which facilitate rapid charge neutralization and floc formation in various water matrices.
PAC is commonly represented by the general formula:
[Al₂(OH)nCl₆₋n]ₘ
where n denotes the degree of hydroxylation (typically n = 1–3), and m indicates the polymerization index. Unlike simple aluminum salts (e.g., alum—Al₂(SO₄)₃·18H₂O), PAC contains polymeric hydroxyl-aluminum complexes, which impart strong coagulation capabilities.
PAC is produced via controlled hydrolysis of aluminum chloride (AlCl₃) in the presence of alkali (e.g., Al(OH)₃ or NaOH). The partial neutralization reaction yields high-charge-density aluminum hydroxide polymers:
AlCl₃ + Al(OH)₃ → [Al₂(OH)nCl₆₋n]ₘ
By adjusting pH and the Al³⁺ : OH⁻ ratio, manufacturers can tailor basicity (ratio of Al₂O₃ equivalent to OH) to specific PAC chemical grades:
Low-basicity PAC (20–40%): More trivalent Al species, stronger charge neutralization, slower hydrolysis.
Medium-basicity PAC (40–70%): Balanced trivalent and tetravalent Al species, moderate speed, broad pH range.
High-basicity PAC (70–90%): More hydroxyl groups, rapid hydrolysis, wider pH adaptability, ideal for high-turbidity effluents.
In water treatment, PAC acts through several synergistic mechanisms:
Suspended colloidal particles carry negative charges that repel one another, preventing aggregation. PAC’s high-positive-charge aluminum hydroxide polymers neutralize these negative charges, destabilizing colloids and enabling aggregation into microflocs.
PAC’s polymeric chains adsorb on particle surfaces and form “bridges” between particles, producing larger, denser flocs. This bridging effect accelerates floc growth and promotes rapid settling.
At optimal PAC doses, aluminum hydroxide precipitates (Al(OH)₃) form a gelatinous network that physically enmeshes suspended solids, further enhancing flocculation and sedimentation.
These combined processes—charge neutralization, adsorption-bridging, and sweep flocculation—explain why PAC is a highly efficient coagulant across a wide range of pH (5–9) and temperatures (5–40 °C).
PAC is available in two main forms: liquid PAC and solid (powdered) PAC. Additionally, grades vary by Al₂O₃ content and basicity.
Al₂O₃ Content: Typically 10–18% (w/w)
Density: 1.20–1.35 g/cm³
Basicity: 40–80% (adjusted by dilution)
Advantages: Ready-to-use without pre-dilution, easy pumping, immediate cloudiness reduction.
Disadvantages: Higher transportation cost per Al₂O₃ unit (requires tanker trucks), limited storage life.
Al₂O₃ Content: 25–30% (w/w)
Appearance: Yellowish to light yellow granular or crystalline powder
Basicity: 40–80% (controlled during production)
Advantages: Lower transportation cost, longer shelf life, easier handling for remote locations.
Disadvantages: Requires onsite dilution (typically 5–10% w/w), extra labor and equipment for dissolution.
Basicity (%) indicates the ratio of Al(OH)₃ content to total Al compound:
Low-basicity PAC (20–40%): Predominantly Al³⁺ species, ideal for low-turbidity feed water (e.g., potable water with <10 NTU turbidity).
Medium-basicity PAC (40–70%): Balanced Al³⁺ and Al(OH)₄⁻ species, suitable for moderate-turbidity effluents (10–200 NTU).
High-basicity PAC (70–90%): Rich in Al(OH)₄⁻ and polynuclear Al species, perfect for high-turbidity industrial wastewater (>200 NTU).
PAC’s versatility makes it indispensable across numerous sectors. Below are key applications:
In potable water plants, PAC is used in rapid mixing and coagulation basins to remove turbidity, color, and trace organics. Its rapid hydrolysis and strong flocculation reduce filter loading and improve effluent clarity (<0.1 NTU).
Case Study: A mid-sized municipal plant in northern Europe replaced alum with 30 mg/L powdered PAC (basicity 60%). Result: 50% lower sludge volume, 20% reduction in filter backwash frequency, and stabilized pH without acid adjustment.
In secondary clarifiers, PAC complements biological treatment by enhancing sludge settling and reducing effluent suspended solids (SS <20 mg/L). PAC also aids in sludge thickening prior to dewatering.
Internal Link: See our Municipal Wastewater Solutions page for detailed case examples.
Industries such as textile, paper & pulp, petrochemical, mining, leather, and metal finishing rely on PAC for:
Color Removal: Textile dye effluent with color metrics >500 ADMI pigment units can be treated with 50–200 mg/L PAC + anionic PAM for color reduction >90%.
Heavy Metal Precipitation: PAC pre-treatment followed by sulfide precipitation reduces metals (Cr, Cu, Zn) to <1 mg/L.
Oil & Grease Removal: In food processing and oil refinery wastewater, PAC disrupts oil droplets for floatation or sedimentation.
After coagulation/flocculation, concentrated sludge often requires dewatering. PAC in combination with anionic polyacrylamide (APAM) yields high cake solids (>25%) in belt filter presses and centrifuges.
In cooling tower blowdown and RO feed water, PAC prevents scale and fouling by removing suspended solids before filtration. Typical doses: 10–30 mg/L PAC + 0.5–2 mg/L APAM.
Optimal PAC performance hinges on correct dosing, mixing, and reaction time. Below are recommended steps:
Collect representative water sample (500 mL). Measure baseline turbidity and pH.
Prepare PAC stock solution (5–10% w/w) by dissolving granular PAC in deionized water. Stir thoroughly until clear.
In each jar, add coagulant (e.g., alum) if needed, followed by graduated doses of PAC solution (0, 5, 10, 20, 50 mg/L).
Rapid mix at 250 rpm for 1 min, followed by slow mix at 40 rpm for 20 min.
Allow settling for 30 min. Measure final turbidity and supernatant clarity.
Select dose yielding lowest turbidity without residual color or sludge carry-over.
Once jar tests determine optimal dose (e.g., 30 mg/L for 200 NTU raw water), implement onsite as follows:
Solution Preparation: Continuously dissolve PAC powder at 5–10% w/w in a dedicated dilution tank equipped with a mechanical agitator. Maintain pH 4–6 for fastest dissolution.
Dosing Point: Introduce PAC solution into the rapid-mix zone immediately after raw water intake or following any pre-chlorination step. Ensure uniform mixing via static mixer or mechanical flocculator.
Mixing Intensity: Rapid mix at 200–300 rpm for 30–60 s; slow mix at 30–60 rpm for 10–15 min (or per clarifier retention time).
Monitoring & Control: Continuously monitor turbidity, pH, and residual aluminum. Adjust PAC dose ±10% to maintain effluent turbidity <1 NTU. Use online turbidimeter and pH probes.
Sludge Handling: Collect settled sludge in a dedicated thickener. Condition with 1 mg/L APAM for sludge flocculation before dewatering (centrifuge or belt press).
Several variables influence PAC efficacy:
pH: Ideal range 5–9. At pH < 5, Al³⁺ species dominate, risk of dissolved aluminum in effluent. At pH > 9, Al(OH)₄⁻ species form, reducing charge neutralization.
Temperature: Optimal 10–35 °C. Low temperatures (< 10 °C) slow hydrolysis, requiring higher doses. Excessive heat (> 40 °C) may degrade floc strength.
Raw Water Turbidity: Higher turbidity (> 300 NTU) calls for high-basicity PAC (≥ 70%). Low turbidity (< 10 NTU) can use low-basicity PAC (≤ 40%).
Organic Matter Content: High DOC (≥ 10 mg/L) can interfere with charge neutralization; co-dose with activated carbon or oxidation (ozone/H₂O₂) recommended.
Coagulant Aid: Adding anionic PAM (0.5–2 mg/L) after PAC improves floc strength and settling.
Compared to traditional coagulants like alum and ferric chloride, PAC offers:
Lower Dose Requirement: Typically 20–50 mg/L vs. 100–300 mg/L for alum.
Lower Sludge Volume: PAC flocs are denser and more compact, reducing sludge by 15–25%.
Broader pH Tolerance: Effective from pH 5–9 without pH adjustment.
Faster Floc Settlement: Settling velocity > 1.5 m/h vs. 0.5 m/h for alum flocs.
Reduced Corrosivity: PAC solutions have lower chloride content, causing less equipment corrosion than ferric chloride.
Improved Water Quality: Produces < 0.2 NTU effluent turbidity and lower dissolved aluminum residuals (< 0.2 mg/L).
Q1: What Is Polyaluminium Chloride Used For?
PAC is used primarily as a coagulant/flocculant in municipal drinking water and wastewater treatment, industrial effluent clarification (textile, pulp & paper, petrochemical, mining), sludge conditioning and dewatering, cooling tower make-up water and RO pre-treatment, and dyehouse effluent color removal.
Q2: How Is PAC Different from Alum?
Alum (aluminum sulfate) must be dosed at higher rates (2–3× PAC), produces more sludge, and works only at pH ~6.5–7.5. PAC functions over a wider pH range (5–9), yields denser flocs, and often requires only half the dose of alum for equal turbidity removal.
Q3: How Do I Prepare a PAC Solution?
For solid PAC: dissolve in ambient water at 5–10% w/w. Example: to prepare a 10% solution, add 10 kg PAC powder into 90 kg deionized or soft water in a mixing tank with moderate agitation until fully dissolved (20–30 min). Store the solution at pH 4–6 to prevent premature polymerization. For liquid PAC: use directly or dilute to desired concentration (e.g., 10 mL liquid PAC in 1 L water for pilot tests).
Q4: Can PAC Treat High-Turbidity Water?
Yes. For raw water with turbidity > 500 NTU (e.g., stormwater or river water), use high-basicity PAC grades (≥ 70%) at 50–100 mg/L. Combine with anionic PAM (1–2 mg/L) to ensure rapid floc growth and settling.
Q5: Is PAC Safe for Drinking Water Applications?
Yes. When residual aluminum is < 0.2 mg/L (measured using the 8-hydroxyquinoline method), PAC-treated water meets most potable water standards (WHO, EPA). Choose low-basicity PAC (< 50%) for minimal aluminum residuals and maintain pH 7–7.5 in finished water.
PAC Product Specifications – Detailed datasheets on our liquid and powdered PAC grades: /products/pac-products
Wastewater Treatment Services – End-to-end solutions for municipal and industrial effluent: /applications/wastewater-treatment
PAC Dosing Calculator – Online tool to estimate optimal PAC dose based on raw water turbidity and pH: /resources/pac-dosing-calculator
Why Combine PAC with PAM? – Benefits of coagulant–polymer synergy for advanced flocculation: /blog/pam-vs-pac
Request a Quote or Sample – Get free PAC samples or custom quotes today: /contact
Polyaluminium chloride (PAC) stands out as a top-tier coagulant for water treatment due to its high coagulation efficiency, wide pH tolerance, low sludge volume, and cost-effectiveness. From municipal plants to heavy industrial effluent streams, PAC helps achieve ultra-low turbidity, rapid floc settlement, and reduced operational costs. By understanding PAC’s chemistry, proper dosing, and best practices, procurement managers and environmental engineers can optimize treatment processes, ensure compliance, and safeguard water resources.
For more details or to discuss your specific project needs, please visit our Contact page or explore our PAC range. Let Tairan Chemical be your partner in advanced water and wastewater solutions.
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