What Is Polyaluminum Chloride (PAC)? Uses, Dosing, Safety

What polyaluminum chloride is and why it’s different from alum

Polyaluminum chloride is a family of polymeric aluminum salts produced by partially neutralizing aluminum chloride. Unlike alum (aluminum sulfate), PAC is already partially hydrolyzed, meaning it contains a distribution of aluminum species (including polymerized forms) that are more effective at forming strong flocs across a broader operating window.

Key practical implication

Because PAC is pre-hydrolyzed, it typically consumes less alkalinity than alum for the same level of coagulation. In many plants, this translates into steadier pH control and reduced need for alkalinity addition when raw water alkalinity is low.

• Alum: simpler chemistry, but often narrower optimal pH range and higher alkalinity demand.
• PAC: stronger charge neutralization and floc formation at lower doses in many waters, especially cold or low-turbidity conditions.

How PAC works in water treatment

PAC removes turbidity and color by destabilizing negatively charged colloids and dissolved organics, allowing them to aggregate into flocs that can be separated. In practical operations, PAC performance is usually explained through three overlapping mechanisms.

Mechanisms that matter operationally

• Charge neutralization: aluminum species neutralize particle surface charge, reducing repulsion so particles can collide and stick.
• Adsorption and bridging: polymeric species adsorb onto particles and connect them into larger aggregates.
• Sweep floc (at higher doses): aluminum hydroxide precipitates can enmesh fine solids and some organics.

A common operational benefit is improved floc strength: under comparable mixing and hydraulic conditions, PAC-generated flocs often withstand shear better than alum flocs, which can improve clarification stability and filter run time.

Typical forms, grades, and specifications you will see

PAC is sold in liquid and powder forms. Product labels usually specify aluminum content (often expressed as Al₂O₃), basicity (a measure of pre-neutralization), density (for liquids), and impurity limits relevant to potable water or industrial discharge requirements. Values vary by manufacturer, but the ranges below are commonly encountered in procurement and operations.

For drinking water, verify that the specific PAC product is approved for potable use under your jurisdiction and that your supplier provides a current certificate of analysis (CoA) showing impurity limits appropriate for human consumption.

Where PAC is most useful

Polyaluminum chloride is primarily selected when operators need robust turbidity removal, improved floc formation, or better performance across variable raw water conditions. It is used in both municipal and industrial systems.

High-value use cases

• Drinking water clarification: turbidity and color reduction prior to filtration, often with more stable performance in colder seasons.
• Surface water with natural organic matter: supports removal of some dissolved organics that drive color and disinfection byproduct precursors.
• Industrial wastewater pretreatment: solids separation prior to DAF, settling, or membrane systems.
• Phosphorus co-precipitation support: can complement biological systems by improving solids capture (site-specific results depend on chemistry and permits).

As a starting point for many waters, PAC doses often fall in the 10–50 mg/L range (as product), but highly turbid events, unusual alkalinity, and high organics can push optimal doses higher. Always validate with jar testing.

How to set and optimize a PAC dose in practice

The most defensible way to determine PAC dose is a jar test tied to measurable outcomes (settled turbidity, filtered turbidity, UV254, color, or downstream filter headloss). Because PAC products vary in basicity and aluminum content, dose optimization should be expressed in both “mg/L as product” and “mg/L as Al₂O₃” for consistent comparisons.

A practical jar-test workflow

1. Define your target: for example, <0.3 NTU filtered turbidity or a specified color/UV254 reduction.
2. Test at least 5 doses spanning low to high (for example, 5, 10, 20, 35, 50 mg/L as product).
3. Keep mixing consistent: rapid mix for dispersion, then controlled flocculation to observe floc size and strength.
4. Record pH before and after coagulation; if pH drifts, include parallel tests with alkalinity adjustment.
5. Evaluate settled water and, when possible, simulate filtration (paper filter or bench filter) to identify the dose that minimizes turbidity and improves filterability.

A common optimization pattern is that clarity improves rapidly up to a point, then plateaus. The operational “best dose” is often the lowest dose that consistently meets targets while preserving filter run time and minimizing chemical cost.

Operational impacts: pH, alkalinity, sludge, and filters

PAC affects more than just turbidity removal. Day-to-day plant performance depends on how PAC shifts pH, how much sludge is produced, and whether downstream filtration remains stable.

pH and alkalinity

Many systems find PAC workable across an operating pH window of roughly 5.5–9.0, but the true optimum is water-specific. If your raw water alkalinity is low, PAC can still depress pH; the difference is that it often does so less aggressively than alum at equivalent performance, particularly at higher basicity grades.

Sludge volume and dewatering

Depending on source water and dose, some facilities report reduced sludge volume compared with alum because effective coagulation can be achieved at lower mass addition. In practical terms, it is reasonable to treat 10–30% sludge reduction as a hypothesis to validate during a controlled trial, rather than a guaranteed outcome.

Filter performance

• If flocs are too small (underdose or poor mixing), turbidity can pass through clarifiers and load filters, shortening runs.
• If overdosed, charge reversal can occur and clarity may worsen; this is often visible as “pin floc” and elevated effluent turbidity.
• The best operating point typically shows rapid settling, resilient floc, and stable filtered turbidity with manageable headloss.

Handling, storage, and safety essentials

Liquid PAC is generally acidic and can be corrosive to incompatible metals. Safe and reliable use requires correct materials selection, secondary containment, and clear operating procedures for transfers and spills.

Practical handling checklist

• Use chemical-resistant tanks and piping appropriate for acidic aluminum salts (confirm compatibility with the product SDS and supplier guidance).
• Maintain secondary containment sized for credible spill scenarios and protect from freezing or excessive heat per supplier recommendations.
• Avoid mixing PAC with incompatible chemicals in shared lines or poorly flushed manifolds; unintended reactions can cause precipitation and plugging.
• Train operators on splash protection and first-aid steps for acidic solutions; follow the Safety Data Sheet (SDS).

If you are switching from alum or ferric salts, plan a staged transition: recalibrate dosing pumps, verify feed-point mixing, and validate treated-water quality under both average and worst-case raw water conditions.

How to choose a PAC product and avoid common failure modes

“PAC” is not one uniform chemical in practice; products differ in basicity, aluminum concentration, and impurity controls. Selection should be driven by your treatment objective and operational constraints, not just delivered price.

Procurement and commissioning checks

• Confirm the product grade (potable vs industrial) and require a current CoA showing aluminum content and relevant impurities.
• Standardize dose reporting: track mg/L as product and mg/L as Al₂O₃ to compare suppliers fairly.
• Verify the feed point provides immediate dispersion (rapid mix) before flocculation; poor dispersion is a frequent root cause of weak performance.
• Run side-by-side jar tests when switching products; even with the same “PAC” label, optimal doses can shift materially.

Troubleshooting signals and fixes

• Rising effluent turbidity after a dose increase: suspect overdosing/charge reversal; step down dose and confirm with jar testing.
• Pin floc and poor settling: verify rapid-mix intensity and injection quill placement; improve dispersion before changing chemistry.
• Unexpected pH drop: confirm alkalinity and consider higher-basicity PAC or controlled alkalinity addition.
• Line plugging or white deposits: check for incompatible chemical mixing, stagnant dead-legs, or insufficient flushing protocols.

The most reliable operational takeaway is simple: PAC works best when dose control, mixing, and monitoring are treated as a single system. Optimizing only the chemical dose without fixing dispersion, flocculation energy, or pH/alkalinity constraints will usually cap performance.