Practical Guide to Polyaluminum Chloride (PAC) for Water Treatment — Dosage, Application & Safety

1. What is polyaluminum chloride (PAC) and how it works

Polyaluminum chloride (PAC) is a pre-hydrolyzed aluminum coagulant widely used in drinking water, wastewater and industrial process water treatment. Chemically it is an amorphous aluminum polymer that combines aluminum species (Al³⁺ and hydrolyzed forms) into a higher-valence polymer that encourages rapid destabilization of colloids and particulate matter. Compared with traditional aluminum sulfate (alum), PAC often performs better at higher turbidity ranges, lower temperatures, and across a wider pH window because its pre-polymerized form produces larger, stronger flocs with faster settling.

2. Key advantages of PAC in practical water treatment

Operators choose PAC when they need reliable coagulation with lower sludge volumes and improved clarity. Practical advantages include:

• Higher destabilization efficiency — removes colloidal organics, turbidity, and some natural organic matter (NOM) at lower doses than alum.
• Wider effective pH range (typically pH 5.5–8.5) — reduces the need for heavy pH correction in many source waters.
• Lower sludge production and easier sludge dewatering compared with many salts.
• Improved performance at low temperatures and when treating algae-impacted waters.

3. Typical PAC product grades and selection (quick reference)

PAC is supplied in liquid or powder forms with varying basicity and active Al₂O₃ (or Al percent). Selecting the right grade depends on source water, required dosing accuracy, and handling preferences.

4. Jar test and dosing procedure — step by step (practical)

A jar test is the standard method to determine optimal PAC dose and coagulant aid needs. Follow these steps for reliable, repeatable results:

• Collect representative raw water and record temperature, pH, conductivity, and turbidity.
• Prepare a series of jars (e.g., 4–8) with identical sample volumes (500–1000 mL).
• Add PAC at increasing doses across the jars (for example 5, 10, 15, 25, 40 mg/L as Al₂O₃ equivalent). Record exact mass/volume used.
• Rapid mix (300 rpm) for 30–60 seconds, then slow mix (30–50 rpm) for 10–20 minutes to form flocs, then allow settling for 30–60 minutes.
• Measure settled turbidity, supernatant clarity, and pH. Compare residuals and select dose that gives target turbidity and acceptable pH change.
• If required, test small additions of polymer (0.05–1.0 mg/L) as a flocculant aid to improve settle and reduce PAC dose.

5. Application methods and common equipment

PAC can be dosed as a concentrated liquid or prepared from powder. Common installation and equipment considerations:

• Dosing pumps: peristaltic or diaphragm pumps with flow-proportional control are preferred for accuracy.
• Tank mixing: stainless or polyethylene mixing tanks with proper agitation to dissolve powdered PAC (if used) and prevent sedimentation.
• Inline static mixers can improve coagulant dispersion before flocculation basins.
• Feed point: inject PAC upstream of rapid mix paddlewheels or static mixers to ensure good contact.

6. Safety, storage and handling best practices

PAC is corrosive to some materials and can be an irritant. Safe handling prevents worker injury and product degradation:

• Storage: keep sealed in a cool, dry, well-ventilated area out of direct sunlight; typical shelf life varies by formulation (check supplier data).
• Materials of construction: use HDPE, polypropylene, or stainless steel (304/316) for tanks and piping; avoid aluminum and galvanized iron for concentrated solutions.
• PPE: gloves, chemical splash goggles, and aprons when handling concentrated PAC; eye wash and shower should be available.
• Spill response: neutralize small spills with soda ash (sodium carbonate) and flush; follow local regulations for disposal of coagulated solids.

7. Troubleshooting common issues and corrective actions

Problem: Poor floc formation or slow settling

Possible causes include underdosing, insufficient mixing energy, low temperature, or excessive organic load. Actions:

• Increase PAC dose incrementally based on jar test results.
• Add low-dose polymer flocculant as a bridging aid (test 0.05–1.0 mg/L).
• Check and adjust mixing speeds/time in rapid/slow mix stages.

Problem: pH drift or increased residual aluminum

PAC may lower pH slightly; monitor residual aluminum especially for potable water. Actions:

• Re-run jar tests while measuring residual Al and pH; choose dose balancing turbidity and residual requirements.
• If residual Al is high, reduce dose or increase settling/filtration efficiency; evaluate use of coagulant aids that lower residuals.

8. Quick dosing guidance table (typical starting points)

The table below gives conservative starting doses for jar testing — always confirm by jar test for your raw water.

9. Final recommendations and supplier checklist

When implementing PAC on-site, confirm these points with your supplier and operations team:

• Obtain the technical data sheet (TDS) showing active Al content, basicity, recommended handling, and shelf life.
• Request material compatibility guidance for tanks, pumps, and piping.
• Perform routine jar tests whenever source water characteristics change (seasonally or after weather events).
• Track residual aluminum and turbidity after treatment to ensure compliance with local potable water standards.