Polyaluminum Chloride Solution: Practical Guide for Water Treatment

Overview: What is polyaluminum chloride (PAC) solution

Polyaluminum chloride (PAC) solution is a pre-hydrolyzed inorganic coagulant containing polymeric aluminum species. It is supplied as a liquid concentrate intended for rapid dosing into raw water, wastewater, or industrial process streams to destabilize colloids and accelerate particle aggregation. PAC solutions are commonly specified by their equivalent Al₂O₃ or Al-content and by solution density and pH. Typical uses include potable water treatment, clarification, sludge thickening, color removal, and tertiary treatment polishing.

Key chemical and physical properties of PAC solutions

Understanding typical properties helps onsite handling, dosing and compatibility decisions. The following table summarizes commonly encountered ranges for commercially supplied aqueous PAC concentrates.

Practical dosing: how to calculate the PAC solution volume

Dosing is typically specified as mass of active ingredient required per unit volume of water (mg/L or g/m³). Most plants receive PAC as a liquid with a known percentage of Al₂O₃ (or % Al). Below is a step-by-step worked example to convert a target active dose into milliliters of PAC solution.

Worked example: deliver 10 mg/L of Al₂O₃ to 1,000 L (1 m³)

Assumptions: PAC solution labeled as 15% Al₂O₃ by mass, density = 1.25 g/mL. Steps:

• Calculate mass of Al₂O₃ required for 1,000 L: 10 mg/L × 1,000 L = 10,000 mg = 10.000 g.
• Convert required Al₂O₃ mass to mass of PAC solution using active fraction: solution mass = 10.000 g ÷ 0.15 = 66.666666... g → round to 66.67 g.
• Convert solution mass to volume using density: volume (mL) = 66.67 g ÷ 1.25 g/mL = 53.3336... mL → report 53.33 mL of PAC solution.
• Practical note: prepare an intermediate dilution or dose with a metering pump; verify with jar tests before full-scale application.

Jar test procedure tailored for PAC solutions

A structured jar test quickly identifies optimum PAC dose and pH adjustments. Follow these practical steps and record results for each condition.

• Prepare multiple 1-L jars filled with raw sample water at the same temperature and turbidity.
• Add PAC solution to jars to create a dose series (for example 2, 4, 6, 8, 10 mg/L as Al₂O₃) using the calculation method above.
• Rapid mix 60–120 s at high speed (100–200 rpm simulated), then slow mix 10–20 min (20–40 rpm) to allow floc growth. Then allow 30–60 min settling.
• Measure turbidity, residual color, and pH in supernatant. Identify the lowest dose that achieves required turbidity and acceptable sludge volume.
• If turbidity improved but residual pH is low, trial small pH adjustments with lime or soda ash and retest—PAC performance often improves near neutral pH depending on raw water chemistry.

Pump selection, feed lines, and on-site dilution

Select metering pumps rated for acidic, viscous fluids and resistant to aluminum salts. Common practical considerations:

• Use chemically compatible tubing (e.g., PTFE, vinyl with chemical rating) and a check valve to prevent backflow.
• For concentrated PAC (high % Al₂O₃), prepare a day-use dilution in a non-metallic tank to reduce pump strain; mix thoroughly before dosing.
• Meter at a point of high turbulence (flash mixer) to ensure uniform distribution; inject upstream of rapid mix when possible.

Storage, handling, and safety practices

PAC solutions are acidic and corrosive to some materials. Implement these safety measures:

• Store in polyethylene, FRP, or coated steel tanks in a cool shaded area. Avoid prolonged exposure to atmospheric CO₂ which can slowly change solution chemistry.
• Provide spill containment and neutralizing materials (e.g., soda ash). Use PPE: acid-resistant gloves, goggles, apron, and face protection.
• Keep incompatible materials (strong bases, oxidizers) separated. Follow local regulations for transport and storage of acidic chemicals.

Common operational issues and troubleshooting

Below are frequent problems when using PAC and practical remedies that process operators can implement.

Poor floc formation or slow settling

Possible causes: underdosing, excessively high natural organic matter (NOM), low temperature, inappropriate mixing energy, or wrong speciation of PAC. Remedy by repeating a jar test, increasing dose in small increments, or combining PAC with a small dose of organic polymer (flocculant) to strengthen floc.

pH drift after dosing

PAC is acidic—large doses can lower pH and harm downstream biological treatment. Monitor residual pH and prepare to add alkalinity (lime, sodium bicarbonate, or soda ash) if necessary. Balance cost vs. operational constraints by minimizing over-dosing.

High sludge volume or poor dewatering

Causes include overdosing, complexed organics, or insufficient polymer conditioning. Solutions: reduce coagulant dose to optimum, test polymers for sludge conditioning, and adjust sludge age or thickening processes.

Environmental and disposal considerations

Sludge generated after PAC treatment contains aluminum hydroxide solids and adsorbed contaminants. Manage disposal according to local regulations:

• Characterize sludge for hazardous constituents before landfill disposal. Consider dewatering to reduce transport volume.
• Explore beneficial reuse options (e.g., soil conditioning) only after regulatory approval and contaminant testing.

Quality control and analytical checks

Maintain confidence in dosing by implementing routine QC checks:

• Verify delivered PAC concentration and density on receipt; keep certificates of analysis.
• Record daily jar tests when raw water quality fluctuates; correlate dose to turbidity, TOC, and temperature.
• Monitor residual aluminum (lab or field test) if regulatory limits require it; adjust process to maintain compliance.

Quick reference: decision checklist before dosing PAC

• Confirm PAC grade, % Al₂O₃, and density from supplier documentation.
• Run jar tests across a realistic dose range and with pH adjustments if needed.
• Verify compatibility with downstream biological or membrane processes.
• Ensure spill control, PPE, and storage requirements are in place prior to receipt and use.

Summary: PAC solution is an efficient, flexible coagulant when its concentration, dosing method, and interactions with raw water chemistry are well understood. Use quantitative dosing calculations, systematic jar testing, and routine QC for reliable, economical treatment results.