How to Use Drinking Water Grade PAC in Municipal Water Treatment

Overview

Polyaluminum Chloride (PAC) for drinking water is a high-purity coagulant manufactured to food-contact safety standards. This grade carries GB 15892 certification (China national standard for drinking water treatment chemicals) and NSF/ANSI Standard 60 certification (North America), ensuring ultra-low heavy metal content — lead ≤ 0.001%, arsenic ≤ 0.0002%, and cadmium ≤ 0.0002%.

With an Al₂O₃ content of 28–30% and basicity of 40–60%, drinking water grade PAC delivers significantly higher coagulation efficiency than conventional alum. The white color powder form does not impart any yellowing to treated water, unlike yellow-grade industrial PAC. For municipal drinking water plants, bottled water producers, and hospital water systems, this grade is the only appropriate aluminum coagulant.

The pre-polymerized aluminum species in PAC (predominantly Al₁₃ tridecameric species) are more effective at charge neutralization than the monomeric aluminum in alum, enabling lower doses, wider effective pH range (5.0–9.0 vs 5.5–7.5 for alum), and faster settling times.

Preparation & Dissolution

Drinking water grade PAC is supplied as a white to light yellow powder (25 kg bags) and must be dissolved before dosing. The dissolution process must maintain food-grade hygiene standards:

1. Use potable water for dissolution: Always dissolve drinking water PAC in potable or process water — never in industrial water containing heavy metals or organics that could contaminate the stock solution.
2. Prepare 5–10% stock solution: Add PAC powder slowly to water while stirring. Dissolve at a ratio of 1 kg PAC per 9–19 L water for 5–10% solution.
3. Stir for 15–20 minutes: Use a clean, food-grade compatible mixer. Solution should appear clear to light yellow. A 1% solution will have pH 3.5–5.0 — this is normal.
4. Filter through 0.5 mm screen if dissolving powder form to remove any insoluble residue (≤ 0.6% for this grade). Undissolved particles in the dosing line can block metering pump check valves.
5. Day tank hygiene: Clean dissolution and day tanks weekly. Biofilm growth in dilute PAC solutions can occur if tanks are not cleaned. Use potable water rinse and inspect for algae or sediment.
6. Liquid PAC alternative: If the supplier provides liquid PAC (Al₂O₃ 10–11%), it can be dosed directly without dissolution. Verify the liquid form also carries NSF 60 / GB 15892 certification separately.

Dosing Guide

Source Water Quality	Typical PAC Dose (mg/L, powder)	Equivalent Alum Dose	pH Range
Clear, low turbidity (< 5 NTU)	3–10 mg/L	8–25 mg/L	6.5–8.5
Normal turbidity (5–30 NTU)	8–20 mg/L	20–50 mg/L	6.0–8.0
High turbidity (30–100 NTU)	15–35 mg/L	40–90 mg/L	5.5–8.0
Very high turbidity (> 100 NTU)	30–60 mg/L	80–150 mg/L	5.5–7.5
High NOM / colored water	10–30 mg/L + activated carbon	—	6.0–7.5
Pre-filter for bottled water	5–15 mg/L	15–40 mg/L	6.5–8.0

Note: Doses above are for 28–30% Al₂O₃ powder PAC. For liquid forms (10–11% Al₂O₃), multiply by approximately 2.7–3x.

Application Procedure

1. Verify certification before use: Before each new batch or lot, confirm the Certificate of Analysis (CoA) shows GB 15892 or NSF 60 compliance and verify heavy metal results. Keep records for regulatory audit purposes.
2. Conduct jar test: Run jar tests at 5, 10, 15, 20, 30, and 40 mg/L. Apply rapid mix at 200 rpm for 1 minute, slow mix at 30 rpm for 20 minutes, settle for 30 minutes. Record settling turbidity, pH, and floc size.
3. Set dose proportionally: Use flow-paced dosing with a turbidity feedback loop where possible. A 3x turbidity increase should trigger approximately 1.5–2x dose increase (not proportional — coagulant demand is not linear with turbidity).
4. Flash mix: Inject PAC stock solution at the flash mixer inlet. G-value ≥ 300 s⁻¹ for 30–60 seconds. PAC's pre-polymerized species are sensitive to flash mixing intensity — under-mixing reduces efficiency more than with alum.
5. Flocculation: G-value 20–60 s⁻¹ for 20–30 minutes. Tapered flocculation (decreasing G with time) produces the best floc strength for drinking water — typical values: 60 → 40 → 20 s⁻¹ in three equal time stages.
6. Sedimentation: Conventional settling 1.5–3 hours, or DAF for algae-laden water (30–45 min). PAC floc is denser and settles faster than alum floc at equivalent turbidity removal.
7. Filter and disinfect: Dual-media filtration (anthracite/sand), then chlorination. PAC enables < 0.3 NTU post-filter more reliably than alum.

Monitoring & Control

Parameter	Frequency	Regulatory Target
Finished water turbidity	Continuous	< 0.3 NTU (WHO/GB 5749)
Residual aluminum	Daily	< 0.2 mg/L (WHO); < 0.1 mg/L recommended
Coagulation pH	Every 2 hours	6.5–8.0
Raw water turbidity	Continuous	Drive dose adjustment
Raw water temperature	Daily	Below 10°C: increase dose 20–40%
Disinfection byproduct precursors (TOC)	Weekly	Monitor NOM removal via TOC reduction
Product lot certification review	Per delivery	GB 15892 / NSF 60 CoA on file
Heavy metals (finished water)	Monthly	Per local drinking water standards

Common Mistakes

• Using industrial yellow PAC instead of drinking water grade: Industrial PAC (28–30% Al₂O₃ yellow powder) may contain heavy metals exceeding drinking water limits. The yellow color itself comes from iron content (up to 3.5% Fe in industrial grade) that can impart color to treated water. Never substitute industrial grade for certified drinking water grade.

• Skipping lot certification verification: Operators sometimes assume all PAC bags from a trusted supplier are drinking water grade. If the supplier changes raw materials or production batches, certification may not apply to new lots. Always request and verify the CoA for each delivery.

• Preparing concentrated stock solutions: A stock solution stronger than 10% is difficult to meter accurately at low doses. It also increases the risk of localized high-acidity zones at the injection point, causing poor coagulation of the surrounding water. Target 5–8% working solution.

• Ignoring temperature effects: Below 10°C, PAC coagulation efficiency drops significantly. The optimal dose at 5°C may be 1.5–2x the summer dose. Winter jar tests are essential — many operators use summer-optimized doses year-round and wonder why finished water turbidity rises in winter.

• Assuming PAC replaces pH adjustment: While PAC has a wider effective pH range than alum, very soft or acidic raw water (pH < 6.0, alkalinity < 30 mg/L CaCO₃) will still require alkalinity supplementation with lime or sodium bicarbonate for optimal coagulation.

Storage & Handling

• Store in a dry, covered warehouse at 5–40°C. Moisture causes caking and can elevate pH, reducing effectiveness.
• Drinking water grade PAC bags must be stored separately from industrial chemicals and biocides — no cross-contamination risk.
• Label storage area clearly: "Drinking Water Grade PAC — NSF/GB Certified." Implement segregation procedures.
• FIFO stock rotation. Shelf life: 2 years in sealed bags.
• Wear chemical-resistant gloves and eye protection during handling. Avoid breathing PAC dust — wear N95 dust mask when opening bags.
• Dissolution area should have a drain connected to wastewater treatment, not storm drains.
• Spill cleanup: collect dry powder, dissolve in water, neutralize to pH 6–9 before disposal.