Determining the correct Poly Aluminium Chloride (PAC) dosage is essential for achieving optimal coagulation performance while controlling chemical costs. Under-dosing leads to poor turbidity removal and inadequate treatment, while over-dosing wastes chemicals and can cause re-stabilization of particles. This guide provides a systematic approach to calculating and optimizing PAC dosage for any water treatment application.
Factors Affecting PAC Dosage
Before calculating dosage, it is important to understand the key water quality parameters that influence coagulant demand:
- Raw Water Turbidity: Higher turbidity generally requires higher coagulant doses. Typical PAC dose ranges from 5 mg/L for low-turbidity water ( 100 NTU).
- pH: PAC performs optimally between pH 6.0-8.0. Outside this range, higher doses may be required.
- Temperature: Cold water (< 5°C) increases viscosity, requiring slightly higher doses or longer mixing times.
- Organic Matter (TOC/DOC): Natural organic matter consumes coagulant. Waters with TOC > 5 mg/L may require 20-50% higher PAC doses.
- Alkalinity: Low alkalinity (< 30 mg/L as CaCO₃) may limit coagulation effectiveness even at higher doses.
- PAC Basicity: Higher basicity PAC products (70-85%) generally require lower doses than lower basicity products (40-55%).
Jar Testing: The Gold Standard for Dosage Determination
Jar testing is the most reliable method for determining optimal PAC dosage for your specific water source. Here is the step-by-step procedure:
Equipment Needed
- 6-place jar test apparatus with variable speed control
- 6 × 1-liter beakers (or 2-liter for high-turbidity samples)
- PAC stock solution (typically 1% or 10 g/L)
- pH meter, turbidimeter, thermometer
- Graduated pipettes or syringes for accurate dosing
Procedure
- Fill each beaker with 1 liter of raw water sample.
- Measure and record initial turbidity, pH, temperature, and color.
- Calculate target PAC doses. A typical range covers 0, 5, 10, 20, 30, 50 mg/L as product.
- Add calculated volumes of PAC stock solution to each beaker. Formula: Volume (mL) = (Target Dose mg/L × 1 L) / Stock Concentration mg/mL
- Start rapid mix: 150-300 rpm for 1-2 minutes.
- Reduce to slow mix: 30-50 rpm for 10-15 minutes.
- Stop mixing and allow settling for 15-30 minutes.
- Carefully sample the supernatant from 2-3 cm below the surface.
- Measure residual turbidity, pH, and color of each sample.
- Plot dose vs. residual turbidity to identify the optimal dose.
Dosage Calculation Formulas
1. From Jar Test to Plant Dose
Once the optimal jar test dose is determined (in mg/L), the plant-scale dosing rate is calculated as:
PAC Feed Rate (kg/h) = Optimal Dose (mg/L) × Flow Rate (m³/h) / 1000
Example: For 30 mg/L PAC dose treating 500 m³/h flow:
Feed Rate = 30 × 500 / 1000 = 15 kg/h of PAC product
2. Liquid PAC Dosing Pump Setting
For liquid PAC (10% Al₂O₃, density ~1.2 g/mL):
Pump Flow (L/h) = [Plant Dose (kg/h) / Liquid Concentration (%)] × (100 / Density)
Example: 15 kg/h of 10% liquid PAC:
Pump Flow = (15 / 0.10) × (100 / 1.2) = 125 L/h
3. Powder PAC Solution Preparation
When preparing PAC solution from powder:
Powder Mass (kg) = Solution Volume (L) × Target Concentration (%) × Density
Typical PAC Dosage Ranges by Application
| Application | Typical PAC Dose (mg/L) | PAC Basicity Grade |
|---|---|---|
| Drinking Water (low turbidity < 10 NTU) | 5 – 15 | Medium-High (60-80%) |
| Drinking Water (moderate turbidity 10-50 NTU) | 10 – 30 | Medium (50-70%) |
| Drinking Water (high turbidity > 50 NTU) | 20 – 50 | Medium-High (60-80%) |
| Municipal Wastewater | 20 – 80 | Medium (50-65%) |
| Industrial Wastewater (general) | 30 – 100 | Medium-High (60-85%) |
| Textile Wastewater (color removal) | 50 – 200 | High (70-85%) |
| Paper Mill Effluent | 20 – 60 | Medium (50-65%) |
| Oil/Water Separation | 30 – 80 | High (70-85%) |
| Swimming Pool Water | 2 – 10 | Low-Medium (40-60%) |
| Lake/River Water Treatment | 10 – 40 | Medium (50-70%) |
Optimization Tips
- Start Low, Increase Gradually: Begin at 10 mg/L and increase in 5 mg/L increments during jar testing to find the precise optimal dose.
- Monitor Streaming Current: Online streaming current detectors (SCD) provide real-time coagulation optimization and can reduce PAC consumption by 10-20%.
- Seasonal Adjustment: Raw water quality changes seasonally. Perform jar tests quarterly and after significant rain events.
- Consider PAC Basicity: Higher basicity PAC may allow 15-25% dose reduction. Test different basicity grades during initial optimization.
- Combine with Polyelectrolyte: Adding 0.1-0.5 mg/L of anionic Polyacrylamide after PAC can improve floc strength and reduce PAC dose by 10-30%.
Common Dosing Problems and Solutions
| Problem | Likely Cause | Solution |
|---|---|---|
| High residual turbidity | Under-dosing | Increase dose; verify PAC product quality |
| Pin floc (very small, weak floc) | Over-dosing | Reduce dose; check mixing intensity |
| Carryover of floc to filters | Over-dosing or poor mixing | Optimize dose and mixing energy |
| pH drop below 6.0 | Over-dosing or low-alkalinity water | Add alkalinity; use higher-basicity PAC |
| Variable effluent quality | Inconsistent dosing | Calibrate dosing pump; automate with SCD |
Need help optimizing your PAC dosage? Contact HydroChemix for free technical support, jar testing guidance, and factory-direct PAC samples tailored to your water quality parameters.