Written by the HyChron Technical Team — water treatment specialists with over 15 years of field experience in municipal and industrial systems. Last reviewed: April 2026
One of the most common operational questions from plants switching to PAC is straightforward but important: once the jar test has identified the optimal dose in mg/L, how do you convert that into actual pump settings, daily chemical consumption, and purchase quantities?
This guide walks through the complete dosage calculation workflow — from jar test result to pump flow rate to monthly consumption estimate — with worked examples for both liquid and powder PAC.
Why Getting the Calculation Right Matters
An error of 20% in dosage calculation affects treatment performance directly. Underdosing leaves particles inadequately coagulated and effluent turbidity above target. Overdosing wastes chemical, increases sludge production, raises residual aluminum, and — at sufficient excess — causes charge reversal that actively worsens treatment performance.
Accurate dosage calculation is also important for procurement planning. Under-ordering PAC results in treatment gaps when stock runs out. Over-ordering ties up capital in excess inventory and risks product degradation beyond shelf life.
Step 1 — Determine the Optimal Dose from Jar Testing
The jar test (ASTM D2035) is the starting point. It produces a dose-response curve showing effluent turbidity at each tested PAC concentration.
From the jar test, identify:
- The minimum PAC dose that achieves your target effluent turbidity
- The dose at which turbidity begins to increase again (charge reversal point)
- Your operating dose = minimum effective dose + 10–15% safety margin, staying below charge reversal
For the full jar testing procedure: How to Dose PAC Correctly in Water Treatment
Step 2 — Convert Dose to Pump Flow Rate
For Liquid PAC
Liquid PAC is typically supplied at 10–11% Al₂O₃ content, with a density of approximately 1.18–1.22 kg/L.
Formula:
Pump flow rate (L/hr) = [Dose (mg/L) × Treatment flow (m³/hr)] / [PAC solution concentration (g/L)]
Worked example:
- Target PAC dose from jar test: 25 mg/L
- Treatment plant flow rate: 200 m³/hr
- Liquid PAC concentration: 100,000 mg/L (10% w/v ≈ 100 g/L)
Pump flow rate = (25 × 200 × 1,000) / 100,000 = 50 L/hr
For Powder PAC (dissolved to working solution)
Powder PAC (28–31% Al₂O₃) is dissolved to a working solution before dosing — typically 5–10% w/v.
Formula:
Working solution concentration (g/L) = dissolution mass (g) / solution volume (L)
Pump flow rate (L/hr) = [Dose (mg/L) × Treatment flow (m³/hr)] / [Working solution concentration (g/L)]
Worked example:
- 10 kg of 30% PAC powder dissolved in 100 L of water = 100 g/L working solution
- Target dose: 25 mg/L, treatment flow: 200 m³/hr
Pump flow rate = (25 × 200 × 1,000) / 100,000 = 50 L/hr
Note: The pump flow rate is the same as liquid PAC in this example because the working solution concentration is equivalent. Adjust for your actual working solution concentration.
Step 3 — Calculate Daily Chemical Consumption
Liquid PAC daily consumption
Daily volume (L/day) = Pump flow rate (L/hr) × Operating hours per day
Example (continuing from above):
- Pump flow rate: 50 L/hr
- Plant operates 24 hr/day
Daily consumption = 50 × 24 = 1,200 L/day of liquid PAC
Daily mass = 1,200 L × 1.20 kg/L (liquid PAC density) = 1,440 kg/day
Powder PAC daily consumption
Formula:
Daily powder consumption (kg/day) = [Dose (mg/L) × Daily treatment volume (m³/day) × Al₂O₃ fraction of target] / [PAC grade Al₂O₃ fraction]
Simplified approach — dose equivalent:
Daily powder consumption (kg/day) = Dose (mg/L) × Daily treatment volume (m³/day) / 1,000
Example:
- 25 mg/L dose, 4,800 m³/day (200 m³/hr × 24 hr)
Daily consumption = 25 × 4,800 / 1,000 = 120 kg/day of PAC as Al₂O₃ equivalent
Actual powder required (at 30% Al₂O₃): 120 / 0.30 = 400 kg/day of 30% powder PAC
Step 4 — Calculate Monthly Purchase Quantity
Safety stock recommendation
Always maintain a minimum of 7–14 days of safety stock — enough to cover supply delays, unexpected demand increases during storm events, or delivery failures.
Monthly purchase quantity:
Monthly consumption = Daily consumption × 30 days Safety stock = Daily consumption × 14 days Order quantity = Monthly consumption + Safety stock − Current inventory
Example (liquid PAC):
- Daily consumption: 1,200 L/day
- Monthly consumption: 36,000 L
- Safety stock target: 16,800 L (14 days)
- Current inventory: 5,000 L
Order quantity = 36,000 + 16,800 − 5,000 = 47,800 L
Step 5 — Account for Seasonal Dosage Variation
PAC dosage requirements are not constant year-round. Raw water quality — turbidity, NOM, temperature — changes with season. A procurement plan based on a single dosage figure will either over-order in good conditions or under-order during high-demand periods.
Recommended approach:
- Estimate dosage for each season based on historical jar test data or raw water quality records
- Calculate quarterly consumption using season-specific dosage estimates
- Adjust safety stock upward before high-demand seasons (storm season, high-turbidity periods)
| Season | Typical Dosage Adjustment vs Baseline |
|---|---|
| Summer (low turbidity, warm) | Baseline dose |
| Autumn (storm events, NOM increase) | +20–40% vs baseline |
| Winter (cold water, low turbidity) | Baseline to +20% (extend flocculation time) |
| Spring (snowmelt, high turbidity) | +30–60% vs baseline |
Reference Dosage Ranges by Application
| Application | Typical PAC Dose | Notes |
|---|---|---|
| Municipal drinking water (low turbidity) | 5–15 mg/L | Jar test essential |
| Municipal drinking water (moderate turbidity) | 15–30 mg/L | Seasonal adjustment needed |
| Industrial wastewater | 20–80 mg/L | Wide range — jar test critical |
| Textile / dyeing effluent | 50–200 mg/L | Depends heavily on dye type |
| Mining / sand washing | 20–60 mg/L | Vary with particle type |
| Sewage treatment (CEPT) | 20–60 mg/L | Jar test for each influent type |
Frequently Asked Questions
My dosage calculation gives a very low pump flow rate — is that correct?
Low pump flow rates are common when treating low-turbidity water with liquid PAC. If the calculated rate falls below your pump’s minimum reliable flow (typically 5–10% of rated capacity), consider diluting the liquid PAC to a lower working concentration, or switching to a more concentrated powder solution to increase the required pump flow rate.
How do I convert mg/L dose to kg per 1,000 m³ treated?
Direct conversion: 1 mg/L = 1 g/m³ = 1 kg per 1,000 m³. A 20 mg/L dose = 20 kg of PAC per 1,000 m³ treated.
Should I calculate consumption based on Al₂O₃ or total PAC product weight?
For procurement planning, calculate based on total product weight (liquid liters or powder kilograms) — that is what you are purchasing. For comparing products of different grades, calculate on an Al₂O₃-equivalent basis to ensure you are comparing like with like.
Conclusion
PAC dosage calculation is a straightforward process once the optimal dose is established by jar test. The key steps — jar test, pump flow calculation, daily consumption estimate, seasonal adjustment — can be completed in less than a day and should be reviewed at every seasonal transition and whenever raw water quality changes significantly.
Getting the calculation right the first time saves chemical cost, prevents compliance risk, and eliminates the guesswork that leads to inconsistent treatment results.
Contact our technical team for a dosage calculation worksheet, jar testing support, and PAC product samples. We respond within 24 hours.
