At a Glance
- PAC dosing calculation scales a lab-derived jar test dose (mg/L) up to a full plant’s daily chemical mass and pump rate, not the other way around.
- Optimum pH for PAC typically falls between 6.5 and 7.5 for most surface waters, but shifts with raw water alkalinity, which is why every plant needs its own pH-dose curve.
- PAC dosage generally runs 2 to 10 mg/L for low-turbidity drinking water and can climb to 50 to 150 mg/L for high-strength industrial effluent.
- The jar test is the only reliable way to confirm a starting dose. Calculations get operators in the right neighborhood; the jar test gets them to the actual optimum for that day’s raw water.
- It depends on your raw water, not a spec sheet. The right dose is the one your own jar test confirms, recalculated whenever turbidity, temperature, or basicity grade changes.
What You’ll Learn
This guide covers the poly aluminium chloride dosing calculation formula, how to scale a jar test result to plant volume, where optimum coagulation pH typically sits, and how to run a jar test that holds up in production.
Introduction
A reliable poly aluminium chloride dosing calculation is the difference between a plant that runs on autopilot and one that keeps chasing turbidity spikes. Get PAC dosing wrong and it costs you twice. Underdose and turbidity or COD breaks through the filters.
Overdose and you’re adding residual aluminium to treated water while paying to dewater sludge you didn’t need to generate. Both are avoidable with the right dosing math and a properly run jar test.
Poly aluminium chloride (PAC) has become the default water treatment chemicals upgrade path for utilities and industrial plants moving off alum, and the market reflects it: global PAC demand is on track to roughly double by 2035, moving from around USD 1.7 billion in 2025 toward USD 3.7 billion, at close to an 8% CAGR, as utilities chase PAC’s wider effective pH range and lower sludge output (market.us).
That growth is driven by exactly the operational question this guide answers: how much PAC to dose, at what pH, and how to confirm it before it hits the plant.
For sourcing PAC to your plant’s basicity and grade specification, Request a Quote to validate performance against your own raw water before committing to a supply contract.
PAC Dosing Basics: Why Basicity Changes the Math
For a primer on poly aluminium chloride basics (product chemistry, basicity ranges, and grade selection), start with our foundational guide before diving into dosing math.
PAC is a pre-hydrolyzed inorganic coagulant, meaning part of the aluminium is already partially neutralized before it enters raw water. That’s why it coagulates faster and across a wider pH band than alum. It’s also why dosing isn’t a single universal number: PAC spans a basicity range of roughly 5% to 65%, and basicity drives cationic charge density and efficiency more than Al2O3 content alone. Switching basicity grade or supplier without re-running a jar test is a common cause of unexplained dosing failure.
Dosing calculations also assume you understand where coagulation ends and flocculation begins in your process train. For that process context, see our breakdown of coagulation vs flocculation: coagulation neutralizes particle charge (where PAC dose matters most), while flocculation builds the neutralized particles into settleable floc.
Dosing Calculation: Formula and Worked Example
The calculation happens in two stages: the jar test establishes the optimal concentration in mg/L, then that concentration scales to your plant’s flow rate.
Stage 1: Jar test dose (mg/L) = mass of active PAC added ÷ volume of the water sample.
Stage 2: Scale to plant flow:
Mass required (kg/day) = Plant flow rate (m³/day) × Optimal dose (mg/L) ÷ 1,000
Worked example:
- Jar test optimal dose: 15 mg/L; plant flow: 2,000 m³/day
- Mass required = 2,000 × 15 ÷ 1,000 = 30 kg/day of active PAC
- For liquid PAC at 40% w/w concentration, specific gravity 1.2: active content = 0.48 kg/L
- Product volume needed = 30 ÷ 0.48 = 62.5 liters/day, or roughly 2.6 liters/hour on the dosing pump
Always confirm exact concentration and specific gravity from your supplier’s TDS before finalizing a pump setting; these vary by grade and manufacturer.
Poly Aluminium Chloride Dosing Calculation for Plant Scale-Up
Once the mg/L figure is confirmed at jar-test scale, the same formula holds regardless of plant size, only the flow rate changes. A 500 m³/day package plant and a 50,000 m³/day municipal works use the identical mass-required equation; what shifts is pump sizing, tank turnover, and how often the dose needs re-verification as flow varies through the day.
Larger plants with fluctuating intake (e.g., monsoon-driven turbidity swings) should treat the calculated mass as a moving target, re-checked against a fresh jar test whenever raw water conditions change materially, not as a fixed daily figure.
Optimum pH for PAC Coagulation
PAC’s biggest advantage over alum is a wider effective pH window, generally pH 5 to 9. But within that window, floc formation and settling speed are still measurably better in a narrower band, typically pH 6.5 to 7.5 for many surface waters. That optimum shifts with raw water alkalinity and hardness, which is why standard guidance calls for plotting residual turbidity against pH at a fixed dose to find the plant’s actual optimum, rather than assuming a textbook value (EPA Ireland Water Treatment Manual). Field studies have found optimal PAC performance as high as pH 8 depending on the water source, confirming this is a starting point for testing, not a fixed spec.
Practical rule: run the pH sweep before the dose sweep. Fixing pH incorrectly first produces a dose curve that looks optimized but isn’t.
Typical Dose Ranges and PAC vs. Alum
| Water Type | Typical PAC Dose |
| Drinking water, low turbidity (<50 NTU) | 2 to 10 mg/L |
| Drinking water, moderate turbidity (50–200 NTU) | 10 to 30 mg/L |
| Surface water, high/monsoon turbidity (200+ NTU) | 30 to 60 mg/L |
| Industrial wastewater (textile, pulp and paper) | 50 to 150 mg/L |
Against aluminium sulfate (alum), PAC generally needs less mass per unit of turbidity removed, roughly 1 kg of PAC can replace 3 to 4 kg of conventional alum, produces less sludge, and holds up better in cold water, though alum still wins on raw unit cost for plants with stable, moderate-turbidity source water and existing alum-calibrated dosing systems.
How to Confirm Your Poly Aluminium Chloride Dosing Calculation with a Jar Test
- Collect a representative raw water sample as close to the plant intake as possible, and test it the same day.
- Set up six jars with equal volumes on a multi-paddle stirrer.
- Run a pH sweep first. Adjust each jar to a different pH at a fixed, mid-range dose. Flash mix (~120 rpm) for 1 to 2 minutes, then slow mix (~20-40 rpm) for 15 to 20 minutes.
- Settle for 15 to 30 minutes and note floc size, settling speed, and clarity.
- Identify the pH with lowest residual turbidity. This is your working optimum pH.
- Hold that pH constant and run a dose sweep across a realistic range (e.g., 5 to 40 mg/L).
- Plot dose against residual turbidity. The point where more PAC stops improving clarity is your optimum dose.
- Scale the confirmed dose to plant flow and set the dosing pump.
- Re-run whenever raw water conditions or PAC grade change materially.
pH first, then dose at the confirmed optimum, is the step most operators skip under time pressure, and it’s the biggest reason a “correct” calculation still underperforms on the plant floor.
Request a Quote once your jar test confirms a target dose and grade, so pricing reflects the actual basicity and concentration your plant needs.
Trust and Compliance
Before any PAC shipment enters a regulated process, confirm the Certificate of Analysis (COA) matches the basicity and Al2O3 spec on the Technical Data Sheet (TDS), and keep the Safety Data Sheet (SDS) on file.
For drinking water, verify certification such as NSF/ANSI 60 in North America, REACH in the EU, or IS 299 in India. Batch-to-batch basicity drift is a real, under-discussed cause of dose curves that stop matching plant performance.
Choosing Between PAC Grades
Choose lower-basicity PAC if your water runs acidic. Choose medium-basicity PAC if chemistry varies seasonally and you need one broadly reliable grade, the default for most municipal plants. Choose high-basicity PAC if minimizing sludge and dosing mass matters more than per-kg cost. Stay with alum if your source water is stable and moderate-turbidity, with dosing infrastructure already calibrated to it.
Not sure which grade fits your raw water? <u>Contact us</u> and our technical team can help you shortlist a basicity grade before you run your own jar test.
Conclusion
There’s no universal PAC dose, only a calculation method and a jar test that together confirm the right one for your water on a given day. Get the pH sweep right before the dose sweep, scale correctly to plant flow, and re-test whenever raw water or PAC grade changes. For poly aluminium chloride (PAC) sourced to the exact basicity and grade your jar test confirms, Request a Quote to validate performance before you commit to volume.
