Weak flash mixing
PAC reacts in seconds. Inadequate rapid-mix energy means part of the dose never contacts the particles and operators compensate by dosing more.
Technical guide
PAC dosage calculation and jar testing
Getting the dose right is worth more than getting the price right. A plant over-dosing by 20% wastes more each year than most buyers ever negotiate off the invoice.
Step 1 — run a proper jar test
Take a representative raw water sample — same source, same time of day as normal operation. Fill six jars with one litre each. Prepare a 1% PAC stock solution: 10 g of solid product, or 10 mL of liquid product, made up to one litre with clean water. At 1%, one millilitre of stock added to one litre of sample equals 10 mg/L dose.
Dose the six jars across a range that brackets your expected optimum — for example 5, 10, 15, 20, 30 and 40 mg/L. Flash mix at around 120 rpm for one minute, then slow mix at 30 rpm for fifteen minutes, then let them settle undisturbed for thirty minutes.
Record for each jar: settled turbidity, final pH, how quickly floc formed, floc size, and settled sludge volume. The optimum is not simply the lowest turbidity — it is the lowest dose that reaches your target turbidity without depressing pH beyond your acceptable band.
Step 2 — convert to plant dosing rate
| What you need | Formula | Worked example |
|---|---|---|
| Product per day (kg) | Dose (mg/L) × Flow (m³/day) ÷ 1000 | 20 mg/L × 5000 m³/day ÷ 1000 = 100 kg/day |
| Solution feed rate (L/h) | kg/day ÷ (solution strength % × 10) ÷ 24 | 100 ÷ (10 × 10) ÷ 24 ≈ 0.42 L/h at 10% solution |
| Monthly consumption (tonnes) | kg/day × 30 ÷ 1000 | 100 × 30 ÷ 1000 = 3 tonnes/month |
| Cost per m³ treated | (kg/day × price per kg) ÷ Flow (m³/day) | (100 × AED 2.50) ÷ 5000 = AED 0.05 per m³ |
For liquid PAC, remember the dose figure refers to product as supplied, not active Al2O3. If you switch between liquid and solid, recalculate — do not carry the same mg/L across.
Troubleshooting
The five mistakes that cause over-dosing
Stale stock solution
Diluted PAC left standing for weeks loses activity. Prepare fresh, and use prepared solution within a week.
Dosing to peak, always
Influent varies through the day. A fixed dose set for peak load wastes chemical for most of the operating cycle.
Skipping seasonal retests
Raw water quality shifts through the year. A dose optimised in January is rarely optimal in July.
Chasing turbidity past target
Once you have hit your target turbidity, more dose buys nothing except cost, sludge, and residual aluminium.
“Optimising a dose from 25 mg/L down to 19 mg/L on a 5000 m³/day plant saves about 11 tonnes of product a year. That is a bigger saving than any price negotiation.”
FAQ
Common questions
Determine the optimum dose by jar test, then apply: kg per day = dose in mg/L × flow in m³/day ÷ 1000. For a 20 mg/L dose at 5000 m³/day, that is 100 kg of product per day.
5 to 10% is the usual working range for solid PAC. Above 10% the solution becomes viscous and dosing accuracy suffers. Use prepared solution within about a week.
At minimum seasonally, and whenever raw water quality changes noticeably — after rainfall, a source change, or a shift in influent load. Many well-run plants jar test monthly.
Yes, substantially. Solid PAC is roughly 28–30% Al2O3 against 10–12% for liquid, so the product dose in mg/L differs by around three times for equivalent treatment. Always recalculate when switching form.
Related
Continue reading
PAC for drinking water treatment
Polyaluminium chloride is the coagulant of choice for potable water clarification across the region — wider effective pH range, lower alkalinity…
PAC for sewage and wastewater treatment
Used in STPs and industrial effluent plants across the UAE for suspended solids removal, COD reduction, phosphorus precipitation and improving sludge…
PAC vs alum: an honest comparison
Alum is cheaper per tonne. PAC is usually cheaper per cubic metre treated. Here is where each one genuinely wins, including the cases where alum is…