PAC Dosage: How to Find the Right Range for Your Water

"What PAC dosage should we use?" is the question we hear most, and the one with the least useful single-number answer. The same poly aluminium chloride can need several times more in a high-solids industrial effluent than in a low-turbidity drinking water plant, and one plant's needs shift between summer and winter, wet season and dry. The reliable approach is to understand what drives PAC dosage, find the optimum with a jar test across a sensible range, convert that into a metering pump setting, and adjust as water quality changes. This guide is for operators at water works, wastewater plants and industrial treatment sites.

What drives PAC dosage

Turbidity and suspended solids. More particles usually means more coagulant. Very low turbidity can be awkward too: with few particles, collisions are rare and floc forms slowly, so dose per cubic metre is not always lower.

Temperature. Cold water is more viscous and coagulation is slower. Winter often needs a higher PAC dosage or more reaction time.

pH and alkalinity. PAC works best within a pH window. It also consumes alkalinity, so in soft, low-alkalinity water an overdose can drop pH noticeably.

Organics and colour. Water high in natural organic matter or colour generally needs more coagulant.

Product strength. Alumina content and basicity vary between suppliers and batches. Change product and you must re-establish the dose.

Process conditions. Mixing intensity, reaction time, clarifier type and whether PAM is used all move the optimum.

Finding PAC dosage with a jar test

A jar test is the most direct way to find the right dose, and shift staff can run one with simple equipment.

Procedure

1. Fill four to six beakers with equal volumes of raw water.
2. Make up a PAC solution of known strength and add increasing doses to each beaker.
3. Mix fast for a minute or two, then slowly for a period. If PAM is part of the process, add it before slow mixing.
4. Let the beakers settle and watch how fast floc forms, how large it grows and how quickly it settles.
5. Measure supernatant turbidity, and pH if you can.
6. Pick the lowest dose that meets the effluent target.

Setting the test steps

Make the first round wide, from clearly too little to clearly too much. Once you have the rough window, run a second round with finer steps inside it. Plant history or supplier guidance can suggest where to start, but your own raw water result is what counts.

Where jar tests go wrong

Results that do not match the plant often come from an unrepresentative sample, test solution left standing too long, mixing that is much harder or gentler than the real tanks, or disturbing the sludge while sampling the supernatant. Fix sampling time, solution preparation and mixing conditions, and repeat the same way each time so results are comparable.

Converting PAC dosage to pump flow

The jar test gives PAC dosage in mg/L. To set the pump:

Chemical needed (g/h) = water flow (m³/h) × dose (mg/L)

Pump flow (L/h) = chemical needed ÷ day tank solution strength (g/L)

Use the diluted strength actually in the tank. For units in more detail, see calculating metering pump flow from water flow. Size the pump for maximum flow at maximum dose with some margin; materials and sizing are covered in PAC dosing pump selection.

Signs of underdosing and overdosing

Underdosing: fine or invisible floc, cloudy clarifier effluent, heavier filter loading and more frequent backwashing.

Overdosing: turbidity stops falling or rises again, loose floc that floats, a clear pH drop, possibly higher residual aluminium, and wasted chemical.

So a higher PAC dosage is not automatically better. When effluent worsens, run a jar test before turning the pump up, to find out whether you are short or over.

Keeping records

A simple log turns jar testing into a tool rather than a one-off exercise. Each time, note the date, raw water turbidity, temperature and pH, the PAC product and batch, the doses tested and the result chosen. After a year you will have your own reference for how PAC dosage moves with the seasons, which makes the first round of each new test much faster. Record the pump setting that went with each chosen dose as well, so a drift in delivered output can be told apart from a change in water quality.

Adjusting PAC dosage in operation

Follow flow. When treated flow changes, the dose in mg/L stays the same and pump output scales with flow. Where flow swings widely, let the pump follow a flow signal.

Follow quality. When turbidity or temperature changes noticeably, repeat the jar test. Do it more often in the rainy season and at season changes.

Prevent crystallisation. PAC solution crystallises at low temperature and when flow stops. Flush heads and lines regularly; see PAC dosing not pumping: crystallisation.

Safety: wear gloves and eye protection when preparing solutions, and rinse any splash on skin with plenty of water straight away.

PAC dosing systems and pumps are listed under our dosing skids.

Conclusion

The right PAC dosage depends on turbidity, temperature, pH, organics, product strength and process conditions, so there is no number to copy from another plant. Find it with a wide-then-narrow jar test, convert it to pump flow using real water flow and tank strength, follow flow automatically and quality by retesting, and keep lines clear of crystals. Send us your flow, raw water quality and current equipment, and contact our engineers to size the right PAC dosing system.

FAQ

What is the most reliable way to set PAC dosage?

A jar test on your own raw water, first with wide steps and then fine steps, confirmed during plant commissioning.

Should PAC dosage go up in winter?

Usually. Cold water slows coagulation, so a slightly higher dose or longer reaction time is often needed. Confirm by testing.

Do I need to retest after changing PAC supplier?

Yes. Products differ in strength and basicity, so the same dose can perform very differently.

What harm does overdosing do?

Turbidity can rise again, pH falls, residual aluminium may increase, sludge volume grows and chemical is wasted.

Does PAC always need PAM?

No. Some waters meet targets with PAC alone. Compare with and without PAM in a jar test.