How to Dose PAC: A Step-by-Step Operating Routine for Water Treatment
If you want to know how to dose PAC on a real plant, the routine runs in this order: check the tank, lines and pump; make up the solution to the design strength; mix and let it settle; prime and calibrate the metering pump; set the dose from a jar test; watch the results on your rounds; and flush the system when it stops. PAC, polyaluminium chloride, is the most widely used coagulant in water and wastewater treatment. This routine fits drinking water plants, industrial effluent stations and small packaged units, and it is written for shift operators and anyone taking over a dosing room for the first time.
How to dose PAC, step 1: pre-start checks
Look at three things before mixing anything. How much old solution is left in the tank, and is there sediment on the bottom? Is the foot valve or suction strainer crusted with crystals? Does the pump discharge gauge match the reading in the last shift log?
People skip this and pour fresh chemical straight in. New solution mixes with the caked residue, and the pump blocks within minutes of starting. If the tank bottom is heavy with sludge, drain it before you make up a new batch.
How to dose PAC, step 2: PAC solution preparation
Mix to the design concentration, not by eye. The design documents or commissioning records should state the make-up strength; work out chemical and water quantities from that.
Powdered PAC
Fill the make-up tank partway with water, start the mixer, then sprinkle the powder in slowly. Tipping a whole bag at once creates lumps that are wet outside and dry inside, and they can take ages to break down. Top up with water to the set level afterwards.
Liquid PAC
Liquid concentrate is usually diluted before dosing too. Add water first, then chemical, with the mixer running; the diluted strength again comes from the design. Dosing neat concentrate forces the pump to run at a tiny setting, hurts accuracy and crystallizes lines faster.
PAC solution is acidic. Wear gloves and goggles, and rinse any splash off skin with plenty of water straight away.
How to dose PAC, step 3: dissolving and settling
"Aging" or "maturing" really belongs to polymer make-up, where long-chain molecules need time to uncoil. PAC dissolves much faster. Once no particles are visible, let the solution stand for a while so insoluble impurities drop to the bottom, and draw from above them. Follow the chemical supplier's guidance on timing.
Many sites run PAC and polymer from the same procedure and give both the same aging time, which is unnecessary. For how the two work together, see our guide to PAC and PAM dosing sequence.
How to dose PAC, step 4: prime and calibrate the pump
With the tank ready, open the pump's suction and discharge valves and confirm the discharge is open before starting. Starting against a closed valve drives pressure up until the relief valve lifts or a line bursts.
After a fresh batch or a long shutdown, the pump head may hold air. Open the bleed valve until solution flows with no bubbles, then close it. Run a calibration column test to confirm that the stroke and speed settings really give the flow you expect. Any routine for how to dose PAC that skips calibration is guesswork.
How to dose PAC, step 5: set the dose and injection point
The dose comes from a jar test. Take today's raw water, run several beakers at different doses, and pick the one with large floc, fast settling and clear supernatant. Convert that dose and the plant flow into pump flow. For typical ranges and how to test them, read PAC dosage: finding the right range.
Inject where the water is turbulent and mixing is strong, such as just upstream of a static mixer or at the inlet of a flash mix tank. Inject into a dead zone and you get good floc in one half of the basin and almost none in the other.
How to dose PAC, step 6: shift checks after dosing
Knowing how to dose PAC also means knowing what to watch afterwards. Check the flocculation basin at intervals: is floc forming, is it even in size, is the settled water clear? If there is an online turbidity meter, follow the trend rather than a single reading.
When raw water changes sharply, for example after heavy rain, repeat the jar test instead of running yesterday's setting.
Check the pump too. Is discharge pressure steady? Any leaks at fittings? Does the tank level fall at the rate your calculation predicts? A level that barely moves usually means the pump has stopped delivering.
How to dose PAC, step 7: shutdown and flushing
A short stop needs no flushing. For a longer shutdown, run clean water through the pump head and lines so PAC cannot crystallize in the check valves. Crystals jamming the valve balls is one of the most common PAC dosing faults; see PAC dosing pump not delivering: crystals and dilution.
Drain outdoor lines before winter shutdowns. Before opening a pump head, stop the pump, close the valves and release pressure.
Conclusion
How to dose PAC comes down to one sequence: check, mix, settle, prime and calibrate, set from a jar test, watch, flush. None of the steps is hard; most problems come from skipping one, usually priming, calibration or flushing. Complete make-up and feed units are on our dosing systems page, and pumps on our mechanical diaphragm pump page. Send us plant flow, PAC form and injection pressure, and contact our engineers to check the pump and make-up setup.
FAQ
How long can mixed PAC solution be stored?
Use it within a reasonable time and follow the supplier's guidance. Long storage changes strength and performance and leaves more sediment.
My powdered PAC has formed lumps. Can I still use it?
Usually more mixing breaks them down. Do not let undissolved lumps reach the pump; remove them or let them settle and drain them.
What is the most common mistake in how to dose PAC?
Skipping priming and calibration. Without priming the pump runs but delivers nothing; without calibration the dial and real output disagree.
The pump is running but the tank level is not dropping. Why?
Most likely air in the head or a stuck check valve. Bleed the head first, then strip the check valves and look for crystals.
How to dose PAC without wasting chemical?
Set the dose from a jar test, adjust when raw water changes, and inject where mixing is strong. Overdosing by habit costs money and often makes settled water worse.