TESTING
The Jar Test, Done Properly: a Ten-Step Procedure for PAC and PAM
The jar test is the cheapest instrument in water treatment and the most often misused. A sloppy procedure gives a dose number that looks precise and behaves randomly in the plant. Done with fixed speeds, timed stages and honest records, a six-beaker run answers the dose question in one afternoon.
Equipment and stock solutions
You need a six-paddle jar tester, six 1-litre beakers, a turbidimeter (even a portable one), a pH meter and a stopwatch. Prepare a PAC stock at 10 g/L - weigh 10 g of solid product, dissolve, make up to 1 litre; 1 mL of this stock in 1 L of sample equals 10 mg/L. For PAM, dissolve the powder at 0.5 g/L with the slow-mixing method described in our preparation guide; freshly made PAM stock works, day-old stock works better.
Label everything. Half of all bad jar data comes from mislabelled beakers or a stock solution that was never actually 10 g/L.
The ten steps
- Fill six beakers with the same raw water, record pH, temperature and turbidity.
- Set a dose ladder: for example 0, 10, 20, 30, 40 and 60 mg/L of PAC.
- Add PAC stock with all paddles running at 300 rpm; mix for 60 seconds. This is the rapid coagulation stage.
- Drop to 50 rpm for 15 minutes - gentle flocculation. Watch when flocs first appear in each beaker.
- Stop paddles, settle 20 minutes. Photograph the beakers at 5 and 20 minutes; photos carry more information than memory.
- Draw supernatant from 2 cm below the surface with a syringe; measure turbidity and pH.
- Pick the beaker with clear water at the smallest dose. If two doses look equal, take the smaller one.
- Check the settled pH: if it fell below 6, the dose is consuming too much alkalinity - repeat with an alkalinity check.
- For polymer, repeat the run with the chosen PAC dose fixed and a PAM ladder of 0.25, 0.5, 1 and 2 mg/L, adding PAM one minute into the slow mix.
- Record everything the same day: dose, turbidity curve, pH, floc appearance. A jar test without notes is an anecdote.
Three mistakes that ruin jar tests
Scaling the dose directly. A jar result of 20 mg/L is a starting point, not a plant setting; hydraulic differences, mixing energy and contact time in the real tank shift the effective dose. Multiply the jar result by 1.2 - 1.5 as an initial plant dose, then trim by effluent data.
Testing water that is not representative. A sample taken after backwash, after a rain event or from a tank that has sat overnight answers a different question. Pull the sample at the actual intake, at the actual hour you care about.
Judging only turbidity. Two doses can give the same final turbidity while consuming very different alkalinity or producing very different sludge volumes. Record pH every time, and scoop the settled floc once to see its texture - dense and gritty versus loose and gelatinous predicts dewatering behaviour.
Frequently asked questions
How much sample water do I need for a full jar test round?
About 10 litres: six beakers at 1 litre, plus spare volume for repeating the promising doses and for the PAM confirmation run. Pull it in one clean container so all beakers see identical water.
Can I run the jar test for PAC and PAM in one pass?
Yes - fix the PAC dose from the first run, then add the PAM ladder one minute into the slow-mix stage of the second run. That mirrors the plant order of coagulant first, polymer after, and shows whether polymer lets you cut the PAC dose.
My jar test says one thing and the plant says another. Why?
The usual culprits are mixing energy and contact time: plant flash-mixers are often far weaker or stronger than 300 rpm on a bench, and short-circuiting in the real tank steals retention time. Match the plant G-value approximately, and treat jar results as a dose corridor rather than a single number.