What a jar test tells you

A jar test finds the polymer dose that conditions sludge best — enough polymer to bind fine particles into flocs that release water, but not so much that the sludge turns slimy and holds water again.

The answer is a dose in pounds of polymer per dry ton of sludge. That unit is used because sludge solids concentration varies day to day, but the mass of dry solids you are actually treating is what the polymer has to work on.

The core formulas

The 2000 converts pounds per ton into a simple gram-per-gram ratio. One pound per dry ton is one part polymer per 2000 parts dry solids.

Making solutions

• Stock is typically 0.25 to 1.0% w/v.
• Working solution is typically 0.05 to 0.25% w/v.
• Dry polymer needs 30 to 60 minutes of mixing to fully hydrate. Undissolved gel invalidates the whole test.
• Make working solution fresh each day. Polymer solutions degrade.

The dilution trade-off

A weaker working solution lets you dose more precisely, but you add more water to the jar. Past roughly 20 to 25% added volume the sludge itself is diluted enough to distort drainage readings. If you hit that, use a stronger working solution rather than accepting the error.

Active content matters more than price per pound

Products are sold as commercial product, but dosed on active polymer. A product at 45% active needs roughly twice the weight of one at 90% active to deliver the same active dose.

This is why the comparison stage ranks on cost per dry ton rather than unit price. A cheap, low-activity product is often the most expensive option in practice.

What to observe

CST explained

Capillary suction time measures how long water takes to wick a set distance through filter paper out of your conditioned sludge. Two electrodes time the wet front between them.

Plot CST against dose and you get a U-shape: it falls sharply, bottoms out at the optimum, then rises again as you overdose. If your CST is still falling at the highest jar, you have not found the optimum — test higher doses.

Signs of overdosing

• Drainage slows down compared to the jar below it
• Sludge feels slimy or stringy
• Floc becomes sticky and would blind a belt
• CST rises again after having fallen

Typical dose ranges

Running a fair comparison

• Use one homogenised sludge sample split across all jars, in a single session. Sludge changes through the day.
• Run replicates — two or three jars per product — so you can see repeatability, not just a single reading.
• Mix identically for every jar. Same speed, same duration.
• Randomise or alternate the order you dose and read the jars so time bias does not favour one product.
• Record the same measurements for every product.

Reading the comparison result

The tool ranks products two ways: best technical performance, and lowest cost per dry ton. When those disagree, that is the real decision — and it should be made deliberately rather than by default.

A product that performs slightly worse but costs meaningfully less may still be right. A product that performs much better may justify a premium if it lifts cake solids enough to cut hauling cost. The tool shows you the trade; it does not make the call for you.

Before you scale up

Bench results are a starting point, not a plant setting. Full-scale equipment applies different shear, different mixing energy, and different residence time. Expect to re-optimise on the machine, usually within a couple of pounds per dry ton of the bench answer.