Polymer Dosing System Sizing: Three-Tank PAM Units vs Batch Make-Down

Polyacrylamide (PAM) is the workhorse flocculant in wastewater treatment and sludge dewatering, and it is also one of the fussiest chemicals to handle. Dry powder clumps the moment it hits water, the solution is viscous, and too much shear breaks the polymer chains. When buyers ask for a polymer dosing system they usually give one number, "how big", without saying whether that means litres of solution per hour or litres of tank. Here is how we size three-tank units and simple batch make-down setups.

How a Three-Tank Polymer Dosing System Works

A three-tank (or three-chamber) unit splits the vessel into three compartments: dissolving, maturing and storage. Solution moves from one to the next, and the compartments are separated, so each stage gets a fairly consistent residence time. The result is a steadier solution strength than you get from a single tank.

A fully automatic polymer dosing system typically cycles like this. When the storage compartment drops to low level, an electric inlet valve opens and a flow meter starts the dry powder feeder, with powder rate set by water flow. At high level the cycle stops, while the mixers keep running. On site you only need water and power, and the operator's main job is topping up the powder hopper.

Many of these units can also make down emulsion polymers, cationic, anionic or non-ionic, into a dilute working solution.

Step One: Calculate PAM Preparation Unit Capacity

Capacity is how many litres of finished solution the PAM preparation unit produces per hour. Common three-tank polymer dosing system models come in steps from 1000 L/h up to 5000 L/h. The calculation has three parts:

1. Work out dry PAM demand in kg/h;
2. Convert that to solution volume at your working strength, where 0.2% equals 2 g/L;
3. Add margin and round up to the next capacity step.

A worked illustration, not a design value, since real dosage comes from jar tests and commissioning: if you need 1.5 kg/h of PAM made up at 0.2%, that is 1.5 ÷ 0.002, about 750 L/h. With margin, the 1000 L/h model is a sensible choice.

What working strength to use

Typical units produce solution somewhere in the 0.3 to 5 g/L range. The weaker the solution, the more volume you make, and the bigger the polymer dosing system gets. Go too strong and the solution becomes very viscous, matures slowly and is harder for the dosing pump to draw. Set the strength from the polymer supplier's advice and your process, then leave it alone.

Maturing Time Decides Whether the Tank Is Big Enough

With capacity sorted, check that tank volume fits the maturing time. Rough rule: maturing compartment volume ≈ capacity × maturing time. If the product needs one hour to mature (an example only), a 1000 L/h unit needs around 1000 L in that compartment.

Different PAM grades dissolve at very different speeds. Some open up quickly, others need a long soak. Send the product name and the supplier's dissolving time along with your capacity, and the polymer dosing system can be checked properly. Short maturing leaves undissolved lumps, often called fish eyes, that block lines and pumps and weaken flocculation.

Three-Tank Unit or Simple Batch Make-Down?

If your daily demand is small and one or two batches a day is enough, a basic batch tank with a mixer will do. The operator weighs powder, sprinkles it slowly into the vortex, waits for it to mature and doses. The equipment is simple, but solution strength depends entirely on the person mixing it.

For larger, continuous demand, such as a dewatering house running every day, an automatic three-tank polymer dosing system makes more sense. Manual batching under pressure means strength drifts up and down, cake dryness follows it, and polymer gets wasted. Base the choice on daily consumption and how often someone would otherwise have to mix.

Mixing, Powder Feed and Water Supply

Mix slowly. PAM is a long-chain polymer, and high shear cuts those chains and visibly reduces performance. A good polymer dosing system uses low-shear geared agitators. If you are building your own batch tank, the NEWDOSE NDM agitator has a 75 rpm option, with sizes for tank volumes from 100 L to 2000 L.

Keep the powder hopper on any polymer dosing system dry. PAM powder absorbs moisture and cakes, the feed becomes uneven and strength drifts. Don't site the hopper in a humid corner, and keep the lid closed.

Water supply is often overlooked. An automatic polymer dosing system doses powder against a flow meter, so unstable water pressure means unstable solution strength.

Size the Dosing Pump on Actual Feed Rate

The PAM preparation unit makes finished solution. The pump delivers what the process actually needs, and the two numbers are not always the same. For flows in the hundreds of L/h, look at NEWDOSE NDWL, ND1000 or ND2000 mechanical diaphragm pumps. Because polymer solution is viscous, mount the pump below the storage compartment level for flooded suction and use a larger suction line than you would for water. Some sites prefer progressive cavity pumps; the choice depends on viscosity and flow.

Conclusion

To size a polymer dosing system, start from hourly PAM demand, convert it to solution at your working strength, add margin and pick the capacity step. Then check the maturing compartment against the product's maturing time. Small, occasional demand suits a batch tank with a slow mixer; larger, continuous demand suits an automatic three-tank unit. Mix slowly, keep powder dry, give the unit a stable water supply, and size the dosing pump separately on the real feed rate.

FAQ

What is the difference between a three-tank and a two-tank unit?

Three tanks separate dissolving, maturing and storage, which gives steadier strength and maturing time. Two-tank units are simpler and suit lower demand. For continuous duty with tight strength control, three tanks is the usual choice.

Can the same polymer dosing system handle emulsion PAM?

Many can, but powder and emulsion use different feed mechanisms, so specify which one you use when ordering.

How long can made-up PAM solution be stored?

It degrades and thins over time, so make it fresh where possible and follow the supplier's guidance.

Why do I keep seeing gel lumps in the solution?

Usually the powder goes in too fast, the inlet water doesn't disperse it, or maturing time is too short. Check powder feed is even and extend maturing if needed.

Can a polymer dosing system be installed outdoors?

Indoors or under a roof is better. Powder hates humidity, solution thickens in cold weather, and sunlight ages the piping faster.

Send your PAM demand, product grade, working strength and daily running hours, and contact us to have the unit and dosing pumps sized together.