Sizing a Polymer Dosing Pump for Sludge Dewatering: From PAM Dose to Pump Flow

Walk into any dewatering house and the two complaints you hear are "the cake is too wet" and "we're burning through polymer". Often the press or centrifuge is fine and the problem is the chemical side: solution strength drifts, and the polymer dosing pump doesn't track sludge feed, so there is too little polymer when the load goes up and far too much when it drops. Sizing has to tie together polymer dosage, solution strength and pump flow in one calculation. Here is how we do it.

Start With the Dry Solids Load

Sludge dewatering polymer dosage is normally expressed as kilograms of active polymer per tonne of dry solids, so the first job is converting wet sludge into dry solids: dry solids = sludge feed × solids content.

An illustration only: a dewatering machine fed 10 m³/h at 2% solids sees about 10 × 1000 × 2% = 200 kg of dry solids per hour, or 0.2 t/h.

Measure solids content, don't copy it from the design basis. How well the thickener is running, and the time of year, can move that figure a long way, and every number downstream moves with it.

Setting the Polymer Dosage

There is no figure you can borrow from another plant. Sludge characteristics, machine type (belt press, screw press or centrifuge), and the charge and molecular weight of the product all change it. The reliable route is a bench test to pick the product, then trimming on the machine while watching cake dryness and filtrate clarity.

Continuing the example, suppose commissioning settles on 4 kg of polymer per tonne of dry solids (again, purely illustrative). That gives 0.2 × 4 = 0.8 kg/h of dry polymer.

A practical trimming method: start the polymer dosing pump in mid range, let the machine settle, then step the dose up or down in small increments, waiting between each change. Log cake condition at each setting and within a few days you will know the working window for that machine. Keeping normal duty in mid range also leaves headroom when the sludge load jumps.

Solution Strength Sets Polymer Dosing Pump Flow

The polymer dosing pump delivers solution, not powder. At 0.2% (2 g/L), 0.8 kg/h of polymer becomes 0.8 ÷ 0.002 = 400 L/h. Dilute to 0.1% and it doubles to 800 L/h.

That is why strength and pump are chosen together. At 0.2%, 400 L/h plus roughly 20% margin is 480 L/h, which points to a NEWDOSE NDWL-530/0.45 (530 L/h, 4.5 bar) or ND2000-500/1.0 (500 L/h, 10 bar). If the site works at 0.1%, you step up to an ND1000-1000/0.4 (1000 L/h, 4 bar) class polymer dosing pump.

Too strong and the solution is hard to draw and mixes poorly with sludge. Too weak and both pump and make-down unit grow. Follow the polymer supplier's guidance and your commissioning results.

The Make-Down Unit Must Out-Produce the Pump

Size the preparation unit and the polymer dosing pump as a pair. The make-down unit's output has to exceed the pump's maximum feed with some spare. In the example the pump needs 400 to 480 L/h, so a three-tank automatic unit in the 1000 L/h class is enough. For small sites running a few hours a day, a batch tank with a slow mixer can work.

Mix gently either way. Sheared polymer chains mean weaker floc and wetter cake.

Sludge Load Changes, So the Polymer Dosing Pump Must Follow

Neither sludge flow nor solids content stays constant, and the polymer dosing pump has to follow them. Two common methods:

1. Electric stroke actuator: NEWDOSE mechanical pumps can take an electric stroke length actuator driven by 4 to 20 mA;
2. Variable speed motor: the AVF code is a VFD-rated motor with a separate cooling fan, for flow control by frequency.

With a sludge flow meter or solids meter, the dose can follow load automatically. Without instruments, check cake and filtrate at least once per shift and adjust by hand.

Calibrate on site

The NEWDOSE mechanical pump manual recommends calibrating at 100%, 50% and 20% stroke, which gives you true output across the whole adjustment range under real conditions. Leave a calibration column connection on the discharge side so you can check the polymer dosing pump whenever you need to.

Piping and Accessories

Polymer solution is viscous, so a few details matter:

• Mount the pump below the storage tank level for flooded suction, with a suction line that is shorter and larger than you would use for water;
• When the pressure difference across the pump is below 1 bar, fit a back pressure valve to stop siphoning;
• Fit a safety valve on a mechanical pump discharge so a closed valve can't overpressure the pump or pipe;
• One polymer dosing pump per injection point, never one pump splitting to two machines.

In cold weather polymer solution thickens and suction losses rise. In unheated buildings, insulate the tank and lines. The injection point matters too: allow enough mixing distance between polymer and sludge, following the dewatering machine supplier's advice.

Conclusion

Run the calculation in one line: sludge feed times solids content gives dry solids; times the commissioned dosage gives polymer per hour; divided by solution strength gives L/h. Add about 20% margin and pick the polymer dosing pump, and make sure the make-down unit out-produces it. Choose stroke or speed control so the dose follows sludge load, and calibrate at several stroke settings after start-up.

FAQ

Is there a standard sludge dewatering polymer dosage I can use?

No figure transfers directly between plants. Sludge and machine type vary too much, so bench tests and commissioning decide it.

The cake is wet. Should I just add more polymer?

Not necessarily. Overdosing can make cake sticky and filtrate cloudy. Check solution strength and maturing time first, then confirm actual pump output.

Polymer dosing pump: diaphragm or progressive cavity?

Both are used. A diaphragm metering pump gives accurate metering and stroke adjustment; for very viscous solution at high flow, some sites choose progressive cavity pumps.

How many polymer dosing pumps per dewatering machine?

For continuous operation, duty and standby. With two machines, give each its own pump rather than sharing one.

How long does made-up polymer solution last?

It degrades over time, so make up what you need rather than large batches, and follow the product data.

Send your sludge flow, solids content, machine type and polymer grade, and contact us to size the pump and make-down unit together.