SLUDGE DEWATERING
Cationic Polyacrylamide for Sludge Dewatering: How to Choose Charge and Dose
Sludge dewatering is where a polymer programme either earns its cost back or quietly gives it away. The polymer has two jobs at once: neutralise the negative charge on the sludge solids, and build a floc strong enough to survive a belt press or a centrifuge. The grade that does both is decided by the sludge and the machine, not by the price list.
Why sludge needs a cationic polymer
Sludge solids - bacteria, cell fragments, extracellular polymer, fine clay, oil-coated particles - carry a net negative surface charge and hold water in the gaps between them. A cationic polymer works in two steps: its positive charge neutralises the surface, and its long chain bridges the neutralised particles into a floc with enough structure to release water under pressure.
Inorganic conditioners such as ferric chloride and lime still work, but they work by adding mass. Polymer works by changing the surface, which is why the dose is measured in kilograms per tonne of dry solids rather than in percent of sludge volume.
Charge density first, molecular weight second
Charge density decides whether the polymer can neutralise your sludge at all. Molecular weight decides how large and how shear-resistant the floc becomes. Get the charge wrong and no amount of polymer will produce a dry cake.
| Charge band | Sludge it usually suits | What goes wrong when it is wrong |
|---|---|---|
| Low, around 10 - 25% | Primary sludge, mineral and lightly organic solids, some paper mill sludge | Weak floc, wet cake, high polymer consumption |
| Medium, around 25 - 50% | Municipal waste activated sludge and mixed sludge - the workhorse band | Often correct; check molecular weight against shear |
| High, around 50 - 60% | Digested sludge, oily and high-organic industrial sludge, food processing | Over-conditioned, sticky gel-like cake, filtrate turns cloudy |
Our range and where it fits
We supply cationic polyacrylamide from 10% to 60% charge density, so the selection question is which band your sludge sits in, not whether a grade exists. Anionic and non-ionic grades cover mineral sludge, clay and coal washing duties where the charge demand runs the other way.
If you are unsure which ion type you need at all, start with anionic vs cationic: which one your process needs before narrowing the charge band.
Dose is per tonne of dry solids, never per cubic metre
Sludge concentration moves hour to hour. A dose set in litres per hour silently becomes an overdose when the feed thickens and an underdose when it thins, which is why so many plants see a good result on Monday and a wet cake on Wednesday.
The arithmetic is short: sludge flow in m3/h multiplied by dry solids as a decimal gives tonnes of dry solids per hour; multiply that by the target dose in kg per tonne.
| Step | Worked example |
|---|---|
| Sludge flow | 80 m3/h |
| Feed dry solids | 1.2% = 0.012 |
| Dry solids load | 80 x 0.012 = 0.96 t DS/h |
| Target dose | 4 kg per tonne DS |
| Polymer demand | 0.96 x 4 = 3.84 kg/h active |
| Solution at 0.2% | 3.84 / 0.002 = 1,920 L/h |
Typical dose bands
Municipal dewatering commonly lands between 2 and 8 kg of active polymer per tonne of dry solids. Digested, oily and high-organic sludges run above that band. Treat these as budgeting figures: the operating number comes from your own bench test, and field practice usually settles 15 - 20% above the bench optimum because plant mixing and shear are never as clean as a beaker.
Match the polymer to the machine
The same polymer rarely wins on two different machines. A grade that produces beautiful floc in a beaker can break apart in a centrifuge feed line, and that is a shear problem rather than a chemistry problem.
| Equipment | What the floc must survive | Typical cake solids | Selection note |
|---|---|---|---|
| Belt filter press | Gravity drainage, then low-pressure squeeze | 18 - 25% | Large, fast-draining, moderately shear-resistant floc |
| Centrifuge | 2,000 - 4,000 G in seconds | 20 - 30% | Fast conditioning and shear resistance matter more than floc size |
| Filter press | 5 - 15 bar, batch operation | 25 - 40% | Dense, incompressible floc that keeps the cake permeable |
| Screw press | Low shear, long residence time | 20 - 30% | Tolerates a slower grade; watch for sticky cake |
The bench test that predicts the plant
A sludge sample, six beakers and an hour of discipline will tell you more than any product sheet. The sequence that produces usable data:
- Take a fresh, representative sample - at least two litres, from the feed, not from the bottom of the tank
- Measure dry solids and pH before anything else
- Make up 0.1 - 0.2% solution of two or three charge densities and let each age for its full hydration time
- Run a dose series per grade: 2, 4, 6 and 8 kg/t DS equivalent, mixed at the same energy for the same time
- Record floc size, settling speed, filtrate clarity and capillary suction time - CST is the single most useful number for dewaterability
- Where a lab press or a Buchner funnel is available, measure cake solids after pressing rather than judging by eye
Six mistakes that look like a grade problem
Most 'wrong grade' complaints traced back to one of these. Check them before ordering a different polymer.
- Overdosing - excess free polymer makes a slippery, gel-like cake that resists compression. Wet and sticky cake with a clear filtrate is the signature; step the dose down in 10% increments
- Over-mixing - flocs are held together by surface charge and chain entanglement, and a high-shear pump after the injection point destroys both
- Solution too concentrated - above about 0.3% the polymer hydrates unevenly and leaves fish-eyes that never dissolve
- Solution too old - a 0.2% solution loses activity within about 24 hours, so make up to daily demand
- Wrong injection point - the polymer needs 10 - 30 seconds of contact before the dewatering machine, and no long transfer line after floc formation
- Hard or recycled make-up water - calcium and residual anions compete with the cationic charge and can cut performance noticeably
Reading the machine, not the jar
Once the plant is running, the machine tells you which direction to move. The table below maps the common symptoms to the first corrective action.
| Symptom | Likely cause | What to do |
|---|---|---|
| Wet cake, clear filtrate | Overdose | Reduce dose 10 - 20%; recheck feed solids |
| Wet cake, cloudy filtrate | Underdose, or charge density too low | Raise the dose, then test a higher-charge grade |
| Good jar, poor machine | Floc shear or wrong injection point | Move the injection downstream, shorten the line, test higher molecular weight |
| Cake sticks to belt or scroll | Overdose or changed feed | Cut the dose; check feed dry solids |
| Cake solids plateau after two dose increases | Charge or equipment limit | Stop dosing up; test a different charge band |
| Consumption creeping up over weeks | Upstream change in sludge character | Recheck dry solids and volatile fraction before changing grade |
Frequently asked questions
Powder or emulsion cationic polymer?
Powder is 88 - 92% active, keeps for around two years sealed, and is the default for container shipments because you are not paying freight on water. Emulsion is 30 - 50% active, hydrates in minutes, and suits plants that cannot run a powder make-up rig, but it has a shorter shelf life and is sensitive to freezing.
Why is my cake wet and slippery instead of dry and crumbly?
That is the classic overdose signature, especially when the filtrate is clear. Reduce the dose in 10% steps until filtrate clarity just begins to fall, then set the operating point slightly above that.
Can I use the same polymer for clarification and for dewatering?
Usually not. Clarification rewards a high-molecular-weight bridging polymer at low dose; dewatering rewards charge neutralisation plus shear resistance. A grade optimised for one duty is rarely optimal for the other.
How long can I keep the made-up solution?
About 24 hours. Activity falls through hydrolysis and biological growth, and a two-day-old solution can lose a third of its performance before anything else in the plant has changed.