Quick answer: A pulp and paper mill produces three very different sludges: primary sludge (fibre and filler from the clarifier, easy to dewater), secondary sludge (biological, from the aeration stage, hard to dewater) and, in recycled-paper mills, deinking sludge (fibre, ink, clay and calcium carbonate). Screw presses and belt presses handle fibrous primary sludge well; a chamber or membrane filter press is the better choice for high-ash deinking sludge, for mixed primary and secondary sludge when a cake above 35–40% solids is needed for boiler fuel or landfill, and for lime mud and green liquor dregs in kraft mills.
The three sludges of a paper mill
Any discussion of pulp and paper sludge has to begin by separating the streams, because the wrong choice of dewatering equipment usually comes from treating them as one material.
| Sludge | Source | What it contains | Dewatering behaviour |
|---|---|---|---|
| Primary sludge | Primary clarifier or DAF on the mill effluent | Short fibre, fines, filler (clay, calcium carbonate, talc), some starch and size | Easy: fibrous, forms an open cake, high dewaterability |
| Secondary (biological) sludge | Activated sludge plant, aerated lagoon, MBBR or MBR treating the effluent | Bacterial floc, extracellular polymer, bound water | Hard: compressible, gelatinous, high polymer demand |
| Deinking sludge | Flotation and washing stages of a recycled-fibre (DIP) line | Ink, coating clay and carbonate, short fibre, stickies; 40–60% ash | Moderate: high ash makes it dense but the fines blind cloth |
| Lime mud and dregs | Kraft recausticizing: calcium carbonate from the white liquor clarifier; green liquor dregs | Fine inorganic solids, alkaline, hot | Inorganic and easy on a filter press; dregs are sticky and alkaline |
Most mills mix primary and secondary sludge before dewatering, typically in a ratio between 2:1 and 4:1 by dry weight, because the fibre in the primary sludge acts as a filter aid and carries the biological sludge with it. That ratio is the single biggest determinant of how well the mixed sludge dewaters, and the mill’s effluent treatment design fixes it. Our guide to when DAF is needed covers the primary stage, and MBR vs MBBR vs SBR the biological stage.
What the cake is for decides the target
Pulp and paper sludge has more end uses than most industrial sludges, and each has a moisture target:
- Boiler fuel in the mill’s bark or biomass boiler: the sludge must be dry enough to contribute heat rather than absorb it. Below about 35% solids sludge is a net energy loss in the boiler; 40–50% solids is the usual target, and higher is better.
- Landfill: most jurisdictions require a cake that passes a paint-filter test and is stable enough to be trafficked, in practice 25–35% solids for mixed sludge. Landfill fees are charged by wet tonne, so every point of solids is money.
- Land application and composting: 20–30% solids is enough; the constraint is contaminants rather than dryness.
- Cement kilns, brick and board manufacture: deinking sludge with its high mineral content is a raw material; the kiln or plant sets the moisture spec, usually 40–60% solids.
The equipment choice follows from the target. A belt press comfortably reaches 20–30% solids on mixed sludge; a screw press reaches 25–35% on fibrous sludge; a chamber filter press reaches 35–45%; a membrane filter press reaches 40–55% on deinking and mixed sludges. If the cake is going to a boiler or a kiln, the filter press is generally the only equipment that gets there without thermal drying.
Equipment comparison for paper mill sludge
| Equipment | Primary / fibrous sludge | Mixed primary + secondary | Deinking sludge | Notes |
|---|---|---|---|---|
| Screw press | 30–40% solids, excellent | 25–35% | 25–35%, screen wear from ash | Low energy, continuous, tolerant of fibre; poor on sludge with little fibre |
| Belt filter press | 25–35% | 20–30% | 25–35% | Continuous, low capital; polymer sensitive; wetter cake |
| Decanter centrifuge | 25–35% | 20–30% | 30–40% | Compact; high power and polymer; fibre can wrap the scroll |
| Chamber filter press | 35–45% | 30–40% | 40–50% | Batch; driest cake among conventional equipment; cloth blinding from fines and stickies must be managed |
| Membrane filter press | 40–50% | 35–45% | 45–55% | Squeeze compensates for the compressible biological fraction; shortest cycle for a target moisture |
A common configuration in larger mills is a screw press or belt press for routine primary sludge and a filter press for the deinking or mixed sludge that has to go to the boiler. See our full filter press vs centrifuge vs belt press comparison and our multi-disc screw press page.
Designing a filter press for paper mill sludge
Conditioning
Primary sludge often needs no polymer on a filter press, or only a little. Mixed sludge needs a cationic polymer at a dose that rises with the biological fraction, typically 2–6 kg of active polymer per dry tonne. Deinking sludge frequently responds to a dual system: a low dose of inorganic coagulant (alum or polyaluminium chloride) followed by polymer. Jar tests and a filter leaf test on your own sludge are worth a day of anyone’s time; they settle the polymer, the cloth and the cycle time.
Cloth
Fibre does not blind cloth; fines, coating clay, carbonate and stickies do. For primary sludge a multifilament or needle-felt polypropylene cloth releases well. For deinking sludge move to a monofilament or mono/multifilament combination with a calendered face and low permeability, and plan for regular high-pressure cloth washing because stickies (adhesives from recycled paper) build up on the cloth surface. Hot alkaline washing removes them; our cloth selection guide and our article on cloth blinding give the detail.
Feed and cycle
Paper sludge is not abrasive, so a centrifugal or progressive cavity pump with a pressure ramp to 6–8 bar is sufficient. Fibrous sludge fills and filters quickly: cycles of 45–90 minutes are common for primary sludge, longer for mixed sludge with a high biological fraction. Membrane squeeze at 12–15 bar for 10–20 minutes takes the last few points of moisture and, more importantly, equalizes the cake so that a boiler receives a consistent fuel.
Cake handling
Fibrous cake is coherent and drops cleanly, but it is bulky and can bridge in a hopper. Design the cake discharge with a wide hopper, a shredder if the cake goes to a boiler feed screw, and enough conveyor capacity for the instantaneous discharge of a 100-plate press, which drops several tonnes in a few minutes. Our page on fully automatic presses covers discharge and cloth wash automation.
Lime mud and green liquor dregs in kraft mills
In a kraft mill the recausticizing plant produces two inorganic solids. Lime mud (calcium carbonate) is normally dewatered on a precoat drum filter ahead of the lime kiln, but where the kiln is down or the mud is sold, a filter press produces a 65–75% solids cake with no precoat. Green liquor dregs, the dark residue from the green liquor clarifier, are sticky, alkaline and hot, and are one of the more awkward filter press duties in the industry: expect a chamber press with a high-temperature polypropylene plate, a cloth chosen for release rather than clarity, and thorough washing. Dregs cakes at 50–60% solids are landfilled or, increasingly, used in soil amendment. Our chemical industry filter press page covers plate materials for hot, alkaline duties.
Water reuse and the sludge that comes with it
Mills closing their water loops, whether to meet discharge limits or to cut fresh water intake, concentrate dissolved solids and fines in the process water. The extra treatment steps that follow (DAF, membrane filtration, evaporation) each produce more sludge or concentrate, and the dewatering system must be sized for it. Our articles on zero liquid discharge and on how much sludge a treatment plant produces give the method for estimating that load.
Frequently asked questions
What is the best way to dewater paper mill sludge?
It depends on the sludge and on where the cake goes. Screw and belt presses are fine for fibrous primary sludge going to landfill or land application. For deinking sludge, for mixed sludge going to a boiler, and for any duty needing a cake above 35–40% solids, a chamber or membrane filter press is the usual choice.
What cake dryness does a filter press reach on paper sludge?
Typically 35–45% solids on primary or mixed sludge with a chamber press and 40–55% with a membrane press; deinking sludge, with its high ash content, reaches the upper end of those ranges.
Why is secondary sludge so hard to dewater?
It is biological floc with a large amount of water bound in extracellular polymer. It is compressible, so higher pressure alone does little. Mixing it with fibrous primary sludge at 2:1 to 4:1, dosing cationic polymer and using a membrane squeeze are the practical answers.
How dry must sludge be to burn in the mill boiler?
Below roughly 35% solids sludge absorbs more heat evaporating its water than it releases by burning; 40–50% solids or better is the usual target for a net contribution. A membrane filter press is the common way to reach it without a dryer.
What is deinking sludge and how is it dewatered?
The reject from the flotation and washing stages of a recycled-paper line: ink, coating clay, calcium carbonate, short fibre and stickies, with 40–60% ash. It dewaters well in a filter press with a calendered monofilament cloth and regular cloth washing, and the cake is often used in cement kilns or board manufacture.
Can a filter press handle lime mud and green liquor dregs?
Yes. Lime mud filters easily to 65–75% solids. Dregs are sticky, hot and alkaline and need heat-resistant plates, a release-oriented cloth and thorough washing, reaching about 50–60% solids.
Tell us your sludge types, their ratio, tonnage and where the cake goes, and we will propose the press, cloth and conditioning for it: request a paper mill sludge dewatering proposal.
References: US EPA — Pulp, Paper and Paperboard Effluent Guidelines, 40 CFR Part 430, US EPA — Pulp and Paper Sector fact sheet, European IPPC Bureau — Best Available Techniques Reference Document for the Production of Pulp, Paper and Board.
This article is part of our guide to wastewater sludge dewatering.