Quick answer: Choose filter press cloth in this order: fibre for chemical and temperature resistance (polypropylene for most slurries, polyester for higher temperature, polyamide for abrasion), then yarn type for the trade-off between cake release and particle capture (monofilament releases, multifilament captures, spun retains fines), then weave and finish for permeability and blinding resistance. Confirm with a filtration test on your slurry, not a catalogue number.
Start with the slurry, not the cloth
Five properties of the slurry decide the cloth before any catalogue is opened:
- Particle size distribution — fine, colloidal solids need a tighter cloth or a precoat; coarse crystalline solids can take an open cloth that filters fast.
- pH and chemistry — strong acid, strong alkali, oxidizers and solvents each rule out some fibres.
- Temperature — polypropylene softens above about 90 °C; polyester and polyamide go higher.
- Cake behaviour — sticky, gelatinous cakes (metal hydroxides, biological sludge) need a smooth release surface; gritty cakes need abrasion resistance.
- Filtrate clarity required — if the filtrate is a product or is reused, capture matters more than throughput.
Step 1: fibre
| Fibre | Strengths | Limits | Typical use |
|---|---|---|---|
| Polypropylene (PP) | Resists most acids and alkalis, low cost, does not absorb water | Loses strength above ~90 °C; attacked by strong oxidizers and some solvents | Sludge dewatering, mining, chemicals, electroplating — the default |
| Polyester (PET) | Higher temperature, good dimensional stability, good abrasion resistance | Hydrolyses in hot alkali; moderate acid resistance | Hot slurries, neutral to mildly acidic process filtration |
| Polyamide (nylon, PA) | Best abrasion resistance and strength, good in alkali | Attacked by acids; absorbs water and can stretch | Abrasive mineral slurries, ceramics, kaolin |
| PVDF, PTFE | Very high chemical and temperature resistance | Cost | Aggressive chemicals, hot concentrated acids |
For sludge dewatering and most industrial wastewater sludge, polypropylene is the answer and the rest of the selection is about yarn and weave. The exceptions are hot process filtration and abrasive slurries.
Step 2: yarn type
The same fibre can be spun into three yarn types, and this choice has more effect on daily operation than the fibre does.
- Monofilament — a single continuous strand, like fishing line. Smooth surface, excellent cake release, easy to wash, resists blinding. Lower particle capture, so the first filtrate can be cloudy until the cake forms. The usual choice for sticky cakes and automatic presses where reliable cake discharge matters.
- Multifilament — many continuous fine strands twisted together. Better capture than monofilament and still reasonable release. The most common general-purpose choice for chemical and mineral slurries.
- Spun (staple) — short fibres spun into a fuzzy yarn. Highest capture of fine particles and clearest filtrate, but the surface holds cake and blinds faster, and washing is harder. Used where filtrate clarity is the priority and the cake is not sticky.
- Mono/multi combinations — monofilament in one direction and multifilament in the other, to get release on the cake side with better capture. A frequent compromise for sludge with fines.
Step 3: weave and finish
Weave sets the openness and how the cloth behaves under pressure. Plain weave is tight and stable; twill is more open and flexible with better cake release; satin has long floats that give the smoothest surface and the best release but less stability. Double-layer weaves put a fine capture layer behind a coarse support layer.
Finishing changes the surface after weaving. Calendering (hot rolling) flattens the surface, closes the pores slightly, improves cake release and reduces blinding; it is standard for sludge cloth. Heat setting stabilizes the dimensions so the cloth does not shrink on the plate. Singeing burns off surface fuzz on spun cloth to improve release.
Permeability and micron rating: which number to trust
Suppliers quote air permeability in litres per square metre per second (or cubic feet per minute per square foot) at a fixed pressure drop, and sometimes a nominal micron rating. Air permeability is the useful one. A low value means a tight cloth: slower filtration, clearer filtrate, faster blinding. A high value means the opposite. The micron rating of a woven cloth is only a rough indicator of the largest particle that passes at the start; once a cake forms, the cake does the filtering and the cloth only has to hold it.
Rules of thumb for a starting point, to be confirmed by test:
- Municipal and biological sludge after polymer conditioning: monofilament or mono/multi PP, calendered, moderate permeability.
- Metal hydroxide sludge: monofilament PP, calendered, tighter than for biological sludge because of fine, gelatinous solids.
- Mining tailings and mineral concentrates: multifilament or mono/multi PP or PA, medium permeability, abrasion-resistant.
- Ceramic, kaolin, stone-cutting slurry: multifilament PP or PA, open weave, high throughput.
- Food, pharmaceutical and fine chemical filtration where filtrate is the product: spun or multifilament, tight, often with a precoat.
Our article on filter cloth micron rating and mesh goes into the numbers, and what causes filter cloth blinding explains the failure that a wrong choice produces.
Sewing and fit: the part that is often ignored
A cloth that is the right material but the wrong fit leaks at the edges, wears at the feed eye and tears at the corners. Check the plate type — recessed chamber, membrane or plate-and-frame — and specify accordingly:
- Feed eye: a centre-feed press needs a cloth with a reinforced neck or a barrel neck; corner-feed plates need eyelets. The neck is where cloth fails first.
- Fixing: latex or rubber edging, ropes, Velcro strips or plate-specific pins. Match the plate maker’s method or the cloth will shift.
- Caulked and gasketed plates need a cloth with a sewn-in cord or gasket, cut to the plate drawing.
- Membrane plates use a cloth on the recessed side only, or a two-piece set; specify which.
Test before you buy in quantity
A laboratory filtration test on a sample of your slurry, with two or three candidate cloths, takes a day and answers the questions a catalogue cannot: filtration time to a given cake thickness, filtrate clarity, cake release and cake moisture. It also gives the data to size the press. Send a sample of the slurry, or of the sludge as it leaves the thickener, and state the pH, temperature and the polymer or coagulant used.
Related: how often to replace filter press cloth, filter press cloth and plates, chamber filter press.
Frequently asked questions
What is the best filter press cloth material?
Polypropylene for most sludge and chemical slurries because it resists acids and alkalis and costs least. Polyester for higher temperature, polyamide for abrasive mineral slurries, PVDF or PTFE for aggressive chemicals. The best material is the one that survives your slurry’s chemistry and temperature.
Monofilament or multifilament filter cloth?
Monofilament for sticky cakes and automatic presses where cake release and washability matter most; multifilament where particle capture and filtrate clarity matter more. Many sludge presses use a mono/multi combination to get some of both.
What does air permeability mean for filter cloth?
The volume of air that passes through a square metre of cloth per second at a standard pressure drop. Low permeability means a tight cloth that filters slowly and clearly but blinds faster; high permeability means faster filtration and cloudier first filtrate. It is a better selection number than a nominal micron rating.
Why does my filter cloth blind quickly?
Usually because the cloth is too tight for the slurry, the slurry has fine or gelatinous solids that lodge in the weave, or the cloth is spun rather than monofilament and holds cake. Precoating, a calendered monofilament cloth, or better conditioning of the sludge are the usual remedies.
Can I use the same cloth on a chamber press and a membrane press?
The fibre and weave can be the same, but the sewn pattern differs: membrane plates take a cloth on the recessed side and often a different fixing. Specify the plate type and size when ordering.
How do I confirm a cloth choice before ordering?
With a laboratory filtration test on your slurry using two or three candidate cloths. It reports filtration time, filtrate clarity, cake release and cake moisture, and the same data sizes the press.
Send a slurry sample and your plate type, and we will test candidate cloths and recommend one with the data: request a filter cloth test.
This article is part of our guide to filter press cloth.