Understanding filtration › 3. Filter media

CHAPTER 3.2

Woven, non-woven, monofilament, multifilament

Two polypropylene cloths can behave in opposite ways: one releases the filter cake on its own, the other retains the finest particles but sticks. The difference lies in the yarn, the way it is assembled and the finish.

Nicolas Ract · 7 min read · October 2026

Three families of yarn

Monofilament

A single extruded strand, thicker, with a smooth surface and high tensile strength. It offers excellent cake discharge and resists blinding well, but retains fine particles less well [1].

Multifilament

Many very fine continuous filaments twisted together. High strength, medium retention and cake release. The more the yarn is twisted, the less it blinds, but the less it retains [1].

Spun yarn (staple fibre)

Fibres cut into short lengths, combed and then twisted, like a conventional textile yarn. Large developed surface area and excellent particle retention, but lower strength and generally poor cake discharge [1].

Summary based on [1]. Many cloths combine several yarns, for example one yarn in the warp and another in the weft, to strike the right compromise.

The weave: how the yarns cross

For woven cloths, the way the yarns cross also changes behaviour. General guidance:

• Plain weave: each yarn passes over then under; a tight, stable fabric with good retention.
• Twill: offset crossings form diagonals; a more supple fabric, a common compromise.
• Satin: long floats of yarn on the surface; a very smooth face, favourable to cake discharge.

Finishing: calendering and treatments

General guidance: calendering compresses and smooths the cloth surface between heated rollers. The cake-side face becomes smoother, which helps cake discharge and limits penetration of fines, at the cost of lower permeability. Other surface treatments exist; they are chosen with the cloth manufacturer, depending on the product.

Non-woven: needle felt

A felt is made of randomly oriented fibres bonded mechanically, without weaving. Its advocates put forward several arguments, published in the trade press by an industry author [2]:

• a much larger open area than a woven fabric (around 70 to 75 % versus 30 to 35 %), hence more flow at equal efficiency;
• good retention of fines, with fine-fibre felts working at the surface without letting solids penetrate into the depth;
• improved cake discharge thanks to a glazed face and, depending on the case, surface treatments;
• better resistance to repeated flexing over the plate edges, where a woven fabric wears.

These arguments come from a source favourable to felts; woven cloths remain the majority on filter presses. The right choice is decided by a trial on the actual product.

Choosing: start from the objective

• The filtrate must be very clear: favour retention (spun yarn, tight weave, calendering, or fine felt), and check that cake discharge remains acceptable.
• The cake sticks and slows down cycles: favour a smooth surface (monofilament, satin, calendering).
• The cloths blind quickly: a smooth, low-penetration yarn, a more gradual filling (see chapter 4.4).
• The cloths wear mechanically: yarn strength and flex resistance matter (see chapter 4.2).