Understanding filtration › 2. The equipment

CHAPTER 2.2

The membrane filter press

Same frame, same cycle, but plates that can squeeze. A flexible membrane, inflated at the end of filtration, compresses the filter cake in its chamber: the cake comes out drier, often sooner. In return, plates and cloths work harder.

Nicolas Ract · 7 min read · October 2026

The principle

A membrane filter press uses the same frame, hydraulic closing system and cycle as the recessed chamber filter press (see chapter 2.1). The difference lies in the plates: behind the cloth, at least one face of the chamber is a flexible membrane. When filtration slows down, water or air is injected between the plate body and the membrane; the membrane inflates, reduces the chamber volume and compresses the filter cake [2][1].

The fact sheet from the U.S. Environmental Protection Agency (EPA) in fact refers to a “variable-volume” filter press, as opposed to the recessed chamber filter press, whose volume is fixed [2].

Membrane plates

Fixed or replaceable membranes

• Fixed membrane: the membrane and the plate body, usually polypropylene, are moulded separately and then welded into a single unit. The absence of a gasket is highlighted as an advantage for food applications [1].
• Replaceable membrane: a polypropylene body takes an elastomer membrane that, according to one manufacturer, can be changed without removing the plate from the press [1][4].

Membrane materials

The manufacturers consulted cite polypropylene, thermoplastic elastomers (TPE) and rubbers such as EPDM, nitrile or fluoroelastomers [1][4]. A review in the trade press specifies that natural rubber is unsuitable for oily slurries or above about 60 °C, and that the food industry requires odourless membranes [5]. As with cloth fibres (see chapter 3.1), the material is chosen according to the temperature, the chemistry of the product and that of the washing agents.

The mixed pack

Membrane plates and recessed chamber plates can be alternated in the same pack: each chamber then has one squeezing face. This arrangement provides squeezing at a lower pack cost than a full membrane pack [1].

Squeezing within the cycle

The cycle follows the steps of the recessed chamber filter press, with one additional step before opening [3]:

• filling and filtration at the feed pump pressure;
• squeezing: the membrane inflates and compresses the cake;
• optionally, cake washing, then air blowing;
• depressurisation of the membrane, opening and cake discharge.

A source from the aggregates sector recommends squeezing with water rather than air above 100 to 125 psi (about 7 to 8.5 bar), following the manufacturer's instructions [3]. Field experience: a compressed gas stores much more energy than water at the same pressure; if a membrane ruptures, the difference is felt.

What you gain, according to the sources

• Dry solids content: the EPA fact sheet indicates a cake that is generally drier than with a conventional filter press, with a more uniform solids content [2]. None of the sources consulted gives a quantified gain valid for all products.
• Cycle time: again according to the EPA, for a target dry solids content the cycle is “usually shorter” [2]. Squeezing takes over from the pump when filtration becomes slow, instead of waiting for the flow to die away.
• A published example: in a trial reported in a trade press review, a squeeze press with 30 mm chambers processed, under its conditions, more than twice the quantity of a conventional press with 8.5 mm chambers [5]. This result applies to that sludge and those settings.
• Cake washing and production throughput are also cited among the advantages, without figures [1].

Capital and maintenance costs are higher; they are justified when a drier cake or faster cycles bring in more [3]. When the cake is disposed of or transported per wet tonne, a few points of dry solids content weigh heavily: that is the calculation in chapter 6.3.

Constraints on the cloths

• Flexing: at each squeeze, the cloth follows the movement of the membrane and then returns. Field experience: cloths folded and unfolded at every cycle fatigue more on the edges and around the feed port; flex resistance matters when choosing the fabric (see chapter 3.2).
• Compression of the cake against the cloth: squeezing pushes fines towards and into the fabric. Field experience: blinding can progress faster there than on a recessed chamber filter; keep track of cycle time (see chapter 4.4).
• Fit: the cloth must follow a moving face. A cloth that is too tight pulls on its fastenings during squeezing; a cloth that is too loose forms folds that mark and wear. Cloth making-up is covered in chapter 3.4.
• Cake discharge: a squeezed cake is more compact. It often drops out better, but a cloth that sticks costs you manual cake discharge.

The membranes themselves

Membranes have a markedly shorter service life than recessed chamber plates, because they flex under high pressure at every cycle; it depends on the number of cycles and the material [3]. Two rules recur in the sources:

• Do not squeeze an empty or underfilled chamber: some designs require a minimum volume of solids before squeezing, otherwise the membrane overstretches and ruptures; there are plates designed to tolerate an empty chamber, at a higher price [3]. One manufacturer, for example, specifies a minimum cake thickness of one third of the chamber thickness before squeezing [4].
• Watch for leaks in the squeeze circuit: Field experience: rising squeeze water consumption, or clear water coming out of one particular chamber during squeezing, often signals a cracked membrane.

Field experience: a punctured membrane lets the squeeze fluid into the cake and wears the cloth facing it. When cloth failures recur on the same plate, check the membrane as well (see chapter 4.2).