Understanding filtration › 2. The equipment

CHAPTER 2.1

The recessed chamber filter press

It is the benchmark machine for pressure filtration: a row of plates clamped tightly together, filter cloths between them, and a pump that pushes the slurry in. Simple in principle, it nevertheless requires an understanding of its cycle to choose and maintain its cloths well.

Nicolas Ract · 7 min read · October 2026

The principle

Vertical plates are lined up side by side between a fixed head and a moving head, pushed by one or more hydraulic cylinders. Each plate has a recess on both faces, so two adjacent plates form a closed chamber. A filter cloth covers each face [1].

The slurry (the “sludge” or the solids-laden “juice”) enters through a port, usually central, that runs through the whole plate pack and feeds all the chambers. The liquid passes through the cloths, drains along the grooves in the plate behind the cloth and leaves through internal channels: this is the filtrate. The solids remain in the chamber and form the filter cake [1][2].

The process is discontinuous: the machine works in successive batches, the cycles. According to the SUEZ Water Handbook, it is the mechanical dewatering device that produces the driest cake, often above 30 % dry solids on sewage sludge [1]. For the concept of dry solids content, see chapter 1.2.

Recessed chamber plates

The sources consulted describe plates mostly made of polypropylene, sometimes of cast iron on large older machines [1]. Some reference points:

• Dimensions range from about 500 × 500 mm to 2,000 × 2,000 mm; a small machine has 20 to 30 plates, a large one 150 to 160 [1].
• Chamber depth sets the cake thickness. For most municipal sludges, the SUEZ Water Handbook recommends 30 mm as a compromise between cycle time and cake mass; about 50 mm for a dense sludge that filters very well [1].
• Pips (bosses) spread over the drainage surface support the cloth and prevent the plates from deforming if a chamber is unevenly filled [1].
• The sloping edges of the recess allow the cake to drop out when the plates are separated [2].

The choice of thickness matters a great deal: a source from the aggregates sector states that going from 25 to 35 mm can double or even triple the cycle time on a slurry of low permeability [4]. The filtration curve explains why (see chapter 1.3).

The cycle, step by step

1. Closing

The moving head clamps the pack, first at low pressure and then at full hydraulic pressure [3]. The closing force must remain greater than the thrust of the slurry in the chambers; on some machines it is adjusted automatically during filtration to keep the seals tight [1].

2. Filling

The pump fills the chambers at a limited flow rate [3]. The SUEZ Water Handbook indicates a filling time of ten minutes at most on sewage sludge, shorter if the sludge filters well [1]. Filling too aggressively drives fines into the still-clean cloth (see chapter 4.4).

3. Filtration

The cake builds up, resistance increases and the pressure rises to its set point, then holds there. Filtration stops when the filtrate flow drops to a set minimum [3]. On municipal sludge, the Handbook cites maximum pressure reached within 30 to 45 minutes and a complete phase of 1 to 5 hours [1]; the fact sheet from the U.S. Environmental Protection Agency (EPA) gives cycles of 1.5 to 6 hours depending on the sludge and its conditioning [2]. In minerals or chemicals, cycles are often much shorter: only a test on the actual product can tell.

4. Air blowing and, where applicable, cake washing

At the end of filtration, water and then compressed air clear the unfiltered slurry from the feed channel (the “core”) [3][2]. Depending on the process, air can also be blown through the cakes to drive out some of the water, or the cake can be washed with a clean liquid to extract a dissolved product [3].

5. Opening and cake discharge

The closing pressure is released, the head retracts and the plates are separated one by one, or in groups. The cake falls under its own weight, with the cloth forming a V between two plates; when it sticks, shaking or scraping systems help to release it [5]. According to one manufacturer, plate shifting typically takes 1 to 5 seconds per plate [3]; the Handbook cites 15 to 45 minutes for a machine with around a hundred chambers, sometimes with manual assistance on sticky cake [1].

Field experience: this is the step at which the condition of the cloths is easiest to see. A cake that stays stuck, a chamber wetter than its neighbours or a dirty sealing edge can be noted at every cake discharge.

These values come mainly from sewage sludge treatment; they vary widely from one product to another.

The role of the cloth

On a filter press, the cloth performs four functions at once:

• retaining the solids, at least until the first layers of cake take over as the filter medium;
• letting the filtrate through without slowing the cycle too much;
• providing the seal between two plates, on the plate edge (see chapter 4.3);
• releasing the cake on opening, otherwise cake discharge becomes manual.

For sewage sludge, the SUEZ Water Handbook describes synthetic cloths, mostly monofilament (polypropylene or polyamide), often laid over a coarser backing cloth that eases filtrate drainage and relieves the fine cloth; tighter multifilament cloths are used for very fine hydroxide sludges [1]. The choice of fibre and fabric is covered in chapters 3.1 and 3.2; fastening the cloth around the feed port in chapter 3.4.

Wear points

• The neck and the feed port: the slurry enters there at high velocity at every filling. Field experience: this is one of the areas where cloths wear or puncture first.
• The sealing edge: crushed at every closing, rubbed at every opening, it wears all the faster when cake remains on it (see chapter 4.3).
• The drainage surface: the cloth is pressed against the pips there; a damaged plate marks the cloth.
• Blinding: it reduces the flow rate without breaking the cloth. For sewage sludge, the Handbook cites cloth washing every 10 to 15 operations with polymer conditioning, every 30 to 40 with mineral conditioning [1]; acid washing to remove lime scale can shorten cloth life [2].
• The plates themselves: chipped edge, warped plate, clogged grooves. They should be checked at every cloth change.

Reading a used cloth and the causes of cloth failure are covered in chapters 4.1 and 4.2. To take dewatering further, the membrane filter press adds a squeezing stage: that is chapter 2.2.