Understanding filtration › 1. Separation basics

CHAPTER 1.2

Filter cake, filtrate and dry solids content

Three terms come up in every discussion about a filter. Defining them precisely, and knowing how to measure the third, lets production, maintenance, the laboratory and suppliers speak the same language.

Nicolas Ract · 7 min read · October 2026

Three definitions

The sludge (or slurry, or feed) is the mixture of solid and liquid entering the filter.

The filtrate is the liquid that passes through the cloth. Depending on the process, it is an effluent to be sent for treatment, water to be recycled or the product itself, such as a juice or a wort. Its quality is judged by its clarity, that is, by the quantity of solids that have passed through.

The filter cake is the layer of solids retained by the cloth, impregnated with the liquid that could not escape from it. Once formed, it is the cake that does the filtering: the cloth merely supports it [3]. The cake is either the valuable product (ore, pigment, crystals) or the waste to be disposed of.

Dry solids content

Dry solids content is the ratio of the mass of dry matter to the total mass of the wet sample, expressed as a percentage. Its complement is the water content, or moisture:

A cake at 30 % dry solids therefore contains 70 % water by mass. You will also come across the terms "dry matter" (DM), "dry residue" or "total solids": for a cake, they refer to the same quantity. Be careful, however, not to confuse dry solids content with the sludge concentration in grams per litre, which relates to a volume.

Field experience: always state which moisture figure you mean. Some sectors, such as minerals, think in terms of moisture; water treatment thinks in terms of dry solids. A misunderstood "20 %" can mean a cake at 20 % or at 80 % dry matter.

Measuring dry solids content: the oven method

The reference measurement is gravimetric: a sample is weighed, dried at 105 °C to constant mass, and weighed again. The international standard ISO 11465:2025, which has incorporated the content of the former European standard EN 12880 on sludges, describes this method (method A, drying at 105 °C) for sludges, biowaste, soils and waste, including filter cakes [1]. In France, NF EN 12880 is listed as withdrawn and replaced by NF EN ISO 11465 of December 2025 [1].

A detailed public method, that of the Quebec Ministry of the Environment laboratory, sets out the steps [2]:

• weigh an empty dish, then the dish with the wet sample;
• dry in the oven at 104 ± 1 °C, in practice overnight;
• leave to cool in a desiccator for at least 4 hours, so that the sample does not pick up moisture from the air;
• weigh the dish with the dry residue; if the specified times are not followed, dry and weigh again until two successive weighings differ by less than 10 mg for a sludge.

The same method notes that a highly mineralised, hygroscopic sample requires prolonged drying [2]. The ISO standard provides another method, by water titration, when the sample contains volatile substances other than water [1], for example solvents.

Taking a representative sample

Field experience: dry solids content varies within the same cake, which is often wetter in the centre of the chamber or near the feed, and drier at the edges. Sample from several points and several plates, mix, and put the sample in a sealed bag as soon as it is taken so that it does not dry before weighing. To follow a trend, always keep the same protocol. Infrared or halogen moisture analysers give a quick result, useful for process control, but they should be checked against the oven method from time to time.

Dry solids with or without reagents

When the sludge is conditioned with lime or ferric chloride, these reagents end up in the dry matter of the cake: according to a fact sheet from the US Environmental Protection Agency, they can account for 15 to 40 % of it [4]. A high dry solids content achieved with a lot of lime therefore does not mean that the water has been better separated from the original solid. To compare two settings or two cloths, compare them at the same reagent dosage.

Compressible or incompressible cake

General guidance: an incompressible cake is made of rigid particles, such as crystals or mineral grains: when the pressure increases, its porous structure hardly changes and the filtrate flow increases with pressure. A compressible cake is made of deformable particles or fragile flocs, such as biological sludges or hydroxides: under pressure, the pores close, the cake resistance increases and the gain in flow diminishes, or even disappears.

Degrémont's Water Handbook (Mémento technique de l'eau) measures this behaviour with a compressibility coefficient, derived from laboratory tests at several pressures (see chapter 1.3), and gives a practical interpretation [3]:

After [3]. Indicative thresholds, established on water treatment sludges.

The same source defines a limiting dry solids content: the value towards which dry solids content tends as pressure is increased. Beyond it, only thermal energy, that is drying, can take it further. Reaching it in operation is generally not cost-effective, as it would require too much reagent and excessively long cycles [3]. Field experience: for a highly compressible cake, membrane squeezing, which compresses the already formed cake, is often more useful than a higher pumping pressure (see chapter 2.2).

Why dry solids content is costly

The mass of cake to be removed equals the dry mass divided by the dry solids content. For the same dry matter, going from 25 % to 30 % dry solids reduces the wet mass by about one sixth: that many tonnes of water that are not transported, not dried and not paid for at disposal. The full calculation, including cloths, shutdowns and cake, is covered in chapter 6.3.

Dry solids content also determines where the cake goes: a French study on sewage sludge dewatering notes, for example, that sludges from belt filters are rarely incinerated because of their low dry solids content [5].