Surface filtration vs. depth filtration: these are the differences between the filtration types

What is surface filtration?

Surface filtration is characterized by the fact that the particles to be filtered are retained on the surface of the filter medium. The decisive factor for separation is that the pore size of the layer is smaller than the particle size of the solid. This type of filtration therefore works in a similar way to a sieve, through whose pores the particles cannot pass.

During the filtration process, the retained solids form a growing filter cake on the surface of the filter medium without penetrating into the interior of the layer. At the start of filtration, the layer therefore consists solely of the filter medium, but as filtration progresses, it also consists of the already formed cake. For this reason, surface filtration is also known as cake-forming filtration.

Surface filtration is used, for example, to filter liquids in the food industry and to treat cooling lubricants. Surface filters are also frequently used in water treatment to remove larger particles such as sand or sediment.

Advantages of surface filtration

• High efficiency with large particles: Surface filters are particularly efficient at removing larger particles
• Easy cleaning: The collected particles are on the filter surface and can be easily removed

Disadvantages of surface filtration

• Lower capacity: As the solids accumulate on the surface of the filter medium, the filtrate flow is increasingly reduced by the growing filter cake
• Not suitable for fine particles: If the particles to be filtered are too fine, they can pass through the pores of the filter

Which Faudi filtration processes use surface filtration?

Surface filtration includes, for example, filtration with cartridge filters and sieve filtration.

Faudi cartridge filters are characterized by their high flexibility and can be used for both surface filtration and depth filtration. In microfiltration using cartridge filters, there is a high differential pressure between the inlet and outlet, whereby larger particles that exceed the filter fineness are retained by the filter medium. Surface filtration with cartridge filters is mainly used in the oil and gas industry and in water management.

Sieve filters are often used for the pre-filtration of coarser particles. They are characterized by the fact that the liquid passes through the filter screen while the particles to be filtered are deposited on it. The sieve can then either be removed and cleaned manually or cleaned by starting a backflushing process.

What is depth filtration?

Depth filtration is a filtration process in which the particles are retained within the filter medium. Depth filtration usually takes place in a relatively thick filter medium layer of “grains” or fibers through which the liquid or gas flows.

The prerequisite for depth filtration is that the solid particles are significantly smaller than the pores of the filter medium. As a result, the particles first penetrate into the interior of the filter medium layer and then adhere to its inner surface. Following the filtration process, the separated particles are disposed of together with the filter medium.

Depth filtration is often used in medicine and pharmacy to remove very fine particles or microorganisms. It is also used in the filtration of cooling lubricants and in the chemical industry, as fine filtration is also necessary here to remove unwanted particles.

Advantages of depth filtration

• High particle holding capacity: As the particles are retained inside the filter medium, a depth filter can hold a larger quantity of solids
• Efficient for small particles: Depth filters are particularly suitable for the filtration of very fine particles

Disadvantages of depth filtration

• Single use of the filter medium: as the particles are trapped inside the filter, the filter medium is disposed of with the separated particles after filtration

Which Faudi filtration systems use depth filtration?

Depending on the type and area of application, cartridge filters are also used for depth filtration, as already mentioned. Depending on the design, the liquid flows through the suspended or upright filter cartridges from the outside to the inside and exits again in a cleaned state. Contaminants and dirt particles are retained in the depth structure of the filter cartridge.

Faudi precoat filters are also used for depth filtration. Here, the contaminated liquids flow from the outside to the inside and are filtered by built-in filter elements. However, the filter elements are first precoated with filter aids. The resulting precoat layer, which is deposited on the filter elements, determines the filtration fineness. The actual filtration only begins after the precoating process.