Mechanical solid-liquid separation – how it works and common filter systems in use

In the production process of most industrial sectors, tasks related to solid-liquid separation inevitably arise. Whether it's the purification of cutting fluids and waste or the separation of liquid components from solids for recycling purposes, the challenges of separating liquids and solids are numerous. Equally numerous are the objectives, processes, and filtration or pressing systems that come into play. In the following, we provide you with an overview of the processes, various filtration solutions, and common application areas. This way, you can find the right solution for your specific use case.

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Filters and Components, Filtration Knowledge

·5 Min. Reading time

Understanding solid-liquid separation

In general, solid-liquid separation refers to all processes that serve the phase separation of disperse solids and liquids. The two-phase mixture to be separated is known as a suspension, slurry or sludge, depending on the liquid or solid content.

Mechanical solid-liquid separation refers to all mechanical processes to achieve the phase separation described. The aim can be either the recovery of the processed liquid or a solid with a low liquid content or as dry as possible.

Mechanical separation processes and their filter systems

To achieve the objectives described above, various methods and systems are used depending on the specific application. Especially when it comes to filtering liquids, there are numerous techniques that require the use of complex equipment. We differentiate between surface filtration and depth filtration, depending on whether particles are retained within a filter layer or settle on the surface of the filter material. Cake filtration combines both approaches. Below, we introduce these mechanical processes for solid-liquid separation with practical examples.

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– Surface filtration (sieve filters, cartridge filters, cross-flow filters)

Surface filtration involves particles to be filtered out settling on the surface of a filter medium. Several filtration techniques operate through surface filtration, including sieve filtration, microfiltration using cartridge filters, and cross-flow filtration.

In sieve filtration, the liquid to be purified passes through a filter sieve and accumulates on it, similar to cake filtration. When a critical pressure loss is exceeded, the filter sieve can be manually cleaned offline, or a backwashing process can be initiated to clean the filter sieve in place. Sieve filters are considered surface filters, mainly used for coarse pre-filtration or as protective filters. The most common configuration among these are sieve basket filters, with sieve cylinder filters being one variant. Usual applications include water treatment in wastewater treatment plants, power plants, refineries, or building technology.


Microfiltrationusingcartridge filters is also a form of surface filtration. The defining feature of microfiltration is the pore size of the filter media, typically ranging from 10 µm to 0.1 µm. In contrast, ultrafiltration, nanofiltration, and reverse osmosis are filtration methods with pore sizes below this threshold. Microfilters operate by utilizing the pressure difference between the inlet and outlet, retaining particles larger than the filter’s fineness. Microfiltration plays a significant role in the mechanical filtration of fluids and is used in industries such as beverage production and the treatment of cooling lubricants.

In microfiltration using cartridge filters, the suspension to be purified flows through the cartridges installed in the filter housing; the solid particles are retained by the filter media inside the cartridges. Thus, surface filtration takes place here.

Cross-flow filtration, also known as tangential flow or cross-flow filtration, is another surface filtration method. Unlike cake filtration, it prevents the formation of a solid layer on the filter material. Here, the suspension flows tangentially over a filter membrane, parallel to the flow direction, at high speed. The resulting turbulent flow prevents cake formation. However, some cross-flow filters allow for thickening and solid recovery.

– Depth filtration (cartridge filters, precoat filters)

In depth filtration, the filtration process occurs inside the filter medium. Various types of filtration systems can be used for this filtration method.

As previously mentioned, cartridge filtration can also be used for depth filtration, with particles being trapped inside the candles. When configured as a depth filter, no filter cake forms. Cartridge filtration finds applications in various industries, from chemicals and pharmaceuticals to food and oil/gas.

Precoat filters, on the other hand, utilize depth filtration to clean suspensions. In a process known as precoating, a filter cake is formed using filter aids, but this cake only prepares for the actual filtration. This type of filtration is suitable for retaining exceptionally small particles and is often used in the treatment of cooling lubricants.

– Cake filtration (cake filters)

Cake filtration is a combinationof surface and depth filtration. It involves solid particles gradually depositing on the filter material’s surface to form a layer known as the filter cake. The suspension must pass through this filter cake, progressively narrowing the passage and retaining finer particles. Over time, this increases flow resistance and pressure loss. Regenerable cake filters, such as FAUDI products, initiate a regeneration process when a certain pressure loss is reached.

Cake filters are used in solid-liquid separation to separate solids from a phase mixture. However, the filter cake can also be further processed and compressed.

Common examples of cake filters include belt filters, such as pressure belt filters, vacuum belt filters, inclined belt filters, and gravity belt filters. These cake-forming filtration solutions all involve the liquid to be cleaned coming into contact with a filter cloth or filter belt, where solid particles are retained. This forms the filter cake, which ensures that finer particles are retained as the filtration progresses. The purified medium flows into a clean tank. These cake-forming filtration systems are used in the treatment of cutting fluids, among other applications.

Filter fineness in mechanical solid-liquid separation

In addition to the various methods and filter systems, filter fineness is a crucial consideration in mechanical solid-liquid separation. Filter fineness determines which particle sizes are retained by the filter medium and which can pass through.

The absolute filter fineness (or geometric pore size) is the diameter of the largest spherical ball that the filter fabric can just allow to pass through. It is determined in a single-pass test, where glass beads are used to simulate “contamination,” and the absolute filter fineness is determined by the largest glass bead that can pass through the filter medium under laboratory conditions.

Nominal filter fineness, on the other hand, is the practically determined filter fineness, which also depends on other influencing factors in reality, such as particle shape and flow conditions. Therefore, it is individually determined based on the suspension to be filtered in conjunction with the filter medium.

When configuring a filter system, filter fineness should be considered to ensure that the filtration task is optimally handled. Our filter systems can be configured for various applications in solid-liquid separation.

Conclusion – solid-liquid separation process

The methods and filtration solutions for solid-liquid separation are diverse. Depending on the objectives and the specific application of mechanical solid-liquid separation, different filtration techniques are suitable, often in combination, to solve the specific separation problem optimally. By using various filter media or filter aids, the systems can be further customized to achieve the desired filter fineness. Need guidance on the right separation method for your application? Feel free to contact us.