Filtration Systems

Filtration systems are used to separate solid particles, macromolecules, microorganisms, or viruses from liquids by means of a filter medium. As the liquid flows through the medium, unwanted components are retained. These systems belong to the category of mechanical separation processes.

Filtration processes executed by filtration systems often use standardized terminology from membrane technology. The end product of filtration – the filtered liquid – is referred to as filtrate. The pressure drop occurring during filtration is described as filter resistance. In surface filtration, a filter cake composed of retained solids forms on the filter medium. In membrane technology, the retained phase is known as retentate, while the permeating phase is called permeate.

Filtration systems and their processes are categorized based on the smallest particle or dissolved macromolecule size they can retain. In pressure-driven membrane filtration, the process is classified as follows:

• Microfiltration retains particles and macromolecules of 0.1 micrometers (µm) or larger.
• Ultrafiltration captures particles between 2 nanometers (nm) and 0.1 µm.
• Nanofiltration retains substances smaller than 2 nm.

The liquid to be separated is pumped through a membrane made of materials such as special-coated paper, metal, or ceramic (ceramic membrane). Each filter material provides resistance to all particles within the mixture.

Contrary to common assumptions, filtration does not solely retain particles larger than the pore size of the filter. While the sieve effect is a frequent mechanism, other factors such as particle inertia, diffusion effects, electrostatic interactions, and blocking effects also play a role. As a result, even particles significantly smaller than the pore size of the filter can be separated.

During the filtration process, retained particles accumulate either as a layer (filter cake) on the membrane surface or within the filter matrix (pores). A thick filter cake typically retains particles completely and significantly increases flow resistance. Depending on the filter material—whether paper, metal, or ceramic (ceramic membrane)—this filter cake or deposit must be regularly removed, for example, by shaking, backwashing, compressed air, permeate flushing, or chemical cleaning. When filtration is paused for cleaning, this method is referred to as dead-end filtration, a technique widely used in many systems.

Drinking Water Treatment and Process Water Purification

For drinking water and process water treatment, undissolved particles are filtered out. In the treatment of raw water from sources such as rivers or lakes, coarse impurities are first removed using rake systems. Screening machines then remove medium-sized particles. The final fine filtration is performed using special filter materials, precoat filters, or fine membrane filters, depending on the application.

Static Filtration

Static filtration can be achieved through either vacuum (suction filtration) or positive pressure (pressure filtration). Sterile filtration, which removes microorganisms, requires filter pores smaller than 0.45 µm.

Dynamic Filtration

A modern filtration method is membrane filtration, which operates dynamically. This is a crossflow filtration process, where the filtrate is drawn off perpendicular to the flow direction.

Summary

Filtration systems are devices designed to separate solid particles, macromolecules, microorganisms, or viruses from a liquid. As the liquid passes through the filter medium, unwanted components are retained. These systems frequently use membrane technology and are applied in numerous industrial and municipal fields, ranging from water treatment to food processing. Depending on the pore size of the filter, filtration is categorized into microfiltration, ultrafiltration, or nanofiltration.