Water treatment: discover the different types of water filtration

Filtration is the process by which of liquid or gaseous substances are purified using various filter processes. It is a mechanical separation process and is based exclusively on a physical basis. Terms such as filtration and filtering are used synonymously in this context. Filtration is essential for water treatment since it is the only way to ensure reliable protection against harmful germs and bacteria.

Water filtration: prioritising water hygiene

Filtration can remove hormones, pesticides, nitrate, nitrite, pharmaceutical residues and any heavy metals from the water. The selection of a specific filtration process depends on the quality of the raw water and the requirements for the end product (tap water, process water, water for medical purposes).

Filters are used in water treatment by local authority water suppliers. The water must be treated in accordance with the provisions of drinking water standards and regulations. More and more private households are now also using additional water filters for hygiene reasons.

Different basic principles of water filtration

Water filtration is a central component of water treatment and is used to remove impurities and make it safe for human consumption. There are various filtration principles that are used depending on the type of impurities and the desired degree of purity of the water.

The three basic filtration principles are:

• Surface filtration
• Depth filtration
• Cake filtration

Surface filtration

Surface filtration is carried out in large filter basins or tanks that are filled with a filter medium such as sand, gravel or activated carbon. Here, the particles are retained directly on the surface of the medium due to their size, as their diameter is larger than the pore size of the filter medium.

This type of water filtration usually only requires a thin layer of filter medium and can also be carried out with very thin membranes. Membrane filters, which have a pore size of just 0.2 μm, are primarily used for sterile membrane filtration in the medical sector in order to provide reliable protection against water-associated germs such as legionella or pseudomonas. Cloth filters are used in sewage treatment plants, for example.

Examples: Membrane filters, filter cartridges, microsieves, cloth filters.

Depth filtration

During the filtration process, the water flows through the filter medium. The larger particles are retained in the upper layers and the smaller particles in the deeper ones. The particle diameter is smaller than the pore diameter so that the particles penetrate the pores and are retained within the filter medium.

In depth filtration, a filter layer of packed granular materials with a considerable depth, typically 1-2 metres, is required to effectively separate the particles from the liquid. This method is particularly suitable for applications with a high dirt load where surface filtration methods such as membrane filtration are not sufficient due to their limited service life.

This type of filtration achieves a high cleaning performance, as very fine particles can be removed.

Cake filtration

Cake filtration combines elements of surface and depth filtration. The retained particles form a filter cake on the filter medium. This filter cake, whose thickness and density can vary depending on the application and particle type, contributes to particle separation or forms the actual filtering layer. It increases the resistance to the water flow and thus leads to an increase in pressure in the filter system.

Cake filtration requires a filter medium that allows the liquid to pass through and retains the solids, as well as a pressure difference. The filter medium can be either flat (such as textile and wire mesh, paper or cardboard) or layered (such as sand or sinter layers).

The method is used in various areas, for example in wastewater treatment plants, where it contributes to the effective separation of solids from the water.

Special filtration processes

In addition to conventional water filtration methods, there are special filtration processes that are particularly suitable for certain applications due to their unique properties and mechanisms. Two of these special filtration methods are membrane filtration and activated carbon filtration. Both processes have proven to be effective in various fields and offer efficient solutions for the removal of contaminants on a microscopic level. In the following sections, we present these two filtration methods in more detail and explain their areas of application, advantages and modes of operation.

Membrane filtration

In the case of the membrane filtration method, the water is filtered through a thin, fine-pored wall (the membrane) which, depending on its nature, can retain even the smallest particles such as viruses or bacteria. Such special filters are referred to as sterile filtration. The majority of membrane filters are made from polymeric materials, i.e. plastics, even though ceramic or metal membranes are also used in some areas.

We also make a distinction between different membrane processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). The first two processes can be described as mechanical fine sieves. They consist of artificially produced membranes with a defined pore diameter. It is important for the function of the membrane that the pores become larger in the direction of the pure water. Nanofiltration and reverse osmosis use dense, diffuse (breathable) membranes.

The principle of reverse osmosis was first discovered in the 1950s during the desalination of seawater. It is a process tha can generate almost 100% pure drinking water from sources such as seawater. In this process, water is pressed through a synthetic, semi-permeable membrane that is only permeable in one direction and only to water molecules.

There are various membrane geometries such as flat sheet membranes, hollow fibre membranes, mono-channels, multi-channel elements, discs and pocket membranes. Hollow fibre membranes, which are employed as point-of-use filters, are primarily used to ensure water hygiene in the medical sector.

Activated carbon water filtration

Activated carbon filtration is essentially based on the principle of adsorption. Pollutants such as dust or heavy metals are removed from liquids or gases with the help of filters containing activated carbon. In water treatment, activated carbon filters are used for the adsorptive retention of organic substances and odours and flavours, or to filter drug residues from the water.

Due to the adsorption technology, activated carbon only has a finite absorption capacity, which means that it needs to be replaced or regenerated after it has been depleted. If the activated carbon is not replaced or regenerated in time, it will be overloaded and the filtrate may contain a higher number of undesirable substances than the raw water. Furthermore, the carbon also acts as a reducing agent and is suitable for removing oxidising agents such as ozone and chlorine from exhaust air or process and wastewater. Activated carbon filters are also available with a membrane core, for example, so that the advantages of both types of water filtration can be combined.

Other chemical and biological processes

In addition, there are various chemical and biological processes for water treatment such as disinfection or biochemical oxidation. The various processes all have their advantages and disadvantages. The choice depends on the specific substances present in the water to be treated and the desired final quality of the water.

Illustrations modified from Förster B, Caesar T: Luftfiltration in Reinraumtechnik (air filtration in clean rooms). Springer 2012