Filtration technologies at a glance

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Updated on: 03/27/2020

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Activated carbon filter

One of the most widespread and oldest filter technologies is that of activated carbon, which is used either as granulate or in block form. Innovative activated carbon monoblock filters are particularly efficient. They are pressed into a solid block in a state-of-the-art sintering process and activated at high temperatures. An activated carbon monoblock filter has an enormous number of very fine pores and channels in a very small space. For example, one cubic centimetre of activated carbon has an area similar to that of a football pitch. Activated carbon monoblock filters are therefore far superior to loose granulate in terms of filter performance and pollutant retention and are used in municipal drinking water treatment as well as in industry.

Basically, activated carbon filters require neither electricity nor chemicals, but clean the water as it flows through the pores of the activated carbon by three different modes of action: mechanically, catalytically and adsorptively.

Easy to understand is the mechanical filter effect based on the principle of a sieve: all ingredients that are larger than the pores and channels of the carbon are retained, e.g. sand, suspended particles, rust particles, asbestos fibres, microplastics or even bacteria and other microorganisms. The catalytic filter effect is achieved by conversion, e.g. chlorine and chlorine compounds are removed. Finally, the adsorptive filter effect ensures by a kind of "suction effect" that substances are deposited on the surface of the activated carbon. In this way, even inorganic and organic trace substances are retained which are smaller than the actual pores and channels, e.g. heavy metals such as lead or copper as well as pesticides, herbicides, fungicides, hormone and drug residues and odour and taste disturbing substances. However, the water remains in its natural form including all valuable minerals, salts and lime.

Short and sweet:

- Depending on the pore size, solids such as sand, suspended particles, rust particles, asbestos fibres, microplastics or pathogens such as bacteria and viruses are filtered.

-Not filtered are all dissolved substances like nitrate or lime

Not all activated carbon is the same

Most water filters use activated carbon for water refinement. These cause a purification of the water and an improvement of the taste. Such an improvement in taste is achieved, for example, with table can filters. They usually consist of an activated carbon filter and an ion exchanger.

However, these filters are often not sufficient for a real improvement in the quality of the water, which not only subjectively improves the taste. It is important to filter out possible pollutants and germs from the water reliably and safely. The choice of a high-quality manufacturing process is decisive for this.

There are major differences: a filter in which the activated carbon is only loosely poured on top of the water does not achieve the efficiency of a costly production process. First-class water filters differ in that the activated carbon is not glued (extruded) in granulated form or with binders to form a mass, but sintered under high pressure and heat. Consumers should therefore pay attention to the manufacturing process used.

Chlorine and chlorine degradation products

Organic compounds

heavy metals

Particles and sediments

asbestos fibres

Pesticides and pesticide residues

germs & bacteria

Parasites

Residues of medicines

Membrane filtration

Activated carbon block filters have a pore size of about 0.45 µm, which is usually sufficient to reliably remove most pollutants, but not pathogens such as bacteria and viruses. Here, a combination with membrane filtration is the best option. Membrane filtration is not only used in medical technology and industry, but increasingly also in drinking water treatment. It is a purely mechanical filtration process using hollow fibre membrane filters, which are particularly fine and reliably retain not only solids and turbidity, but also more than 99.99% of all bacteria, legionella and parasites (microfiltration with defined pores of approx. 0.15 μm size) and, in the case of ultrafiltration, even viruses (ultrafiltration with defined pores of approx. 0.02 μm size). In this way, even pathogens such as legionella can be almost completely removed from the water. The natural equilibrium of the water is maintained and is not changed, the use of disinfectants or the irradiation of the water with UV light is not necessary.

Reverse osmosis

The reverse osmosis treatment process was originally developed for the desalination of seawater. The process is relatively complex, but offers a high degree of purification of almost 99.9%. Reverse osmosis is based on an important principle of nature - osmosis. The osmosis principle regulates the water balance of a cell by balancing the internal pressure of the cell. Two solutions with different particle concentrations, separated by a semi-permeable membrane, strive to balance their particle concentration, thus creating the so-called osmotic pressure.

Design of a membrane cartridge. Section through the cartridge

Water treatment via reverse osmosis reverses this process. Under pressure, the tap water is pressed against a semi-permeable membrane of the osmosis plant with extremely fine pores through which only the water molecules themselves pass. The osmosis system removes all impurities from the water, such as pathogens, heavy metals, nitrates, pesticides or other pollutants. However, reverse osmosis has a major disadvantage: almost everything is radically removed from the water, including valuable ingredients such as minerals and salts, leaving almost only the water molecules (called permeate or osmosis water). In addition, more waste water is produced than treated water. Although osmosis water can be drunk, it tastes bland due to the lack of minerals. Moreover, it is acidic and therefore aggressive due to a lowered pH-value. Therefore, at least the minerals are often artificially added again afterwards to obtain drinkable water. While the technique of reverse osmosis is useful in technical applications under controlled conditions, e.g. in medical technology and industry, it is rather unsuitable as a filtration method for drinking water in the household. A filter method that leaves the drinking water in its natural composition and structure is more sensible here. 

Ion exchanger

What has already been described above for the table-top can filter is also available as a central large-scale plant for the household: decalcification plants that work with the technology of ion exchange. Here, substances such as calcium and/or magnesium ions, which are responsible for the water hardness, but also nitrates or heavy metals are taken from the water and replaced by other ions. Calcium is usually exchanged for sodium, thus reducing the lime content. However, such ion-exchange systems are actually not real water filters, microorganisms and most pollutants remain in the water (as with the table can filter, see above). 

Distillation

Water distillation, i.e. the evaporation of water with subsequent condensation, is one of the oldest ways of treating water. Tap water can be distilled as well as ground water or contaminated water from natural occurrences. In nature, distilled water is produced by evaporation of water from lakes, rivers and seas. This process removes all pollutants and impurities, but also minerals, trace elements and salts from the water, resulting in a hygienically clean product. Distilled water is mainly used as a solution in chemistry, medicine and biology. However, distillation is a very energy-intensive and therefore quite expensive process (approx. 15 to 20 cents/l). In addition, it takes a very long time, for example, an 800 watt device needs about 4 to 5 hours for 3 litres of water.

Contrary to earlier assumptions, distilled water can be drunk. In Asia, drinking distilled water is even considered to be particularly healthy. However, since distilled water contains hardly any minerals, it supplies the body with fluid, but not enough vital minerals and trace elements. To quench one's thirst with distilled water alone over the long term can lead to disturbances in the mineral balance.