Activated carbon filters for water: function, performance and choice
How an activated carbon filter purifies water: adsorption, block filter versus granules, what it filters out of drinking water and what to look for when buying.
An activated carbon filter for water binds chlorine, microplastics, hormones, pharmaceutical residues and certain heavy metals. Good activated carbon block filters remove up to 99% of these substances and eliminate unpleasant odours and metallic tastes from tap water. Such filters are not only used in the home. Swimming pools, the beverage industry and wastewater treatment also rely on activated carbon as a filter medium. This guide explains how an activated carbon filter works, what it filters out of drinking water and what to look for when buying.
What is an activated carbon filter?
An activated carbon filter is a water filter that uses activated carbon to bind pollutants to its porous surface. Its effect is based on adsorption. As early as the end of the 18th century, it was known that charcoal attracts substances and decolourises liquids. In 1901, a patent marked the beginning of modern activated carbon production. Since then, medicine, industry and water treatment have used the material. You can find more background in the article What is activated carbon?.
How does an activated carbon filter work?
Adsorption refers to the accumulation of foreign substances on a surface. Pollutants and toxins stick to the activated carbon and accumulate in its pores. The term comes from the Latin “adsorbere”, meaning “to suck in”. It is precisely this effect that turns activated carbon into a filter. A single gram of high-quality activated carbon has an internal surface area of around 1,500 square metres. The pores vary in size. Since its 2015 report, the International Union of Pure and Applied Chemistry (IUPAC) has divided them into 3 classes. Micropores measure up to about 2 nanometres, mesopores 2 to 50 nanometres and macropores over 50 nanometres. Across this huge surface, the water travels a long way, and dissolved substances get caught along the way.
How is activated carbon made?
Activated carbon is made from plant-based raw materials in two steps. First, the starting material is carbonised in the absence of oxygen. This process is called pyrolysis. What remains is almost pure carbon. In the second step, hot gas or a chemical process activates this carbon in a furnace. This creates the highly porous structure. Today, coconut shells or certain hardwoods are usually used as the raw material. The Active cartridge from Alb Filter consists of sintered activated carbon made from coconut shells.
Granular activated carbon or activated carbon block filter?
An activated carbon block filter removes up to 99.9% of contaminants, loose granular activated carbon significantly less. The difference lies in the design. With loose granules, for example in a jug filter, the water flows around the grains instead of through them. This limits the filtration performance. A jug filter does remove chlorine as well as taste and odour from the water. Loose granules, however, let microplastics, pharmaceutical residues, hormones and limescale largely pass through.
An activated carbon block filter, by contrast, compresses the carbon into a solid block, often called a carbon block. The water has to pass through the fine channels. Particularly high-quality block filters are made of sintered activated carbon. During sintering, the block is heated strongly and compacted under pressure. This increases the number of pores and enlarges the internal surface area.
| Feature | Granular activated carbon | Activated carbon block filter |
|---|---|---|
| Type | loosely packed grains | solid, compressed block |
| Water flow | flows around the grains | flows through the channels |
| Filtration performance | limited | high, up to 99.9% |
| Typical use | Jug filter | Under-sink and tap filters |
| Minerals | are partly removed as well | remain in the water |
What does an activated carbon filter remove from water?
An activated carbon block filter binds numerous substances: chlorine, pesticides, hormones, pharmaceutical residues and heavy metals such as lead. It works in three ways at once. Mechanically, the block retains coarse particles such as sand, rust, asbestos fibres, bacteria and legionella. Catalytically, it breaks down chlorine and chlorine compounds and removes the taste of chlorinated water. Through adsorption, it binds dissolved substances such as hormones, pesticides and heavy metals in its pores.
| Substance group | Filtered |
|---|---|
| Chlorine and chlorine compounds | yes |
| Microplastics | yes |
| Hormones and pharmaceutical residues | yes |
| Pesticides and herbicides | yes |
| Heavy metals such as lead and copper | yes |
| Bacteria, germs and legionella | yes (with a fine pore structure) |
| Minerals and trace elements | no, they are preserved |
You can find more on the individual substances in the guide Heavy metals in tap water and in the overview of substances in tap water.
Does an activated carbon filter also remove limescale?
Limescale is the term for dissolved calcium and magnesium in water. Pure activated carbon does not filter out this limescale. Only the combination with an ion exchanger lowers the water hardness. A suitable tap filter does not remove 100% of the limescale. Depending on the water hardness, however, it reduces it significantly. Anyone who mainly wants soft water should therefore look for a filter with an additional ion exchanger.
Do minerals remain in the water?
Important minerals remain in the water: calcium, magnesium, sodium and potassium. A good activated carbon block filter is designed to leave these substances in the drinking water. This is exactly what makes it suitable for use at home. Loose granular activated carbon behaves differently. It partly removes valuable minerals and trace elements from the water as well. Sintered activated carbon, on the other hand, reliably keeps the minerals in the tap water.
Activated carbon filters on the tap and for the water pipe
An activated carbon filter sits directly on the tap, under the sink or in the water pipe. Installation usually takes less than 10 minutes and often requires no tools. A filter on the tap is suitable for the kitchen and can be retrofitted quickly. An under-sink filter disappears into the cupboard and supplies filtered water via its own tap. Both versions work without electricity or an electrical connection. The guide Water filters for the kitchen provides an overview of the systems; models for the cupboard can be found under Under-sink water filters.
How useful is an activated carbon filter?
For most households, an activated carbon filter is a sensible solution. It costs little compared with other filter systems and is highly effective. A good block filter removes more than 99% of many pollutants and improves taste and odour. The water flows clear and fresh straight from the tap. Anyone who filters buys bottled mineral water less often, cutting both costs and their CO2 footprint. Still mineral water in single-use bottles has an environmental impact 90 to 1,000 times greater than tap water. For special requirements, such as removing nitrate, reverse osmosis and ion exchange filters are also an option.
Risk of germ growth and cartridge change
Activated carbon has a limited storage capacity. Depending on the model, the filter cartridge is changed every 4 to 6 months or after around 5,000 litres. Changing it on time is important. Otherwise its effectiveness declines, and accumulated substances can get back into the water. Permanently installed block filters are considered germ-proof if their pore size is around 0.3 micrometres or less. Tested products can be recognised by certificates from TÜV or university institutes. You can read more about this in the article on the risk of germ contamination in jug filters.
What to look for when buying
Four criteria count when buying: design, efficiency, sustainability and replacement interval. First, check whether it uses granules, a block or sintered activated carbon. Ask about the efficiency the manufacturer guarantees. Look for recyclable and renewable materials. And finally, work out how high your own water consumption is and how often the filter cartridge needs to be changed. An overview of suitable models can be found under Drinking water filters.
Related to this topic
Alb Filter FUSION – sintered activated carbon as a block filter
The FUSION under-sink drinking water filter combines a sintered activated carbon block with a second Nano filtration stage. It binds chlorine, microplastics, hormones and heavy metals and leaves the minerals in the water – permanently installed, without electricity and without waste water.
Frequently asked questions
What does an activated carbon filter remove from water?
An activated carbon filter binds many substances from the water: chlorine, microplastics, hormones, pharmaceutical residues and heavy metals. Activated carbon block filters also capture pesticides, asbestos, plasticisers and PFCs (per- and polyfluorinated chemicals). Minerals and trace elements, on the other hand, are retained.
How does an activated carbon filter work?
An activated carbon filter works by adsorption. The ultra-fine pores of the activated carbon form a very large surface area, around 1,500 square metres per gram. The water takes a long path through these pores, and pollutants and toxic substances adhere to them.
Does an activated carbon filter remove limescale?
Activated carbon on its own does not filter out limescale. Only in combination with an ion exchanger does an activated carbon filter reduce water hardness. Depending on the degree of hardness, the limescale content falls significantly, but such a filter does not achieve complete decalcification.
Can an activated carbon filter remove hormones?
Yes. Many municipal waterworks do not remove hormone residues at all, or only inadequately. A high-quality activated carbon block filter binds hormones such as estradiol and ethinylestradiol as well as pharmaceutical residues, for example traces of antibiotics.
How often does the filter cartridge need to be changed?
Depending on the model, an activated carbon filter cartridge lasts about 4 to 6 months or around 5,000 litres. After that, its storage capacity is exhausted. It is best to stick to the manufacturer's specifications, as there is no visible indicator of saturation.
What is sintered activated carbon?
Sintered activated carbon is a compacted form of activated carbon. During sintering, the block is heated to a high temperature and compressed under pressure. This increases the number of pores and the internal surface area. As a result, the filter binds more pollutants while leaving minerals in the water.
