Activated Carbon Filter

Carbon water filters have been a standard part of water treatment for quite a long time now (for centuries actually since the time of ancient Egyptians).

And here, we’ll tackle everything you need to know about activated carbon water filters in detail including how they work and the various types available plus their advantages and disadvantage.

Today, they are widely used, in small-scale and large-scale water treatment/filtration systems to remove contaminants and unpleasant tastes and odor from both tap and well water.

They come in several different types but the ones that are most often used are granular activated carbon filters (GAC), carbon block filters, and radial flow GAC carbon filters. 

Each has its advantages and disadvantages but the medium and the mechanisms for removing the contaminants are closely related.

They utilize activated carbon which has a very microporous structure with a large surface area. It works by trapping and holding organic chemical molecules present in the water flowing through it.

The effectiveness of these filters, particularly their excellent adsorption capacity and the fact that they are recognized as being safe, are reasons why they are most widely used in home and commercial water treatment systems.

What is activated carbon?

Activated carbon is a form of carbon or material made out of carbon molecules that have been processed, treated, or activated by heat or steam to make them extremely porous and increase their surface area and adsorptive properties.

It’s made from carbonaceous (carbon-based) materials like wood, bamboo, coal, peat, or coconut shells, and can either be in the form of a fine back powder or granules.

How does activated carbon work?

Activated carbon works by trapping chemical contaminants and other toxins in the water through a process known as adsorption.

Being extremely porous, it’s filled with millions of microscopic and submicroscopic pores that together create a very dense network of holes.

These millions of pores provide vast surface space for water to pass through and they are what allow the activated carbon to filter out/grab contaminants like toxic chemicals and hold them in place.

This is accomplished via adsorption which is a distinct process where the organic compounds/chemicals basically adhere to the surface area of the activated carbon instead of getting pulled inside the pores like is the case with the absorption process.

The surface area of the activated carbon has a negative electrical charge hence attracts and holds the positively charged molecules of the organic chemicals and other toxins in the water – they stick only to the surface, they don’t enter into the activated carbon media and no new substance is created afterward.

The process is almost the same as dusting furniture at home where the dust clings only to the dust cloth’s surface.

Everything happens on the surface level and as such, the surface area in activated carbon is what matters the most and its effectiveness depends on how porous it is.

The porous it is, the greater the surface area and the more capacity it has to capture and hold more contaminants within and without it.

How is activated carbon is made?

Activated carbon is basically derived from carbonaceous or carbon-rich organic materials such as wood, coconut shells, nutshells, coal, and peat.

It’s produced through two key processes, carbonization, and activation.

Carbonization Process

This is the first process where the raw material with carbon content is thermally decomposed (pyrolyzed) at extremely high temperatures (600 to 900°C) in an inert environment (such as a tank without oxygen).

It results in a change of chemical composition where elements such as hydrogen, oxygen, sulfur, and nitrogen are eliminated from the source material.

Raw carbon-based materials are essentially full of other substances like organic matter which tend to fill the pores in the carbon. Thereby, the carbonization process equally removes these substances hence leaving behind pure (or almost pure) carbon.

The pores that were filled with these substances become empty which helps increase the surface area of the carbon.

The Activation Process

The charcoal or carbonized material must now be activated in order to fully form the pore structure or microscopic pores across its surface.

Depending on the kind of source material used, the activation process can be done using one or a combination of these two main methods; thermal (physical/steam), or chemical activation.

Thermal Activation

Also known as physical or steam activation, the thermal activation process involves oxidizing or subjecting previously carbonized material to oxidizing atmospheres at high temperatures (600 to 1200°C) using hot gases (like carbon dioxide, argon, nitrogen or argon), air or steam, or a combination of these.

The process takes place in an inert atmosphere.

The carbonized material is oxidized using the hot gases and then followed by air which helps burn out the gases.

The high temperatures at this point change the internal structure of the carbon material by reducing the size of the pores.

Steam is usually then introduced to help open up the pores.

This process creates an extremely elaborate network of millions of interconnected microscopic pores in the carbon, thereby vastly increasing its surface area and giving it more capacity to attract and hold contaminants/organic chemicals.

It generates a highly-porous carbon than regular carbon.

Chemical Activation

Chemical activation involves adding chemicals to the source material before the process of carbonization.

The chemicals typically used are strong bases (like potassium hydroxide or sodium hydroxide), acids (like phosphoric acid), or salts (like zinc chloride).

This is followed by carbonization where the material is exposed to heat at temperatures of around 450 to 900°C and activates quickly.

The activated agent is washed out and recovered.

Like thermal activation, this process results in millions of interconnected series of pores inside the carbon.

Because it only requires low temperatures, the chemical activation method is usually a more efficient and effective way of activating carbon.

Much more pores are created which increases the carbon’s usable surface area greatly.

Besides that, chemical activation usually produces powdered carbon which often doesn’t require further crushing.

The chemically activated carbon can as well be taken through another round of steam activation to give it additional properties.

The downside though, is that this technique is problematic sometimes as, for instance, zinc traces may be left in the end product. However, it’s much preferable because of the lower temperatures as well as the short time required for activation of the raw source materials.

Both activation methods are, however, carried out in rotary kilns, fluidized bed furnaces, fluid bed reactors, multiple hearths, or in a shaft.

After the activation process, the end product (powder or granules) from the source materials can be used for filtering.

It’s the activation process that grants the carbon the filtration properties useful for filtration applications.

It creates more pores (and a more elaborate network of pores) which results in an extremely large surface area that allows the activated carbon to have more capacity to attract and hold chemical impurities (organic chemicals) in the water.

In fact, just a single pound of activated carbon can provide a surface area of 3,000 square meters (32,000 square feet) while a teaspoon can roughly provide an area equal to the size of a football field.

It’s more porous than standard/regular carbon.

Why do we use activated carbon in water filtering?

Water treatment plants usually treat water with both chlorine and chloramines.

These disinfectants and other organic chemicals can still linger in the treated water hence tainting it with an unpleasant chemical flavor.

For this reason, activated carbon is widely used in both small- and large-scale water treatments for both municipal and well water because it has remarkable odor, taste, chlorine, and organic compounds (plus other chemicals) reduction capabilities.  

It’s specifically processed to give it an extremely porous structure resulting in a large surface area which provides contaminants maximum possible exposure to active sites within the carbon so that more of the chemical contaminants can be adsorbed/removed.

The porous texture of the carbons traps microscopic particles and has a negative electrical charge that attracts positively charged molecules of organic chemicals and toxins that make water taste and smell bad.

They adhere to the surface space of the activated carbon and only clean water gets to flow out.

Activated carbon is also so effective that it can also be used to filter air (air purification).

The enormous surface area alone equally makes it useful for various different applications such as sewage treatment, smoke removal, and removing odors from refrigerators or a strong smell from the air.

It’s a common product in hospitals and emergency kits too.

What is an activated carbon water filter?

An activated carbon water filter is a specialized water filter cartridge where activated carbon has been used as the medium to make water clean by capturing chlorine and other organic chemical contaminants.

The activated carbon is either in powdered block or granular form compressed at high pressure into a cartridge which can be of different sizes based on where it’s meant to be used.
How does an activated carbon water filter work?
When water enters an activated carbon water filter, it passes through the carbon media where the contaminants are adsorbed and held within the surface of the carbon.

The clean water then continues to flow down through the media until it reaches the outlet.

The carbon attracts the chemical contaminants at the molecular level just like how magnets attract and hold metal fillings.

Once the surfaces are full, the whole filter must be discarded and replaced, usually, every 2 to 6 months, based on how often it’s used, or less if used daily and the water has high levels of chemical contaminants.

However, the performance or level of filtration provided will depend on the type of activated carbon you use, including factors like the amount of activated carbon in the cartridge, the flow rate of the water, and the amount of contact time as well as the particle or molecule size.

Basically, activated carbon can remove particles ranging from 50 to 0.5 microns in size.

All activated carbon filters are usually rated for CTO (Chlorine, Taste, and Odor) removal though, or called CTO filters which means at the most fundamental level, they can remove chlorine, taste, and odor.

The key difference between them comes down to the pore sizes.

Carbon molecules with large pores will capture big and heavy molecules like the organic chemical molecules while those with small, fine pores are able to trap smaller and lighter contaminants.

What’s actually removed by activated carbon filters?

Activated carbon filters are able to clear out a vast number of contaminants from drinking water.

The CTO-grade cartridges, for instance, are around 10 microns in pore size – they are best at removing chemicals like chlorine, which affect the aesthetic quality of water. They help improve the taste, odor, and color of the water.

There are 5-micron activated carbon filters too and these, aside from chlorine, remove more chemicals such as VOCs like herbicides and pesticides.

One-micron filters, on the other hand, can remove more than 100 different chemicals including all 14 identified pesticides and 12 herbicides as well as all 32 identified organic contaminants.

They remove chlorine, trihalomethanes, oils, chlorinated compounds, and VOCs like benzene, toluene, xylene, and trace-pharmaceuticals, plus more other organic chemicals.

In addition to that, they remove sediment, color, unpleasant odors, and tastes from the water.

Sub-micron carbon block filters have pores that are less than one micron in size hence they are capable of removing additional particles/contaminants from water including metals like lead, mercury, radon, arsenic, and asbestos.

These contaminants don’t really adhere to the carbon surface but they can’t fit through the small pores hence get trapped and cleared from the water.

The pores of certain carbon block filters are too small for even cysts to pass through.

That said, note that activated carbon filters on their own they are not able to remove every contaminant.

Some salts, fluoride, dissolved minerals (like calcium and magnesium), metals, and microorganisms like bacteria and viruses slip through them.

In most filters though, the activated carbon is often combined with a secondary media or element such as silver which gives the carbon filter additional bacteriostatic properties.

Adding silver creates a media that’s able to kill bacteria which is essential because some carbon filters can have bacteria/mold growing in them. 

Certain iron-reduction carbon filters also include KDF media (Kinetic Degradation Fluxion media) which is made of ground zinc and copper to create a chemical reaction.

Adding the KDF media helps convert ferrous (dissolved) heavy metals like iron to a ferric (solid) state where they are then captured in the matrix of the activated carbon. 

There are three most popular types of activated carbon water filters:

  • Granular activated carbon (GAC)
  • Carbon block (in solid block form or certain shape)
  • Radial activated carbon (organize GAC to allow the water to pass through it in a particular direction).

All three are made from activated carbon but they differ in terms of structure, pore size, and more as you’re going to find out below.

GAC – Granular Activated Carbon Filters

What is Granular Activated Carbon Water Filter?

GAC filters also referred to as “fixed-bed carbon filters” are typically loose granules of activated carbon usually placed in cylindrical containers.

They are basically millimeter-sized granules (0.2mm to 5mm) and are not held together hence form a coarse mesh that allows water to flow through the cartridge quickly and easily, as the contaminants get filtered out by the carbon particles in the process.

How it is made

Like all activated carbon filters, GAC is made from raw organic source materials that are rich in carbon content such as coal, coconut shells,  nutshells, woods, and a few others.

The filter is made by pulverizing solid pieces of activated carbon into much smaller grains which are sifted through sieves in order to separate them into uniform sizes.

This process allows the manufacturers to separate the GAC into large, small, and dust-like activated carbon.

The loose activated carbon granules (usually 0.2mm to 5mm in size) are then placed in cylindrical cylinders of different sizes to form the GAC filter cartridges.

The loose distribution of the granules plus their relatively large size results in a coarse mesh and is what particularly differentiates GAC from carbon block filters, and consequently, provides a smaller external surface area.

How does it work

In granular activated carbon filters, incoming water gets directed through the collection of loose activated carbon contained in the cartridge.

As the incoming water flows through, the porous carbon particles adsorb the contaminants in it leaving clean water to pass through to the outlet.

Due to the loose distribution of the granules and their large size, the process is slightly different from that of other activated carbon filters like the carbon block.

The loose distribution allows the carbon granules to shift which can result in channeling meaning that the water can sometimes cut a path through the carbon particles where there’s the least resistance and through which more water will flow.

The granules aren’t quite restrictive as the carbon block filters hence the water gets through the activated carbon particles at a faster rate.

The consequence is that the water will have little contact with the activated carbon itself resulting in less filtration.

The channeling effect plus the large pore size of the granules also provides a small external surface area and can allow contaminants to pass through.

GAC filters as well can harbour bacterial growth because stationery water tends to remain behind as the incoming water flows through frequently traveled “channels”.

The relatively stagnant and contaminated water in these areas forms an ideal setting for bacterial growth.

Application

The rate of diffusion is faster in GAC filters and as such, they are well suited for adsorption of vapors, gases, and aesthetic impurities like particulates and foul taste and odor. They typically get used in large-scale applications like in municipal, commercial and industrial water treatment systems.

Because they allow for faster filtration of water, they are also often used in residential water treatment systems like a whole-house filter system (at the point of entry either into a home or business), as a point of use filter such as an under-sink unit, or in pour-through units like a water filter pitcher.

GAC filters are effective at removing chemicals like chlorine and hydrogen sulfide that give water a bad taste and odor which are common complaints in tap water.

They are generally best suited for applications that require basic filtration and a high flow rate.

Cost

GAC filters are inexpensive and readily available for use. The estimated average annual cost of an under-the-sink or whole-house GAC filter can be over US$100, that is, for a family that uses about 25 liters per day.

Eventually, the ability of GAC filters to remove contaminants is used up and they require to be replaced, usually every 9 to 12 months. However, if the water use and/or contaminant levels are high, they may need more frequent change-outs.

Pour-through GAC filter units can cost about US$10 and their replacement filters cost around US$2 each – they typically need replacement after every 40 gallons of water or a month.

The cost of granular activated carbon filter used in larger units or on a large scale tend to begin at about US$330, and they can generally cost as high as about US$2500.

They need changing about once a year and their replacement filters can cost around US$0.07 per gallon of water treated.

On the other hand, the initial capital expenditures for a municipal, commercial or industrial treatment plant that utilizes granular activated carbon range from US$600,000 for a 2-mgd (mega gallon per day) plant to around $4,000,000 for a 20-mgd plant.

Advantages

  • They are suitable for removing chlorine as well as foul taste and odor
  • They are also suited for the adsorption of vapors and gases since their rate of diffusion are faster
  • They are not restrictive like carbon block filters – water flows through the loose granules at a faster rate
  • They offer clean handling and often last longer than carbon block filters
  • They can be regenerated and reused – the carbon granules can be re-activated, unlike the finely powdered granules of the carbon block filters
  • They are inexpensive and readily available in the market

Disadvantages

They are not as effective and efficient as carbon block filters – the activated carbon granules are large and offer smaller surface area resulting in less filtration
They are only suited for removing gases, vapor, chlorine, and bad odor and tastes
The sieve effect is not potent due to the large particles that are loosely distributed – small molecules of contaminants can pass through the carbon particles
They are subject to channeling – water tends to cut a path through the granules bypassing some carbon particles and leaving them underutilized
GAC filters are equally known for growing bacteria – as the water channels through the granules, some areas of relatively stagnant and probably contaminated water remain behind creating ideal settings for bacterial growth

Carbon block filters
What is a carbon block filter?
Carbon block filters are carbon filters that consist of finely powdered granules of activated carbon (often one micron or less in size) held together by a binding agent that only takes up about 15% of the surface area.

The binding agent holds all the individual granules together, keeping them firmly locked in place so they don’t at all move about each other.
How it is made
As mentioned above, carbon block filters are made using finely powdered granules of activated carbon sourced from carbon-rich materials like coconut shells, wood, or coal.

A binding agent is used to hold the individual granules/particles together in order to keep them locked rigidly in place. The combination of the binding agent and the granules is heated and then formed into blocks.

Each carbon block comes as a cartridge that’s formed to specific dimensions and has end caps on the block that direct water through its pores.

What actually distinguishes carbon block filters from the rest is the size of the“mesh” which the carbon creates.

A carbon block filter can contain particles that range from 0.045mm to 0.18mm in size. This means they are 4 to 27 times finer than GAC filters – their surface area is about 10 times more than that of GAC filters.   

Moreover, in high-quality carbon block filters, the structure of pores is often uniform across the entire block which is important in mechanical filtration as it prevents channeling – there’s no path of least resistance.

Water will not favor any single path over another hence the entire carbon block filters equally which maximizes its filtration capabilities and life.
How does it work
Carbon block filters use three different processes to filter water. They utilize mechanical filtration which works like a screen door or a sieve. It involves impure water flowing through the activated carbon block itself where water is forced through the tiny pores of the carbon block.

As it travels through the block, the tiny pores strain out impurities down to about 0.5 microns. All the unwanted elements are kept and clean water is left to go through.

The other process is physical adsorption. Activated carbon filters provide about 10 times more pore surface area compared to GAC filters because they are ground to finer granules.

The large surface area provides much more space for dissolved organic chemicals to accumulate. Their negative electrical charge attracts and holds the positively charged molecules of the organic chemicals present in the water as it flows through the carbon block.

The molecules stick to the surface of the carbon particles, they don’t pass through the pores.

Moreover, the binding agents that hold the carbon granules together are usually specially designed such that they keep the surface of the activated carbon clear hence it doesn’t get clogged with particulates quickly which allows it to adsorb continuously.

The final step is the electrokinetic adsorption where some carbon block filters come wrapped in a special outer membrane that water can pass through it and in the process, the wrap/membrane gains a net positive charge which attracts negative ions of impurities and removes them from the water.

Through these three steps, carbon block filters are able to clear out a wide range of contaminants along with organic chemicals. They can remove metals such as lead, asbestos, arsenic, radon and mercury.

Sub-micron carbon block filters can even remove microbiological contaminants including waterborne pathogens like cysts – the tiny pores that measure less than a micron are too small for pathogens like cysts to pass through.
Application
The carbon block filter is typically more efficient at eliminated a wide range of contaminants from the water and as such, its scope of application is quite broad. It’s used in many fields and areas.

In water treatment, for instance, it’s used in commercial and community water treatment plants as well as waste and sewage water treatments.

Similar to the GAC filters, carbon block filters are also commonly used in home/residential filtration systems such as in whole-house systems, point of entry (POE) systems, and point of use (POU) RO systems.
Cost
Like the GAC filters, carbon block filters are affordable water filtration types and their cost of maintenance is quite low too. They can cost around US$10 to US$200 or more to purchase depending on the size (capacity) and the application.

The replacement filters cost slightly lower than that, usually around US$10 to US$150.

Generally, carbon block filters offer one of the best cost-to-value ratios compared to many water filtration systems – very few systems other than RO and UV light filters can remove impurities such as heavy metals (like lead), VOCs, particulates, or microscopic cysts within a short time.  

Furthermore, while other systems may require chemical additives or high power consumption, carbon block filters do a majority of the filtration using a natural mechanical process.

They are also highly customizable as you can fit a unit right to your existing space or existing filter system which saves you the money you would use on remodeling or changing your plumbing system.

Advantages

More effective and efficient than GAC filters – the large surface area, tiny pores, and fine particles strain out impurities down to about 0.5 microns
Offers a massive surface area due to the finer granules – traps and holds all unwanted organic chemicals
They are capable of removing a wide range of additional contaminants like metals (lead, asbestos, arsenic, radon, and mercury) and waterborne pathogens like cysts
The compact structure of the activated carbon granules prevents channeling
The binding agents help keep the surface of the activated carbon clear – it doesn’t get clogged with particulates quickly which allows it to adsorb continuously
Some carbon block filters are wrapped in a special outer membrane that can attract negative ions of impurities and removes them from the water as it passes through the block

Disadvantages

They have a lower flow rate – water takes more time to pass through the carbon block due to the compact structure of the granules. They are too tight hence significantly restrict the flow
May require more frequent replacements if the water has high levels of contaminants or particulates

Radial Flow GAC Carbon Filters
Radial flow GAC carbon filters are a special type of GAC filters that consist of polypropylene outer shell, core, end caps, and a bed of granular activated carbon as the filtration media.

The unique thing about them is the radial flow direction where water flows through the filter media from the outside to the inside radially like a carbon block filter.
How it is made
The radial flow GAC carbon filters are made using granular activated carbon sourced from carbon-based materials like coconut shells.

The bed of activated carbon granules is the main filter media and it’s sandwiched between an outer shell and the core both of which are made of polyethylene.

The cartridges also feature end caps which are equally made of polyethylene.

The outer shell can be constructed using a 20 to 70-micron porous polyethylene.

Typically, the filters are produced as continuous lengths of tube, rod, slab, flexible flat sheet, or any other complex shapes.

The finished filter elements with the unique radial design basically combine the benefits of GAC filters (increased flow rate) and the surface area of carbon block filters resulting in high adsorption capabilities and particulate reduction.
How does it work
Radial flow GAC carbon filters are designed to work much more like both carbon block and GAC filters.

The water flows in a radial direction much like carbon filters. It flows from the exterior of the filter to the inside passing through the bed of activated carbon granules.

This means the whole exterior surface of the filter gets in contact with the incoming water, not just one edge like in a GAC filter, but the use of granules of activated carbon helps increase the flow rate like is the case with GAC filters.

These two effects lead to low-pressure drop and the addition of the polypropylene outer shell, core, and end caps also increase the filter’s surface area and contaminant reduction capabilities.

As such, radial flow GAC carbon filters tend to have a higher adsorbent capacity than conventional GAC filters. They outperform them in almost every application – they are especially effective in reducing chlorine taste and odor, as well as organic chemicals.

The unique radial design does also help reduce the release of carbon fines in the filtered water which is a common problem with GAC-style filter cartridges.

There’s no channeling either like in GAC filters since the bed of activated carbon granules in radial flow GAC carbon filters is a rigid structure packed in a rigid tube which prevents movement of the granules or the formation of channels.
Application
These types of activated carbon filters are ideal for both residential and commercial use. They are used in whole-house filtration systems, point of entry (POE) filtration, point of use (POU) systems, commercial RO pretreatment units, and any other high flow rate applications.

They are particularly suitable for removing chlorine, odor, taste, and color.
Cost
Compared to the other activated carbon filters, the cost of purchasing radial flow GAC carbon filters is a bit high as they average between $30 and $200 or more based mainly on the capacity and the brand.

The capacity of most units usually ranges from 10, 000 gallons to 100, 000 gallons of water which is equal to a lifespan of about 2 to 12 months.

Radial flow GAC carbon filters from high-end brands like Home Master and APEC Water also tend to cost a little more.

The cost of maintenance will depend on how often you replace the filters based on your water usage and the level of contamination, but the cost of a replacement filter is almost the same as the cost of a new radial flow GAC carbon filter.

Advantages

Very effective at filtering out chlorine, organic compounds, and bad taste and odor – the radial flow direction and improved surface area increases the adsorption capacity
They are structurally rigid which prevents channeling or bypassing
There’s no release of carbon fines like in the GAC filters

Disadvantages

Like the other activated carbon filters, they are not effective against heavy metals, microbes, inorganic compounds, and total dissolved solids
They may require frequent filter changes if the incoming water has high contamination levels

Conclusion
Carbon water filters are very effective and reliable water treatment systems, especially for removing chlorine, VOCs, and other organic chemicals such as pesticides and herbicides.

There are those that are even much more effective and versatile like the carbon block filters which are able to remove additional contaminants like metals (lead and asbestos) and microbes.  

All carbon water filters are not equal.

Some perform way much better than others in terms of the number of contaminants they can remove, and some are purposely designed for selected special applications.

Overall, if your tap or well water has a foul taste or odor, or has traces of chlorine or other organic chemicals, then getting a carbon water filter can help clear them and improve the taste and odor of your water.