How do Carbon Filters Work to Remove Toxic Chemicals in a Drinking Water Supply?
Whether you buy a bottle of water or turn on the faucet at home for a drink, the last thing you want to worry about is that it might be contaminated with bacteria or toxic chemicals. In developed countries and cities that have advanced drinking water treatment facilities set up, they have methods for dealing with those harmful pollutants. It’s the same with bottled water companies. For instance, companies like Aquafina and Glaceau Smartwater use technology like reverse osmosis to purify the water they market.
However, there is far more to a purification process than just reverse osmosis. Before that, the raw water needs to go through a rigorous filtration process to remove contaminants that can cause issues with the RO membrane. Toxic chemicals are one such contaminant. One of the most effective ways to remove these pollutants is with activated carbon filters.
To the average person, filters may bring to mind coffee. A thin paper filter keeps the coffee grounds out of the final product. Activated carbon may be less familiar, but it is being marketed heavily for home filtration, like in the ever popular Brita filter. If you open one up you will find a black powder inside. However, the question remains: how does a simple carbon filter work to reduce/eliminate such toxic substances?
To answer that question, let’s break things down from the very beginning.
What is Activated Carbon?
In a super simplified explanation, activated carbon is either powdered charcoal or coal. Charcoal is produced by burning organic material in the absence of oxygen. Coal, on the other hand, is formed over millions of years deep underground. After an activation process (hence the name) the powder from either of these sources can then be used for filtering. The activation process grants it certain valuable properties that become useful for filtration applications.
Sources of Activated Carbon:
Coal is one source mentioned above — more specifically bituminous coal — but the more common organic charcoal producing sources are wood and coconut shell. Each source has benefits and disadvantages, but coconut shell activated carbon has proved to be especially beneficial in more ways than one. Here are just a few things about each source of Activated Carbon:
- Readily activated and easy to regenerate
- Non-renewable fossil fuel that must be mined
- Can affect taste due to inorganic ash
- Contains certain chemicals that can leach into filtered water
- Larger pores than other activated carbons
- Makes for lower density activated carbon
- Can remove taste and color
- A renewable resource when compared to coal, but trees take many years to grow
- Meso- and macro-sized pores do not work for fine filtration applications
- Highly renewable and readily available resource
- Greener source than wood or coal, no mining required and no deforestation
- High porosity
- Creates less inorganic ash that affects filtration quality
- High abrasion resistance so it lasts longer before requiring replacement
The activation process is what gives activated carbon its filtration qualities. The act of activating carbon powder is either by physical methods or chemical reactions to burn away the carbon and volatiles to creates pores within the particles. Physical activation uses hot gases and air while chemical activation uses acids, bases, or salts.
Pores, Pores, Pores
Porosity is the bread and butter of filtration no matter the method. It is what gives the filter media the ability to capture contaminant particles. Two of the most important aspects of porosity are pore size and pore density. The importance of size is simple enough to understand, smaller pores can capture smaller particles, resulting in a higher quality effluent. Pore density refers to the number of pores within a particular unit of area on the particle itself. The presence of pores increases the overall surface area of the media and therefore, higher pore densities equate to larger surface areas. Just a pound of some activated carbons can have a surface area of over 100 acres.
ADsorption and ABsorption
How do activated carbon filters work? Using the culmination of the aspects described above. The carbon source determines the effect of the activation step that results in the formation of pores. It is these pores that allow the powder to filter out contaminants like toxic chemicals by increasing surface area. This is accomplished in two processes: adsorption and absorption.
Adsorption is similar to dusting your furniture at home, the dust clings to the surface of the dust cloth. Absorption is more akin to a sponge, pulling the contaminants inside the pores of the sponge. Greater surface areas increase the capacity of activated carbon to collect and hold the pollutants within and without it. Other medias adsorb contaminants as well but do not absorb them. Of course, absorbed particles can’t simply be backwashed out, hence the need for reactivation to burn them out.