A charcoal filter is a common part of most gravity fed and many force-fed water filters. Charcoal is the product of the destructive distillation of wood. This process yields wood alcohol, acetic acid, several burnable gases, and a few other products. The solid residue that results from this process is what we know as charcoal.

Charcoal is an odorless and tasteless solid, porous, black, and brittle.  Charcoal is normally too dense to float in water, yet sometimes it does. How can this be? It is because charcoal has the remarkable ability to adsorb solids and gases. Charcoal does this so well that it can adsorb enough gases to make it float.

The filters usually use activated charcoal, coal, or carbon. This means the charcoal has been processed to make it extra porous. The result is that just one gram of activated carbon has a surface area of between 500 m and 1500 m! When you consider that it takes 454 grams to make one pound and that a tennis court has 260 m, you can see it is very porous! The fact is, the greater the surface area, the more likely it is that impurities will touch the charcoal as they pass by.

The secret of charcoal’s filtering ability is in the fact that it is so good at adsorbing (not absorbing). “Adsorption is the concentration of a gas, liquid, or solid on the surface of a liquid or solid with which it is in contact.” Just one cubic centimeter of charcoal is able to adsorb 90 cc of ammonia gas though it adsorbs other substances far better.

Pollutants in the water pass through the filter and come in contact with the activated charcoal. These substances are attracted to the charcoal by what is called van der Waals forces. Wiki says, “In physical chemistry, the van der Waals force is the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules.”

It gets more technical that that but, suffice it to say, molecular forces bind some compounds to the charcoal. Activated carbon does not bind well with some chemicals. These include  alcohols, glycols, ammonia, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.

his is good news and bad news. It is good news because our bodies need the minerals in water. We don’t want them filtered out. Fluorine is said to be good for dental health so many don’t want that filtered either. However, others on the list above are unwanted and the filter must have other substances to remove these.

In summary, water and contaminants pass through the activated charcoal filter and, because of the filter?s porosity, the substances will likely come in contact with the carbon. The van der Waals forces will cause the substances to be attracted to the charcoal where they will remain until the filter is washed or replaced. For the contaminant, it is dead end road. For the person drinking the water, it is refreshing and healthy.

The best line of activated charcoal filter we are aware of is the Berkey Water Filter. Whether you choose their Berkey Light Water Filter or another model, each set of filters they ship with can be re-cleaned to purify up to 6,000 gallons of water.