We live in a world inundated with water filters, I bet nearly everyone reading this has, at some point, owned a Brita filter or some other such product. But do you know what these filters do? Do you know what they remove?

Most commercial filters will state what they can remove in their NSF testing results (that’s the National Science Foundation) which are normally included in the package. The most common varieties are certified to remove mercury, chlorine, copper, and cadmium. Unless specifically stated, water filters are not removing lead, pesticides, bacteria, or viruses. 

But how does a water filter work? Let’s build one to find out. To be clear, this is not a filter you can or will want to use in your home to purify the water from your sink. This is simply an excellent visual aid that helps explain how water filters work. To begin, we will cut a plastic soda bottle in half. Invert it onto itself (turning the top into the now open bottom), and begin layering. Our first layer is a packing of cotton, which is primarily there to keep the remaining ingredients inside the filter.

The star ingredient of any water filter is activated charcoal. Activated charcoal is a form of charcoal that has had its surface area increased exponentially. It is “activated” by running hot gasses and pressurized air through it. The air causes thousands of tiny pores to open up all over the surface of the charcoal, which allows for incredible absorption of chemicals, minerals, and contaminants. 

Believe it or not, just one gram of activated charcoal has surface area in excess of 5,400 square feet! Activated charcoal is the main ingredient in Brita filters and all other commercially available filters. It can be made from either wood or, increasingly more commonly, coconut shells. Burning coconut shells into activated carbon is a great way to repurpose what would otherwise be waste in a world where coconut water and milk are ever more popular.

Add about one inch of activated charcoal on top of the cotton balls and proceed with the next ingredients. What are the other ingredients in our filter? We’re using sand and gravel, both of which can be found referenced in ancient Egyptian and Sanskrit documents as useful materials to purify water.

Layer an inch of sand on top of the charcoal. Do so carefully, as sand is much heavier and will sink straight through the charcoal if it is just dropped into the filter. Sand is a worth adversary for many contaminants. It also has been used for centuries. In 1627 Sir Francis Bacon hypothesized that water desalinization could be achieved through sand filtration. Though he was wrong (salt cannot be removed by mere grains of sand), this marked the first scientific exploration into the field of water purification. In 1804 we find evidence of sand filters being used to purify water supplies in Scotland. Sand will keep out small and minute particulate matter, which includes dirt, soil, etc.

Finally, on top of this all, we will add some gravel. Small stones, while not particularly useful in filtering out microscopic elements, are perfect for removing twigs, leaves, and other large particulate matter before it reaches and clogs up the much more refines layers below. 

Water filtration is one of the reasons our cities and countries have flourished since the mid 1800’s. Prior to that era we were plagued with infectious diseases which spread quickly through city water supplies. This ensured that citizens would die out before reaching old age and that cities would remain small. As we learned to clean our water and make it safe to transport through pipes, our cities blossomed and our average age of demise climbed steadily.

Now that your filter is finished, time to take it for a spin! Pour some murky water through it (made with either mud and dirt from outside or just a hefty squeeze of food coloring). Add the water slowly, it will take a while to make its way through the layers of the filter. Once it starts dripping you'll see that nearly all of your contaminants (for coloring and/or mud) have been removed!​