How Do Water Purifiers Work?

How does my water filter work? What can I do to purify water in an emergency? Learn about physical filtration, chemical filtration, and more.

Sabrina Stierwalt, PhD
4-minute read
Episode #261

water purifier at home

Research from the World Health Organization sets a daily water requirement of 2.5 liters per person per day for drinking, and 15-20 liters per person per day for cooking and hygiene. After all, our bodies are between 55-75% water, depending on your age and sex. But one week after Hurricane Maria devastated the island of Puerto Rico, the Pentagon reported that 44% of Puerto Ricans lacked access to potable drinking water. That’s 1.5 million U.S. citizens. Even more troubling, as of October 20th, a full month after Hurricane Maria, one million Puerto Ricans were still without reliable drinking water.

When our water is not clean and potable, we face serious health risks like cholera and diarrheal diseases. 76 cases of leptospirosis, a bacterial infection that arises from exposure to water contaminated with animal urine, have been reported in Puerto Rico since the hurricane.

We only get one supply of water to sustain all of life on Earth. The water we drink today is the same water the dinosaurs drank. So how do we rid the water we drink from the contaminants it may have picked up on this journey through the water cycle? How does water purification work?

The methods used to purify water depend on what contaminants are being removed, in particular the size of the particles and how they react chemically. Generally, filtration techniques fall into two main categories: physical filtration and chemical filtration.

What is Physical Filtration?

Physical filtration acts much like the sieve you use to separate your pasta from the water in which you cooked it. As water passes through a screen or in some cases layers of sand, the largest particles can be caught by the screen and thus filtered out. Membrane-like materials can remove smaller particles based on the size of the pores of the membrane. For example, nanofiltration can remove particle sizes down to 0.0001 to 0.005 microns like viruses, pesticides, and herbicides. Adding coagulants like lime to water can cause particles to clump together so that they can be more easily filtered.  

Another form of physical filtration is the process of reverse osmosis where water is pushed through a membrane at pressure. That pressure results in a blockage by the membrane of particles dissolved within the water while the water passes through. Reverse osmosis can remove metal ions, and is also the technique used in most desalination plants to remove aqueous salts.

What is Chemical Filtration?

In chemical filtration, water is passed through filters of active materials that remove impurities chemically, for example by adding other chemicals, like chlorine or other reducing agents, to prevent certain reactions from happening.

When the water in our homes is properly sanitized, it also goes through a disinfection process, where a relatively safe chemical is added to the water to kill off any remaining microorganisms. Chlorine is a commonly used disinfectant as it can effectively kill off contaminants at fairly low concentrations.


About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.

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