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.
How Does My Home Water Filter Work?
The filters typically found in our kitchens tend to use a combination of physical and chemical filtration techniques, although the kind of filter found in a pitcher tends to be different than the kind that attaches to your faucet. For example, typical home filters like those found in Brita and ZeroWater pitchers use activated carbon granules similar to charcoal. Charcoal is a porous surface with many crevices for capturing particles but of course have to be replaced once those cracks become clogged with impurities.
Typical home filters like those found in Brita and ZeroWater pitchers use activated carbon granules similar to charcoal.
These filters also typically employ an ion exchange stage of filtration where atoms in the water are broken apart into ions or charged particles. The problematic ions are then removed by the filter and replaced by less troublesome ions. This is the process used in water softening: magnesium and calcium ions are trapped by the water filter and replaced with sodium ions.
How Do I Purify Water in an Emergency?
So what can you do if you find yourself in need of safe drinking water? A crude solution is to filter the water through a clean cloth, paper towel, or coffee filter to remove the largest particles. Boiling water can also help kill off certain bacteria but will not remove chemicals. Distilling water, which requires capturing the steam produced during the boiling process and condensing it back into liquid form, can also help to remove some contaminants, but others like volatile organic compounds boil off before water does (i.e. at lower temperatures) and so will remain in the resulting steam.
Chlorine bleach tablets, like those found in camping stores, can be added to water for disinfection and come with precise instructions of chlorine to water ratio. If you don’t have access to such tablets, regular, unscented chlorine bleach can be used. Household bleaches come in different percentages of the active ingredient sodium hypochlorite, so the Environmental Protection Agency offers specific ratios of bleach to water based on the type of bleach you have.
In Puerto Rico, the long term clean water situation is complicated, especially since a large percentage of the island is still without power, and power is necessary to run water sanitation systems. In the future, we may be able to use desalination techniques to turn some of our salt water into fresh drinking water but the process remains costly on a large scale. For now, access to clean water is of the utmost importance on the island before citizens are forced to choose between contaminated water and no water at all.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
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