Understanding Ozone: From Stratospheric Ozone Holes to Water Disinfection

Featured Water Professional
Dr. Brian Redmond, Licensed Professional Geologist, Professor

Popular science said the polar ozone holes were a really bad thing in the 1990’s and early 2000’s - but what about now? Now we use ozone to disinfect water, but what makes it good here? Are there any risks to using Ozone to disinfect water? Can we see it, smell it, taste it, feel it? Are some people sensitive to it? Is there a best-practice that any home-owner or RV dweller can use to employ Ozone in their water treatment activities?

The ozone holes were, indeed, a big problem some 20-30 years ago but that was because there was still a lot of ozone-killing chemicals being released into the atmosphere, primarily things like freon which was a popular refrigerant used in refrigerators and air conditioners. Since then, the legal production and use of freon and its ilk has greatly diminished. Unfortunately, these old refrigerants have a long half-life in the atmosphere but as they slowly diminish and are not replaced, the ozone holes have been slowly closing.

As a real-estate agent would say: location, location, location.  Ozone in the stratosphere, the ozone layer, is a good thing – it blocks much of the ultraviolet radiation from the sun.  For most of Earth’s history there was no ozone layer because there was little or no oxygen in the atmosphere from which to make it.  The unblocked UV radiation levels at the Earth’s surface were high enough to be lethal to many forms of life although a few feet of ocean water was enough to protect marine life.  This is why life on land had to wait so long, until about 400 million years ago, for the accumulation of enough atmospheric oxygen to form a protective ozone layer with an ozone content of on the order of 10 ppb.  A few feet of ocean water was enough to protect marine life from the UV but that doesn’t work on land and that is why land dwellers had to wait until the ozone layer formed. Today’s ozone holes in the polar regions are not devoid of ozone; they represent a depletion of ozone of about one third.  

Ground-level ozone is a bad thing because ozone is a very powerful oxidant.  A standard measure of oxidizing power is something a chemist calls electrochemical potential, the bigger the number, the more powerful the oxidant.  The electrochemical potential for ozone (O3) in, is 2.07 which, compared to most other compounds, is quite high.  Ozone has about five times the oxidation power of oxygen and twice that of chlorine.  

Oxygen will break chemical bonds and combine with compounds to produce new compounds which is not a good thing for many organic compounds in living organisms – such oxidation can kill the organism.  When oxygen first appeared in the atmosphere some 3 billion years ago, it killed many of the microbes (microscopic-sized organisms including bacteria) exposed to it who had not developed mechanisms to resist such oxidation.  Some microbes, and their descendants, including us, were able to safely harness the energy released by oxidation (respiration), unlocking a huge new source of energy, making possible the eventual rise of multicellular organisms.  

Chlorine gas (Cl2), with an electrochemical potential of 1.36, is a more powerful oxidant than oxygen (O2) at 1.23.  This is why chlorine-containing bleach can, well, ‘bleach’ and disinfect many materials by oxidizing them including drinking water.  Water treatment plants will ‘disinfect’ the drinking water (the chlorine kills any microbes in the water) and keep a lower level of ‘residual’ chlorine in the water so that as the water travels from the treatment plant to the taps of a home, any microbes that might somehow get into the water on the way are killed.  

Chlorine gas is not good for humans either which is why it was used in WWI.  Fortunately, the residual chlorine in drinking water, although high enough to kill microbes, will not bother humans although some object to the chlorine taste.  The same principle works for swimming pools.  You might note that people who spend a lot of time in the chlorinated water of swimming pools often get bleached hair by the end of the summer.  

An even more powerful oxidant is ozone (O3), with an electrochemical potential of 2.07, which is even more efficient at killing microbes in drinking water.  Like chlorine gas, ozone exposure is not good for humans and so the OSHA PEL (Permissible Exposure Limit) for ozone exposure is a very low 0.1 ppm (in air) averaged over eight hours.  Why is more ozone exposure dangerous?  Consider what happened to the WWI soldiers who were gassed by chlorine.  Those who survived had damaged lungs and some were even blinded.  Ozone is even stronger.  

0.1 ppm is also the limit for ozone in water in an enclosed treatment system.  That ozone level maintained for at least five minutes (contact time) will kill any microorganisms in the water.  

Being so reactive, ozone does not last long in either water or the air.  At ground level in the air, it will typically linger for from half an hour to four hours.  In water, it may last about ten minutes.  How long it is present depends on the amount originally present, the temperature, the pH (for water), and the humidity (for air).  The greater the temperature, pH, and humidity, the faster the ozone will disappear.  

This means that, unlike chlorine, there is no significant ‘residual’ ozone in ozone-treated drinking water which is why treatment plants use chlorine to disinfect the water, not ozone.  However, bottled water producers commonly use ozone to treat their water, ensuring that the bottle is tightly sealed after the ozone disinfection.  No residual chlorine means no chlorine taste in the bottled water which is a definite plus for some people.  

An individual homeowner could use an ozone treatment system designed such that the water is treated just before the water is used.  However, great care must be taken to ensure that no ozone escapes the enclosed treatment system.  It is probably best that a professional install such a system and that it be regularly checked.  If you should ever smell any ozone from the treatment system, shut it down immediately and ventilate the room.  

Which brings us to the question of whether you can see, smell, or taste ozone.  You can’t see it but it does have an electric smell and, if you could taste it, would probably be like the experience you would have placing your tongue between two terminals of a battery.  

Ozone can be produced by large electric sparks and what bigger spark is there than lightning?  If you have ever been near a lightning strike you might have noticed a sharp, electric smell.  That is ozone.  Electric arc welders also produce ozone as does malfunctioning electric equipment and short-circuits.  The ozone odor threshold for humans is about 0.01 to 0.05 ppm which is half or less of the OSHA standard of 0.1 ppm.  However, if you smell it, you won’t know how much higher than the odor threshold it might be so if you smell it, it is best to shut down its source if you can or, if not, leave the area.  

Additional Info

Ozonation as a Drinking Water Treatment - Is it in Your Tool Box?

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