For nearly 100 years, ozone has been used all over the world for water sanitation. It is nature’s strongest oxidizer and has a byproduct of pure oxygen. Chlorine, on the other hand, is a very effective, time-proven chemical, but is ineffective against hard-to-kill microorganisms such as cryptosporidium. It also has some unpleasant byproducts. Together, however, these two substances can complement each other’s strengths and work to improve the condition of modern swimming pools.
The Origins of Ozone
In the natural world, ozone (O3) is created when oxygen molecules are subjected to ultraviolet (UV) rays from the sun or by an electric charge from lightning. Some oxygen (O2) molecules are split into single oxygen atoms, which then combine with other O2 molecules to form ozone.
Man-made ozone manufacturing techniques replicate nature two ways—by forcing oxygen or ambient air past a UV light source or by sending a spark (known as a ‘corona discharge’) through a stream of oxygen or dry air. Compared to UV ozone systems, corona discharge systems create much more ozone at a higher concentration by weight, making them better suited for commercial and high-volume residential pools and spas. Both types of ozone generators are highly reliable.
Ozone reacts very quickly. It is 100-times stronger than chlorine but has a relatively short half-life. Chlorine, on the other hand, has a long half-life, which allows it to function as a long-term residual sanitizer in the pool basin. When sized properly, the swimmer will not detect any chlorine; the small amount required will be ‘free available’ chlorine, which has not combined with contaminants that create chloramines. Furthermore, the pool water will have an oxidation-reduction potential (ORP) of more than 700, with less than one part per million (ppm) of chlorine. The water is as pure and clear as bottled water and safer than pools that use chlorine only or combinations other than ozone and chlorine.
There are several different sanitation methods for pools. The most common types are chlorine, bromine, ionization, UV, ozone or various combinations of each. Chlorine is the most widely used sanitation method, but is becoming less desirable because of the harmful byproducts it produces and its difficulty inactivating cryptosporidium. Chlorine is a relatively strong oxidizer and bactericide, but it creates chloramines and trihalomethanes (THMs), which cause irritation to the eyes, skin, throat and lungs. Chloroform, a compound found in THMs, is a known animal carcinogen (cancer-causing agent). Salt chlorine generation is another option; while it produces the same byproducts, it eliminates handling issues one encounters with liquid chlorine. Bromine is an alternative to chlorine and works particularly well in warmer water, and therefore, in spa applications. It is also a good choice for indoor pools with low bather loads. The challenge with bromine is its overall oxidizing capability. It is also weaker than chlorine and ozone
Combining Ozone and Chlorine
It is often more effective to combine sanitation methods. Using ozone as the main pool sanitizer and chlorine as a secondary sanitizer can increase bather comfort while decreasing hazardous chemicals and byproducts. Ozone performs the ‘heavy lifting’ in oxidation, while chlorine acts as a residual sanitizer. Using ozone in conjunction with chlorine is also a natural, pre-emptive approach to chloramine management, which also improves water quality. Introducing ozone prior to chlorine further reduces chances of chloramine formation. The ozone pre-oxidizes the contaminants that form chloramines and oxidizes chloramines that have already formed. The use of ozone also reduces pool maintenance, because it breaks down filter-clogging greases and oils, which extends filtration cycles, eliminates scum buildup along the waterline and makes water balancing easier.
Using ozone with lower levels of chlorine will also yield higher ORP levels and less chemical usage (both residual sanitizer and pH control) compared to other sanitizer options. Ozone can also combat microbes that have become resistant to conventional sanitation methods, which can be more quickly and effectively oxidized. Ozone is also more capable of destroying enteric bacteria, amoebic cysts, viruses and spores compared to conventional sanitizers, making commercial and residential pools safer for swimmers of all ages. As a micro-flocculent, ozone can also make pool water visibly clearer, providing a more pleasant swimming experience.
Another common sanitizer combination is chlorine as the primary sanitizer and UV as the secondary sanitizer. UV is very effective at controlling microorganisms, with the added benefit of taking up very little space in the equipment room. However, owners and operators need to be aware this method requires 20 to 50 per cent more chlorine, because the UV consumes hypochlorous acid (HOCl), causing the UV system to consume a great deal of power in the process. When shopping for a UV system, one should select an
system that is NSF-approved with medium pressure, and an energy-saving mode.
What Safety Precautions or Concerns Need to be Considered When Dealing with Ozone Gas?
Ozone is one of the safest and strongest oxidizing agents known to man. Unlike its competition (chlorine), throughout the last century that ozone has been in commercial use there has never been a fatality linked with the gas. This in part is due to the manufactures, designers and installers of ozone equipment whom have built, specified and installed ozone systems with safety precautions to protect the consumer. Low levels of ozone can be smelled by the human nose, at levels far below the Occupation Safety and Health Administration’s (OSHA) rated level for ozone exposure, listed at 0.1 ppm ozone over an eight hour period.
Those exposed to ozone often experience a shortness of breath, and headaches. Usually leaving the area exposed with ozone to an area with fresh air will begin to subside symptoms. However, those that are subjected to an elevated exposure should seek medical attention immediately.
How Important is Sizing the Ozone Equipment?
There are a number of elements that go into the design of an ozone system, of these a key element to a pool or spa’s sanitation process is the sizing or in more correct terms the dosage of ozone that is applied by the ozone system. Ozone must be applied with the proper dosage for the size of the water feature (in volume), the amount of anticipated bather load (number of swimmers; often rated as low, medium or high), and other potential load that would cause reaction with an oxidizer, such as dirt and other organic and inorganic compounds that can be found in pool waters
A properly sized ozone system is typically regarded as the primary sanitizer. This is to say that the ozone will take the place of typical sanitizers such as chlorine by providing as much as 95% of the oxidation required for a given feature. To achieve this level, ozone is typically dosed between 0.3 to 0.5 parts per million (ppm). In doing so the secondary sanitizer (chlorine) will be available for the remaining 5% and to have a residual effect for sanitation purposes in the body of water. In residential pools or spas, if chlorine is used as the secondary sanitizer in combination with ozone, chlorine levels can often be reduced to 0.2 to 0.5 ppm, without having the side effects and by-products of chlorine disinfection, such as chloramines. In commercial pools and spas the same rule is applied for sizing the ozone system, however the amount of chlorine or secondary sanitizer used must comply with local health department regulations. Even though these levels must still be achieved it is the amount of chlorine used to achieve this level that will be reduced greatly with the use of ozone, and without the side effects and by-products of chlorine disinfection.
If a pool or spa is not dosed between 0.3 to 0.5 ppm of ozone there will still be some benefits to applying the ozone system, however, the lower the dosage the more chlorine takes over as the primary sanitizer and ozone becomes a secondary sanitizer. As this change in hierarchy occurs so will the by-products of chlorine disinfection, increasing chloramines levels.