By Kelly Johnson, MWS

Ozone (chemically written as O3) is a great swimming pool and spa oxidizer. Though not a stand-alone sanitizer or oxidizer, we will learn that it supports traditional pool and spa sanitizers, such as chlorine. Ozone does many things well and in many cases better than chlorine. Ozone generating systems are electro-mechanical devices that generate and inject ozone gas into swimming pool or spa water.

A residential ozone generating system consists of three basic parts. The first component is the generator, which is either a corona discharge (CD) or an ozone-producing ultraviolet (UV) lamp. The second component is the Venturi injector and the third is the air dryer, which is needed where high-humidity conditions exist, to prevent nitric acid buildup in the ozone cell to prevent damage to the generator.

Corona discharge ozone generation. This component consists of an electrical enclosure that includes a high-voltage power supply, ozone-producing cell and ancillary items that control these components. Ambient air is drawn into the oxygen concentrator, which dries and removes moisture and other trace gases and contaminants; some of it is converted into ozone gas. CD technology will produce greater quantities of ozone versus UV.

Ultraviolet ozone generation. This component consists of an electrical power supply and an ozone-producing UV lamp (185 nm). Ambient air is drawn into the ozone generator and some of it is converted into ozone gas. This ozone gas is introduced into the water, commonly through Venturi injection, in the recirculation flow after the filter (and heater) and before the chlorine feeder.

Gaseous ozone dissolved in water is referred to as aqueous ozone. Ozone provides antimicrobial oxidation for supplemental treatment or secondary disinfection, oxidation of organic and inorganic contaminants, chlorine byproduct reduction and some algae reduction. Ozone has the added benefit of destroying chloramines, as well as reducing chlorine consumption, specifically the need for chlorine shock oxidation. N-CL is a chloramine and when oxidized by ozone it becomes nitrogen, which then gases off the pool surface as free chlorine (FC)F, which is therefore left in the water to destroy bacteria, algae and other pool contaminants. Destruction of a chloramine is related to both a monochloramine or a dichloramine as trichloramines are not typically going to be found in a pool or spa environment. Ozone is considered a secondary disinfectant, can also act as a micro-flocculant and is an anti-foaming agent.

What ozone does in swimming pools and spas
Once the ozone has been dissolved in the water, an oxidation reaction occurs upon any collision between an ozone molecule and an oxidizable substance. Organic contaminants are destroyed and many dissolved metals become insoluble. Ozone is capable of killing all known microbes (including Cryptosporidium and Giardia), destroying organic contaminants that may create chloramines (an odorous, irritating byproduct of a chlorinated pool or spa) and breaking down existing chloramines. This oxidation occurs immediately at the ozone-gas injection point and continues in the return lines.

An ozone generating system is considered a pesticidal device by the US EPA Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Companies that produce ozone generators must register each production facility with the US EPA and each facility will be given an agency establishment number. The company must also submit annual reports to US EPA, including the number of units produced and distributed. WQA members can obtain more detail about their obligations under FIFRA by contacting the WQA Government & Regulatory Affairs Department.

Ozone generating systems do not leave a substantial ozone residual in pool or spa water. All ozone generating systems must be used in conjunction with a US EPA registered sanitizer, therefore providing a residual disinfectant to prevent infection of the swimmer or bather.

Ozone applications
Ozone generators will always be used in conjunction with more traditional chemical treatments, such as chlorine. Ozone has a short half-life, coupled with the hydraulics in a swimming pool and filter run-times established by the homeowner, thus dictating the need to use traditional chlorine products for residual disinfection to prevent infection. Ozone is not an effective algicide or algistat, so a pool algicide or algae suppressant (like a Quat-based algicide, copper algicide or sodium tetra borate) is always a great addition for overall pool care, with or without the use of ozone. For residential pools, ozone is appropriate for use in conjunction with chlorine, but is not recommended for use with bromine, because of the potential to produce bromate. UV-generated ozone systems can be used with biguanide chemistry (PHMB), a non-chlorine sanitizer used by some pool owners.

Another plus in using ozone in a pool or spa environment is the reduction in the use of chorine or non-chlorine-based oxidizers, known as pool shock. Ozone will oxidize these organic materials, therefore reducing body oils and greases (another body contaminant). This helps with reducing the load on any pool or spa filter including sand, diatomaceous earth (DE) or cartridge-type filters. Results in using ozone is the reduction in traditional sanitizers and shock-chlorination products, less load on the filtration system and in many cases, a better clarity to the water. Ozone will not cause discoloration of bathing suits. Operational costs for traditional pool and spa sanitizers and oxidizers are, therefore, reduced.

Preventative maintenance is important on pool and spa ozonators. This will include the annual replacement of the UV bulb and ozone tubing. As needed, the Venturi injector will need to be replaced as the connection points on these plastics will become brittle. On most CD ozonators, the indicator light will go out, indicating replacement is needed.

Many in the pool and spa industry do see the application of ozone working best with a spa versus a pool, as we get very quick turn-over time of water volume in a spa versus a pool, providing proper or better distribution of dissolved ozone gases. Ozone has been well established by most spa manufacturers and consistently for many years, where as the application of ozone for residential swimming pools is somewhat more limited. Pool builders and pool owners are given more opportunities to choose from other alternative sanitizers or oxidizers for secondary sanitation. There are a lot of parallels in applying ozone in water treatment versus pool and spa water applications. Sizing run times and maintenance are similar. But having an open body of water exposed to the elements with swimming pools and spas will always make ozone a secondary sanitizer.


  1. The Association of Pool & Spa Professionals /APSP.
  2. “Ozone Generating Systems Fact Sheet–Revised.” June 2018.
  3. APSP Recreational Water Quality Committee.

About the author
Kelly Johnson, MWS of Ponca City, OK-based Quality Water Services, has been in the pool and spa business for 40 years, previously working for BioLab, Inc., a chemical manufacturer in Atlanta, GA for 10 years. He also owns three retail/service pool and spa stores since 1998. Johnson is a third-generation water-treatment dealer serving Oklahoma and Kansas since 1945. He is a member of the Association of Pool & Spa Professionals and the WQA Board of Governors.


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