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Ozone in Pools: All You Need to Know

Ozone is used to supplement disinfectants like chlorine or bromine in pool or spa water. Ozone (O3) is a colorless, odorless gas used for water sanitation. It consists of three atoms of oxygen and is a highly reactive form of oxygen. Ozone is one of the strongest known oxidizers, capable of destroying organisms by tearing open the cell wall, a process known as cell lysing. It is used to purify water by oxidizing contaminants.

How is Ozone Produced?

Ozone can be produced through two methods: UV and CD. In the UV method, UV rays split an oxygen molecule into two oxygen atoms. The unstable oxygen atom combines with any nearby oxygen molecules, resulting in ozone. This process occurs at a specific wavelength of 185 nm. In the CD method, ozone is generated by a dielectric, similar to a car’s spark plug. When electricity is applied, a continuous spark or corona is created. If air with oxygen at near 21 percent or pure oxygen is sent into the same space as the corona discharge, ozone is formed.

What is an ozonator?

An ozonator is a device using electricity and oxygen to create ozone and deliver it to a body of water for sanitizing purposes.

How do Ultraviolet Ozonators Work?

Ultraviolet (UV) ozonators work by passing air over UV lamps, which split the oxygen molecules in the air. The resulting oxygen atoms seek stability. These UV lamps emit light at 185 nanometers (nm) and have been used for years to generate ozone from ambient air. However, UV systems produce less ozone concentration than CD systems.

What is Corona Discharge?

Corona discharge is a method of producing ozone by passing electricity through oxygen and water.

How is Ozone Dissolved in Water?

Ozone is a gas and must be dissolved in water to work. The solubility of ozone in water depends on several factors, including the size of the bubbles, the concentration of ozone, the water temperature, the contact time, the water balance, and the TDS. If ozone is not dissolved into the water, it will not kill and oxidize. For instance, if all the ozone produced created one bubble that burst at the top of the water, not much ozone would dissolve in the water. However, if the same amount of ozone produced a million bubbles, a lot of ozone would dissolve in the water.

How to Introduce Ozone into Water?

Ozone can be introduced into water through several methods including using an air pump and a diffuser stone, employing a special Venturi injector, or utilizing contact chambers that allow 2-4 minutes of contact before returning. In certain setups, ozone is added to a side stream of the circulation system using a separate booster pump. The side stream water is drawn from the main circulation line, post-filter and pre-heater. The ozone is applied to this side stream, and the ozonated water is returned to the main circulation flow post-heater and pre-disinfectant injection.

What is the role of ozone in water treatment?

In water treatment, ozone directly oxidizes or eliminates contaminants such as chloramines, combined chlorine, disinfection by-products, organic materials, microorganisms, and micro-contaminants. Ozone also decomposes into the much more potent hydroxyl radical (•OH), which rapidly oxidizes and decomposes organic materials.

How Does an Ozone Unit Impact Water Chemistry?

An ozone unit impacts water chemistry by aiding in the flocculation of particles in the water, making them easier for the filter to remove. However, it’s important to note that ozone has a neutral pH and doesn’t significantly affect other water balancing parameters. Regular pool maintenance, such as shocking the pool and brushing and vacuuming, is still necessary.

How is Ozone Introduced in Some Installations?

In certain setups, ozone is added to a side stream of the circulation system. This process uses a separate booster pump. The side stream water is drawn from the main circulation line, post-filter and pre-heater. The ozone is applied to this side stream, and the ozonated water is returned to the main circulation flow post-heater and pre-disinfectant injection.  In these installations, the ozone is introduced into a side stream of the water circulation system. This side stream is drawn from the main circulation line after it has passed through the filter and before it reaches the heater. A separate booster pump is used in this process. Once the ozone has been applied to the side stream, the ozonated water is then returned to the main circulation flow. This occurs after the water has passed through the heater and before the disinfectant is injected.

What are Some Additional Ozone Injection Systems?

Besides venturi injection, some systems may use a contact column. This device ensures thorough mixing and increased contact time to prevent gaseous ozone from reaching the pool or spa area. In certain setups, ozone is added to a side stream of the circulation system using a separate booster pump. Commercial pools using ozone as a sanitizer require additional equipment, including a contact tank, mixing tower, injector, booster pump, ozone monitors, and other components.

How to Measure Residual Ozone in Water?

Residual ozone in water can be measured using test kits, test strips, or electronic probes. However, achieving a residual of ozone is challenging. If a residual is present, it indicates that all oxidizer and sanitizer demands have been met.

What is the Ideal Residual of Ozone?

The ideal residual of ozone is suggested by some manufacturers to be between 0.01 to 0.05 ppm. However, others emphasize the importance of dosage level, agreeing that a dosage of 0.1 to 0.4 ppm of ozone is appropriate.

What is the Half-Life of Ozone?

Under laboratory conditions, ozone has a half-life of 3 to 18 minutes. In pool or spa water, it can last from a few seconds to possibly 30 minutes. This duration depends on the sanitizer demand and the type or amount of other sanitizers or oxidizers used.

What are the Limitations of Hydroxyl Radicals and Ozone in Pool Water?

The limitations of hydroxyl radicals in pool water are that they exist for 10 microseconds or less, so there’s no risk of any reaching the pool. The limitations of ozone in pool water are that its half-life is influenced by pH and decreases as pH increases. Ozone also reacts with chlorine, making it unavailable for hydroxyl radical generation. In most pool or spa water, ozone’s half-life is about 5 to 10 minutes.

What Should You Know About Using Ozone in Pools and Spas?

Ozone is used to supplement disinfectants like chlorine or bromine in pool or spa water. However, bathers should not be exposed to ozone in the water. UV lamps, available in low and medium pressure output levels, are used in this process. Low pressure lamps require more units for high flow rates due to their lower power density. Medium pressure lamps, though having a lower effective UV light output, require fewer lamps due to their higher power density. These lamps have a long lifespan, lasting between 4,000 to 9,000 hours.

Can Ozone Stand Alone?

No, ozone cannot stand alone. It doesn’t provide a lasting residual to the water due to its short half-life. Also, the usual concentration, residual, or dosage rates used in U.S. pools and spas are not sufficient to kill algae.  Ozone is a highly reactive form of oxygen with three atoms, represented as O3. It is one of the strongest known oxidizers, 25 times faster at killing than hypochlorous acid, the most potent form of chlorine. However, it is unstable and reacts immediately, breaking down and leaving little to combat algae on pool walls. This requires a residual of chlorine, bromine, or another sanitizer. Furthermore, the usual concentration, residual, or dosage rates used in U.S. pools and spas are not sufficient to kill algae. Therefore, ozone cannot stand alone as a sanitizer in pools and spas.

Why is Ozone Depletion Important?

Ozone depletion is important because all ozone should be depleted before the circulation water returns to the pool. This is because ozone, being heavier than air, could collect at the water’s surface, causing discomfort to the eyes and respiratory systems of swimmers. Therefore, the method of ozone-to-water contact is crucial.  Ozone is a highly reactive form of oxygen that is used to purify water by oxidizing contaminants. However, it is unstable and reacts immediately, breaking down. If not properly depleted, it could cause discomfort to swimmers. It is also important to note that ozone cannot stand alone as it doesn’t provide a lasting residual to the water due to its short half-life. Therefore, it is crucial to ensure that all ozone is depleted before the water returns to the pool.


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