Ozone has been used for decades in pool and spa water treatment as a secondary sanitizer and oxidizer. Its popularity, however, is increasing due to its identified benefit of helping defeat chlorine-resistant microorganisms (such as cryptosporidium) as pointed out by the Centers for Disease Control and Prevention in the recently released Model Aquatic Health Code.
Despite the long-term use and acceptance of ozone, there remains some confusion about its chemical properties, generation techniques and application methods.
Chemically, ozone (O3) consists of three atoms of oxygen, making the arrangement very reactive compared to its diatomic relative, oxygen (O2). Ozone contributes its oxygen to other molecules it comes in contact with in a process called oxidation. This can be thought of as “chemical burning,” with the result being simply carbon dioxide and water. This process is valuable in pools and spas, where a myriad of organic contaminants can exist that need to be “burnt” away via oxidation. Left alone, these organic contaminants will consume the sanitizer, make the water cloudy and potentially pose health concerns with accidental ingestion.
Oxidation also can be accomplished with chlorine, another strong oxidizer, most commonly with shocking or super-chlorination. However, ozone is a stronger oxidizer, so it generally does a better job of eliminating organic contaminants.
Ozone also is beneficial as an oxidizer of inorganic compounds, particularly metal ions, which are dissolved in the water, causing them to form oxide precipitates that are more readily filtered out.Additionally, studies have shown that ozone will form micro-flocculants of soluble organic compounds.
One benefit of using ozone as an oxidizer is that it does not create odorous and known carcinogenic byproducts like chlorine is known to do. In fact, ozone is effectively consumed during oxidation, leaving no chemical products except for the carbon dioxide and water from the “burnt” organic contaminant. Ozone also has the benefit of providing sanitizing properties — killing bacteria, viruses, algae and protozoans. In providing both oxidizing and sanitizing benefits, it can dramatically reduce the need for chlorine. Ozone equipment manufacturers tout that overall chlorine reduction can be 50 percent or more. Actual chlorine reduction is solely dependent on each specific situation and will vary considerably.
Two well-established techniques exist in the pool and spa industry for producing ozone. The first is corona discharge, in which a very high voltage (e.g. 5000-10,000 volts) is placed on one or more pairs of metal electrodes, separated by an air gap. The air between plates becomes ionized, and when the ionized air discharges electrical energy, the surrounding oxygen in the air can be converted to ozone.
In the second method for producing ozone, air is passed across an ultraviolet lamp. To make ozone, the lamp must emit a wavelength of 185nm, at which point oxygen in air is converted to ozone. A common configuration for a UV-based ozone generation system is one or more UV lamps placed inside a container. Air is continuously brought into the container, ozone is made and ozone-rich air is removed from the container.
In both methods, the ozone gas created must be injected into the pool or spa water. Two primary methods are well-known in this area. The first is through the use of a gas diffuser. Diffusers, similar to those used to aerate fish tanks, can be used to introduce ozone, under pressure, through thousands of tiny pores, creating very small gas bubbles. The result is a greater ozone/water contact area, reducing the time required to dissolve ozone into the water. This is of paramount importance. Failure to dissolve all the ozone means the user will not receive the benefit of all the ozone created, and that a serious health consequence could occur.
A second method for injecting ozone is to use a venturi. These specially designed devices create a suction (i.e. vacuum draw) when water passes through them. Placed into the circulation-system plumbing, a venturi can be used to draw ozone gas into the water stream. The very small, constricted throat of the venturi device, where the ozone enters, causes an immense amount of turbulent action that breaks apart the ozone gas into fine bubbles, helping it dissolve into the water. Both ozone gas introduction methods have no moving parts and are effective when designed and installed properly.
My ozone generator can beat up your ozone generator!
If you are of the mindset that more is better, then you perhaps have already fallen victim to the game of one-upmanship.
Ozone systems historically have been sold on the basis of the grams of ozone per hour that a given system generates. So logically one would assume that a system that makes 2 grams of ozone per hour is better than one that makes 1 gram of ozone per hour. And one that makes 5 grams per hour is better than one that makes 2 grams per hour. Logical, right?
Well, the amount of ozone gas created is not what it is important, nor what you should care about. What truly matters is how much of the created ozone actually goes into the water. Remember, undissolved ozone is not only useless, but it forces the user to install additional equipment to remove it so bathers are not harmed. This additional equipment, often referred to as ozone gas removal or ozone destruct systems, take out the undissolved ozone gas bubbles from the water and convert it into oxygen, usually using activated carbon, before sending it out to the atmosphere. These destruct systems are large, expensive and add to the overall cost and pool equipment footprint.
So, back to the example above. Would you rather have a 1-gram-per-hour ozone system where all the ozone was dissolved in the water, or a 2-gram-per-hour ozone system where only 1 gram per hour made it into the water and the other 1 gram per hour was required to be treated with an expensive ozone destruct system?
So how much ozone do you need? Remarkably, very little. As a sanitizer, ozone’s effectiveness, like that of chlorine, is measured by its CT value. This is the required concentration (C) of ozone acting over a period of time (T) needed to kill a microorganism. Ozone, being so potent, requires only a small concentration in the water to be effective and a great assistant to chlorine. As an oxidizer, a greater amount of ozone is required to convert the entire mass of organic contaminants to carbon dioxide and water. However, studies have shown that when even a small concentration of ozone is introduced into pool water just ahead of a UV system, the benefits of ozone are greatly magnified. This benefit results from an interaction between UV and ozone that leads to the formation of another potent oxidizer/sanitizer, called hydroxyl radicals. So, importantly, the amount of ozone you really want is simply the amount that is required to perform the job you need it to perform.
Ozone can be a great supplemental sanitizer and oxidizer to chlorine when designed and installed properly into a pool and spa system. Its ability to destroy microorganisms and eliminate organic matter assures it a rightful place at the table in the water treatment of pools and spas.
Ray Denkewicz is global product manager of Hayward Industries, in Elizabeth, N.J.