Oxidation and Sanitation with Ozone in the Pool and Spa Environment

Authors: Ronald Barnes and Ronald Brook

Discussions on ozonation with customers frequently result in the use of the term "alternative sanitizer". However, this term is somewhat misleading because ozone actually performs two functions which are critical to proper pool and spa water treatment - oxidation and sanitation. Failure to consider both functions can result in the growth of pathogens which induce unpleasant effects ranging from itchy skin to life threatening illnesses.

Sanitation

Appearance is a poor indicator of water sanitation. In fact, pool or spa water can have excellent clarity, no odor or taste, and still be contaminated with undesirable bacteria, viruses, and fungus. True water sanitation can only be judged by various chemical and biological tests. With respect to sanitation, the public health department usually sets levels for bacteria which it considers safe. A sanitizer is typically defined as an agent which can kill 99.9 percent of all growing bacteria. The importance of the efficiency in which this task is performed cannot be underestimated since billions of bacteria are introduced to pool or spa water each time a bather enters. Outdoor pools and spas may also be contaminated with various pathogens, pyrogens, and nuisance organisms (see Sidebar 1) from sources including rainwater, animals, and the air. What Makes a Good Sanitizer?

Several factors influence the effectiveness of a sanitizer in pool and spa water. They include: concentration of the sanitizer, intensity and nature of the sanitizer, kill time, kind and number of organisms to be removed, pH, and the amount of organic loading (see Sidebar 2) in the water. It should be no surprise then that there is not a single sanitizer (or product) which will be ideal for each factor listed. Ideally, kill time will be short (30 seconds or less) and sanitizer concentration low. Chlorine and ozone meet these criteria.

What is Oxidation?

Oxidation is frequently overlooked as an important part of pool and spa water treatment. However, one should consider that as a general rule of thumb 90 percent of chlorine or bromine in a pool or spa application is used to oxidize organic loading (which forms chloramines or bromamines) and inorganic loading (which can form carcinogenic or pyrogenic compounds).

Technically, oxidation is the process of electron transfer. Atoms or molecules which lose electrons are oxidized. In simple terms, it can be thought of as a form of burning. Roughly speaking, organic material can be thought of as wood in a fire. Just as burning reduces wood to ashes, oxidizing agents in water reduce organic loading to carbon dioxide and water (or inert compounds) which are the equivalent of ashes. The oxidizer can be equated to the oxygen which feeds the fire and allows it to burn.

Why is Oxidation Important?

The oxidation of organics and inorganic compounds is important for a variety of reasons. First, sanitizers are neutralized by organic matter in the water. For example, the oxidation of ammonia by chlorine results in the formation of chloramines. This depletes the reserve of chlorine available for sanitation, and causes many of the undesirable effects typically associated with chlorine (i.e. odor, irritation). Second, organic matter can provide protection to the very pathogens (see sidebar) which you wish to kill. Coatings of organic matter can present a barrier to entering the cell by sanitizers which rely on disruption of cellular metabolism to work. Finally, organic matter provides the nutrients (see sidebar) required by pathogens and nuisance organisms to reproduce and grow.

Inorganic compounds, pyrogens, and metals can cause staining, corrosion, and reactions which are harmful to bathers. Oxidation of inorganic compounds, pyrogens, and metals reduces them to inert compounds which either precipitate or coagulate. They are then removed from the pool by filtration.

Ozone

As noted previously, ozone is a powerful oxidizer and sanitizer. In fact, it is the most powerful oxidizer and sanitizer readily available (see Sidebar 3). Despite this, ozone is still not a stand alone solution in pool and spa water treatment. The reason for this is that ozone is very-unstable. However, this instability also gives ozone its potency and quick reaction time (on the order of 1/10th of a second with organic contaminants and pathogens). This high reactivity also causes ozone's only real drawback, namely it is impossible to establish an ozone residual without considerable expense - typical European corona discharge type installations using the expensive DIN standard still do not achieve an ozone residual in the pool. This problem can be overcome by using chlorine or bromine as a residual sanitizer and injecting ozone generated by very-ultraviolet light via a bypass venturi (commonly called sidestream injection). The process can be enhanced by using pressurizable VUV ozone lamps connected to 1/8 - 1/3 HP compressors. Pressurizable systems are typically used on commercial and large residential installations.

Ozone and Sanitation

There are three primary reasons why ozone is an especially powerful sanitizer. It is non-selective, it kills by rupturing the cell membrane (also called lysing), and it has a short reaction time. The term non-selective means that ozone will react with any pathogen which it contacts. This is not the case with all products termed as alternative sanitizers and should be kept in mind when selecting one.

The method by which ozone kills (lysing) is important because pathogens cannot develop a resistance to it (in the case of organisms such as algae, ozone only destroys the ability of the algae to reproduce, however, they cannot develop resistance). Most alternative sanitizers kill by entering the cell and interfering with metabolic processes - essentially poisoning them. As is the case with bacteria which develop a resistance to antibiotics, most organisms can, or already have, develop a resistance to alternative sanitizers which act in this manner.

Before examining the final factor, a quick look at how microorganisms die in the presence of a sanitizer is helpful. When exposed to a lethal dose of any sanitizer, they are killed in equal percentages per unit of time. Therefore, the effectiveness of a sanitizer can be correlated directly to sanitizer concentration. This effectiveness is more commonly known as a CT value (more commonly seen in the drinking water industry) and will vary from organism to organism. An additional benefit of ozone in this context is that it is pH neutral. Changes in pH will not degrade its performance and conversely, it will not affect the pH of the pool or spa.

Ozone when considered with these factors in mind is a very potent oxidizer. The time required to kill most pathogens in the pool or spa environment when run in conjunction with chlorine or bromine will be on the order of a few seconds at ozone concentrations of .1 - .3 mg/L. In practical terms, this means that a very- ultraviolet ozone generator 1.2 lbs.-O3/day ozone can treat a 1 million gallon pool. A typical installation is shown in Diagram 1. Ozone and Oxidation

Although ozone is a powerful sanitizer, it primarily acts as an oxidizer in the pool and spa environment. Bather or organic loading exerts an ozone demand which can only be overcome in the bypass loop, which is the reason why ozone should be used with chlorine or bromine. Ozone reactions result in several outcomes. Simple molecules will be broken down into carbon dioxide and water. More complex molecules may be broken down into a variety of configurations which precipitate and are removed by filtration. Byproducts of these reactions may also be oxidants (i.e., hydroxyl radicals, ozonites, etc) and cause further reactions. In short, an initial reaction of ozone with organic molecules, may cause dozens of sub-reactions which are also beneficial. Finally, when used with chlorine or bromine, ozone reacts with the combined forms (i.e. chloramines and bromamines) to produce the free forms of these halogens (hypochlorous or hypobromous acid) which act as the residual sanitizer.

There are several results from these reactions. First, the oxidation of organic loading deprives pathogens from a primary source of nutrients. Second, it eliminates a potential source of protection. Third, it increases the oxidation-reduction potential of the pool or spa water, commonly called ORP. ORP is measured in millivolts (mV) and is a fairly common method for controlling sanitizer levels in commercial pools and spas. A detailed discussion of ORP is beyond the scope of this article, but it is essentially a measure of the sanitation/oxidation capability of the water in the pool or spa. A reading of 650 mV is commonly considered an acceptable minimum for the inactivation of pathogens. This is easily achievable using a combination of ozone and a halogen.

Putting it All Together

Now that we have covered the basics of oxidation and sanitation we can take a look at the operations of a complete system. Our example will consider the use of a compressor driven VUV system on a commercial pool (see diagram 1). Starting with a hypothetical pool using ozone and chlorine with low organic loading, we will first examine the effects of bathers entering the water in terms of oxidation and sanitation without ozone.

When bathers enter the water, bacteria, perspiration, make-up, oils, and other types of organic matter will enter the water. Chlorine will then begin to oxidize the organic matter and kill the bacteria which has entered the water. Combined chlorine, including chloramines will be formed. The combined chlorines will eventually reach a point where eye irritation and odor problems are unacceptable. If loading is high enough, the reserve of chlorine could be depleted to the point where the free chlorine is insufficient to meet the loading introduced by new bathers. In the process, some bacteria and nuisance organisms will be killed, but a percentage will be resistant enough to survive and may establish a foothold in the pool. In a normal situation, the operator would shock the pool to free up the combined chlorines and make-up for any losses which would put the pool back into a pristine condition. If an ozonator is added to the pool, the situation changes drastically. Chlorine still acts as the first line of defense, but ozone enhances the power of the system so that the net effect of the system is greater than one would expect from viewing each method independently. As water flows through the bypass loop, combined chlorine is converted to free chlorine and organic matter is oxidized. The oxidized organic matter is either converted to carbon dioxide and water or precipitant which is readily filtered from the system. The oxidation of organic matter also raises the ORP of the water, contributing to sanitation in the main body of the pool. Finally, after the ozone demand exerted by the organic loading is satisfied, pathogens and nuisance organisms in the water will be killed or neutralized in their ability to reproduce.

The net result of the addition of ozone will be a reduction in the amount of chlorine required to maintain a chlorine residual, the elimination of the negative effects of chloramines, and a reduction in the frequency of shocking required (reducing chemical requirements). Finally, a reduction in pathogens by 99.9 percent due to the increased killing power of the chlorine since it is kept in the free state (chloramines take up to 25 times longer to kill bacteria) and the increase in ORP due to the ozone will virtually eliminate the threat of waterborne diseases and nuisance organisms. V.U.V Ozone in Pools and Spas

A common misconception in the industry is that ozone generated by very-ultraviolet light cannot be used on pools. When used in conjunction with chlorine, VUV ozonators, just like corona discharge ozonators, can be used to treat pools of up to 1.2 million gallons. A simple way to gain perspective on this matter is to consider the use of VUV ozonators on spas. The spa environment is considerably more difficult to treat than pools due to the heavy bather loading, low total reserve of sanitizer, high levels of organic loading, and an environment conducive to the growth of pathogens and nuisance organisms. Despite this, the combination of a small ozone generator and a halogen is able to produce excellent water quality. The same results can be obtained in residential and commercial pools with larger VUV ozonators because pool water is a less difficult treatment problem. A more detailed discussion of VUV technology will follow in the May issue.

Summary

In summary, ozone acts primarily as an oxidizer in the pool and spa environment. Ozone assumes most of the oxidative role of chlorine or bromine, thus allowing them to operate more effectively as a sanitizer. While ozone contributes to sanitation in this role, it is not its main function. So the next time your discussing ozone, think in terms of oxidation and sanitation, and leave the term alternative sanitizer on the shelf.

Sidebar 1 - Ideal Sanitizer

An ideal sanitizer would possess these properties. A single agent which meets all criteria does not exist (ozone comes about as close as you can get). The list is useful for evaluation of proposed pool and spa treatments:

Factor Ideal Criteria 1. Toxic to Microorganisms -Ability to kill microorganisms in low concentrations. -Non-Selective, kills broad range of organisms in varying water conditions.

2. Solubility -Soluble in water to extent necessary for effective use.

3. Non-Toxic to Bathers -Will not harm bathers at concentrations sufficient to kill microorganisms.

4. Capacity to Avoid Reaction -Non-Reactive with organic with Organic Loading loading. 5. Non-Corroding/Non-Staining -Does not damage surfaces or stain swimsuits.

6. Odorless -Either odorless or inoffensive

Sidebar 2 - Organic Loading

Organic loading (also known as bather load) is a generic term for various compounds which enter pool and spa water. Any compound falling in this category will contain carbon atoms and generally hydrogen in its molecular structure. A partial list of of organic loading in a pool or spa will include: body oils, body grease, dirt, perspiration, mucous, urine, and fecal matter. Organic loading supplies a food source for pathogens and nuisance organisms, all of which require carbon to to grow and reproduce. Ozone reacts readily with organic loading.

Sidebar 3 - Pathogens, Pyrogens, and Nuisance Organisms

A pathogen is any agent (typically a virus or bacteria) capable of causing a disease. A well known pathogen which fits in this category is the bacteria which cause Legionnaire's disease. The ability of a given pathogen to cause disease is a function of its virulence. The more virulent the pathogen, the less organisms that are required to cause disease. The goal of sanitation in pool and spa water is to reduce the number of all pathogens which can occur to levels which are unlikely to infect the bather.

Pyrogens may be generally defined as inorganic molecules which are capable of causing adverse reactions or disease.

Nuisance organisms include algae and fungus which due to cause disease, but are unsightly or undesirable in pool or spa water. A common problem in spas - itchy, infected skin - is caused by Pseudomonas aeruginosa bacteria which can also be classified as a nuisance organism (although it is sometimes classified as a pathogen).

Sidebar - 4 Relative Strength of Oxidizers

Ozone Peroxide (Acid) Permanganate Hypochlorite (Acid) Chlorine Bromine

Author Information: Ronald Barnes is the President and CEO of Prozone International. Ronald Brook is the Vice-President of Engineering. Prozone is located in Huntsville, AL and manufactures very-ultraviolet and corona discharge ozone generators.