Properly maintained pools can be a great source of fun and relaxation for inground pool owners across the United States. As open bodies of water, however, pools are subject to contamination, not only from active bathers but also from the natural environment in which they reside.
While chlorination tends to be the primary method by which homeowners manage swimming pool water contamination, other advanced water treatment methods exist which are also quite effective. For instance, ultraviolet (UV) light and ozone are two such technologies which are excellent supplementary water treatment methods for pools. This article discusses how UV and ozone can be employed to complement chlorine in the battle against the inevitable—pool water contamination.
Sources of contamination
Broadly speaking, there are three distinct sources for contaminants entering any pool: the source water, bathers, and the environment.
Where did the water come from?
In most cases, source water for pools comes from the drinking water supply and, as such, is safe and low in contaminant levels. However, the municipal supply of drinking water can have significant concentrations of chloramines—as much as 3 mg/L (3 parts per million [ppm])—due to the fact many drinking water management plants use monochloramine as part of the water treatment regimen.
Monochloramines are used because they are chemically more stable throughout the water distribution system, helping to keep the supplied drinking water free of bacteria and biofilm. Further, they are less objectionable than other forms of chlorine in terms of taste and odor.
For many, groundwater or well water is the major source. Generally, this water is clean and free of disease-causing micro-organisms because the soil acts as a filter. Since this is not always the case (e.g. a leaking septic system, livestock waste), well water should be tested regularly.
In addition to the potential for micro-organism contamination, groundwater can vary considerably in the levels of inorganic compounds such as calcium (Ca), iron (Fe), manganese (Mn), hydrogen sulfide (H2S), and organic contaminants such as tannins (from decayed wood and leaves). These types of contaminants pose no serious health consequences, but can be annoying in pools as they can create water imbalance (calcium), stains (iron and manganese), water discolouration (iron, manganese, and tannins) and odours (hydrogen sulfide).
Table 1: Unintentional contamination caused by bathers
|Sweat||1 L (0.26 gal) per hour (active swimming)|
|Urine||50 mL (1.7 oz)|
|Fecal matter||104 mg (0.14 g)|
|Skin cells||Six million enterococci cells shed every 15 minutes|
When it comes to contamination, bathers represent a more significant origin than the source water. Bathers, whether accidentally or intentionally, release a myriad of contaminants into the pool, including: sweat, saliva, nasal mucus, urine, fecal matter, cosmetics, body oils, suntan lotion, soap and detergent residues, skin, blood, and vomit. Some of the contamination represents a real health hazard, but in all cases, a higher demand is placed on the sanitizer, requiring more chlorine to be used (relative to a contaminant-free pool). Further, because many of the contaminant sources contain nitrogen (N) (e.g.urea in urine and sweat), chloramines are formed.
Chloramines are undesired byproducts of spent chlorine that are known to irritate skin, eyes, and the respiratory tract.
Many people would be surprised at the level of contamination caused by a single bather. Scientific studies have demonstrated that active swimmers perspire 1 L (0.26 gal) of sweat every hour, release about six million skin cells every 15 minutes, and accidentally release 50 mL (1.7 oz) of urine and 104 mg (0.14 g) of fecal matter per bathing event (see Table 1). Naturally, some of the released contamination is accidental; however, it is well-known many swimmers (17 per cent in one report) intentionally urinate in the pool. Fortunately, urine is sterile and poses no microbial health issue (provided the bather does not have a urinary tract infection); however, urine contains a significant quantity of nitrogen compounds (i.e. urea) which leads to the formation of chloramines.
The environment represents the third source of pool water contamination. In outdoor pools exposed to the elements, water can become contaminated from windblown debris, rain containing microscopic algae spores, insects, bird and rodent droppings, pets (e.g. dogs), and water from unsanitary sources (e.g. rain runoff). Indoor pools, including those protected by a screen enclosure, are less susceptible to environmental contaminants.
Impact of contamination
Perhaps the most serious issue with pool water contamination is the potential for illness due to microbial introduction. Swallowing just a little water that contains these germs can make a bather ill. In fact, studies have shown the average swimmer ingests 50 mL (1.7 oz) of water for every hour of swimming.
Pathogenic microbes, which include bacteria, viruses, protozoans, and fungi, are of greatest concern in pools as they have been associated with numerous recreational water illnesses (RWIs).
In the U.S., the Centers for Disease Control and Prevention (CDC) regularly monitors pool-related health outbreaks and have identified Cryptosporidium as one of the leading causal agents with nearly 10,000 reported cases in 2008 alone. Diarrhea is the most common reported illness associated with pathogenic contaminants, but otitis externa (i.e. swimmer’s ear), as well as skin rashes and respiratory infections, also occur frequently.
Unfortunately, young children, elderly persons, and individuals that are immunocompromised can become very sick (and even die) when infected with waterborne microbes. Contrary to popular belief, chlorine does not kill all microbes instantly. Some microbes, such as Cryptosporidium, are very tolerant to chlorine, requiring several minutes to several hours of chlorine exposure before microbial populations are reduced to safe levels.
The World Health Organization (WHO) has categorized various types of micro-organism hazards that can exist in recreational water as fecally derived or non-fecally derived. Recognizing the variety of micro-organisms that can exist in pool water, a prudent strategy is always to employ multiple disinfectant types, where practical, to ensure sanitization success.
UV and ozone as chlorine’s companion
Table 2: Chlorine effectiveness against common pool micro-organisms
|Micro-organism||CT Value (ppm minutes)||Per cent inactivation||Temperature||pH|
|Cryptosporidium||15,300||99.9||25 C (77 F)||7.0|
|Giardia||15||99.9||25 C (77 F)||7.0|
|Adenovirus||0.75||99.99||5 C (41 F)||7.0|
|E. Coli||less than 0.25||99.99||23 C (73.4 F)||7.0|
|Norovirus||0.07||99.99||5 C (41 F)||7.0|
|Shigella dysenteria||less than 0.05||99.9||25 C (77 F)||7.0|
Table 3: UV light’s effectiveness against common pool micro-organisms
|Micro-organism||IT Value (mJ/CM2)||Per cent inactivation|
UV and ozone have been used in pool water treatment for decades, but only recently has the interest level in these technologies escalated.
The renewed interest is primarily a result of the clear identification of chlorine resistant micro-organisms, such as Cryptosporidium, and to a lesser extent, Giardia, as the causal agents in most RWIs.
UV and ozone are also more widely accepted for their role in chloramine destruction. Further, recent developments within the drinking water and pool industries has pointed to the synergistic oxidation and sanitizing benefits achieved when UV and ozone are used in combination.
Chlorine’s effectiveness as a sanitizer is measured by its CT value, where ‘C’ stands for concentration and ‘T’ for time. As logic would dictate, the higher the concentration of chlorine or the longer exposure time it has to a micro-organism, the more effective it is at eradicating it. Established CT values of chlorine for some common pool water microbes are shown in Table 2.
For instance, Cryptosporidium requires 15,300 ppm of chlorine acting on it for one minute in order to effect a 99.9 per cent reduction in the organism. Alternatively, a lower concentration of chlorine can be used, but the time required for inactivating the organism will be lengthened (e.g. one ppm chlorine acting for 15,300 minutes has the same CT value as 15,300 ppm for one minute).
In any event, it should be clear that destroying Cryptosporidium with chlorine requires impractical chlorine levels or impractical periods of time. While chlorine is a very good sanitizer, and has been a mainstay in the pool industry for decades, it is not the perfect end-all solution to pool water microbial treatment. Additionally, as has been stated previously, chlorine combines with other chemistries in the pool water to form chloramines—also known as combined chlorines. These chlorine forms are irritants to bathers and advanced treatment methods are needed to reduce their levels.
Using UV and ozone together as part of an auxiliary water maintenance program can have many benefits. UV, as a standalone supplement to chlorine, for example, provides tremendous protection against a wide range of micro-organisms. Similar to chlorine, the effectiveness of UV can be observed by its IT value, where ‘I’ stands for intensity and ‘T’ for time. For micro-organisms of interest to the pool industry, the IT value has been established (see Table 3) and, fortuitously, UV easily destroys Cryptosporidium. The recognition of UV’s value to Cryptosporidium inactivation occurred back in the ’90s after a drinking water issue in Milwaukee, Wis., left 69 people dead from Cryptosporidium contamination. UV light was soon determined to be very effective, requiring only small doses of light energy to destroy it.
UV’s mechanism of action is the breaking of DNA and RNA bonds within the target micro-organisms. Coincidentally, the energy required to vibrate certain bonds within DNA and RNA to the point at which they break, leading to micro-organism inactivation, can be generated by any UV lamp that emits ultraviolet light at the 254 nanometer (nm) wavelength. Both low-pressure and medium-pressure UV lamps are available and used for this purpose. The pros and cons of these two different lamp types are covered elsewhere.
Referring to Table 3, the minimum established UV dose required by the U.S. Environmental Protection Agency (EPA) for 99.9 per cent Cryptosporidium inactivation in drinking water is listed as 12 mJ/cm.2 This amount of UV is lower than the dose required to inactivate Pseudomonas aeruginosa to the same level (99.9 per cent). What is most interesting and relevant about these data is the National Sanitation Foundation (NSF), a respected authority for qualifying commercial pool equipment in the U.S., requires a 99.9 per cent or greater destruction of Pseudomonas aeruginosa for certification of UV pool equipment. By extension, devices that pass the 99.9 per cent requirement for Pseudomonas aeruginosa inactivation would have successfully inactivated 99.9 per cent or greater of Cryptosporidium—a U.S. drinking water requirement.
It should be self-evident that not all micro-organisms are readily destroyed by UV light. For example, adenoviruses can require a UV dose eight to 14 times greater than Cryptosporidium. In short, like chlorine, UV is not the end all solution to sanitization, but as a partner to chlorine in the fight against Cryptosporidium, the leading causal agent in pool disease outbreaks, it is a great choice.
UV light is also a great partner to chlorine in that it helps destroy chloramines. Chloramines, formed when chlorine combines with nitrogen (N)-containing compounds such as urea from sweat and urine, are broken down by the UV light. Most commercial facilities monitor chloramines as a measure of water quality. Generally, in a pool, chloramines should not exceed about 0.5 ppm. Pool operators should shock or super-chlorinate the pool to destroy these contaminants when that concentration is exceeded. The benefit of adding a UV system to pool water treatment is the chloramines are better held in check and less frequent shocking is needed.
Ozone represents another important strategy in enhancing pool water quality. Chemically, ozone (O3) consists of three atoms of oxygen. This arrangement is very reactive compared to its diatomic relative, oxygen (O2). As an oxidizer, ozone contributes its oxygen to other molecules it comes in contact with in a process called oxidation. Oxidation can be thought of as ‘chemical burning,’ whereby the result of the oxidation is simply carbon dioxide (CO2) and water. This process is valuable in pools and spas/hot tubs where a myriad of organic contaminants can exist, which need to be ‘burnt’ away via oxidation.
Table 4: Ozone effectiveness against common pool micro-organisms
|Micro-organism||CT Value (ppm minutes)||Per cent inactivation||Temperature||pH|
|Cryptosporidium||3-10||99||25 C (77 F)||6-7|
|Giardia lamblia||0.5-0.6||99||5 C (41 F)||6-7|
|Adenovirus||0.9||99||25 C (77 F)||7|
|E. Coli||0.02||99||5 C (41 F)||6-7|
|Legionella pneumophila||0.05-1.5||99||25 C (77 F)||7|
Oxidation is necessary in pools because the organic contaminants consume the sanitizer, make the water cloudy, and potentially pose health concerns with accidental ingestion. Generally, ozone does a better job than chlorine at eliminating organic contaminants because it is 2.2 times more powerful as an oxidizing agent than hypochlorite (ClO−).
Like chlorine and UV, ozone is very effective at chloramine destruction and microbial inactivation. For chloramine destruction, ozone’s mechanism of action is the oxidation of the chemical bonds leading to chloride and nitrate formation. For microbial destruction, ozone’s mechanism of action is oxidation of the cell wall or membrane of the micro-organism, leading to cell leakage and ultimate cell death.
With respect to most bacteria and viruses, less than 1 mg/L (1 ppm) of ozone is needed for a 99 per cent inactivation (see Table 4). Analogous to chlorine, the sensitivity of micro-organisms to ozone is measured by ozone’s CT value—ozone concentration multiplied by the exposure time. Unlike chlorine, however, ozone does not provide a long-lasting residual and, therefore, cannot be used as the primary sanitizer. Owing to its disinfection and oxidation capabilities, ozone makes for a logical complement to chlorine.
The benefits of UV and ozone as destroyers of chloramines and micro-organisms are not lost on major international authorities on water sanitization, including the CDC, EPA, and WHO. All of these organizations recognize and recommend UV and ozone as supplementary technologies to chlorine for drinking water and pool water treatment.
Taking UV and ozone to another level
Microbial inactivation studies, employing a combination of UV and ozone, have been reported for decades in water treatment.
In these research investigations, which span multiple water treatment applications, researchers have observed a synergistic effect when UV and ozone are used in combination for disinfection and oxidation purposes. That is to say, the observed results were greater than the expected contributions of their parts. The synergistic action, as observed by many investigators, has been attributed to the formation of hydroxyl radicals when UV light interacts with ozone. The use of hydroxyl radicals in water treatment is commonly referred to in scientific literature as ‘advanced oxidation.’
Synergy of UV and ozone for disinfection and oxidation
In 2006, Magbanua and co-workers clearly and systematically delineated the synergistic effect of a UV/ozone combination against E. coli.. In separate independent tests, the researchers studied the efficacy of UV and, then, the efficacy of ozone against E. coli. In doing so, the team was able to establish the requisite dosages of these two disinfectants to affect a specific level of microbial reduction.
In a second series of tests, the team began to use UV and ozone together to determine if the resulting microbial reduction was simply the sum of the contributions from each disinfectant. The results were nothing short of astounding.
It was discovered the individual UV and ozone doses required to destroy E. coli to the same extent (e.g. 99.9 per cent), could be reduced by a factor of 18 and four, respectively, when the two disinfectants were used together. In essence, there was a synergistic effect on the microbial reduction of E. coli when UV and ozone were paired together in a dual disinfection strategy.
According to Magbanua et al, the synergy associated with UV/ozone water treatment is attributed to the presence of supplementary hydroxyl radicals. Hydroxyl radicals are extremely fast-reacting, potent, non-selective chemical species. In fact, their oxidation power is recognized as being far more potent than chlorine gas, hypochlorous acid (HClO), or ozone. Further, the reactivity of hydroxyl radicals has long been recognized as being extremely fast—in some instances as much as one million times faster acting than ozone for bond breaking via chemical oxidation. For these reasons, the inactivation rate of waterborne pathogens is much greater due to the additional oxidizing power provided by the supplementary hydroxyl radicals.
The benefits of pairing UV with ozone does not stop with disinfection performance. While UV has virtually no oxidizing ability, the resulting hydroxyl radicals created from UV and ozone are tremendous oxidizers. Again, as is the case with disinfection, the literature is studies that reflect the superior oxidation performance of hydroxyl radicals formed from UV and ozone.
Table 5 provides a list of some of the reported research on advanced oxidation in water treatment and the applications in which it has been employed. It is clear the breadth of the applications for the trio of technologies within water treatment is vast owing to the potency of the combination for disinfection as well as oxidation. In recognition of the tremendous advantages that a combination UV and ozone system brings to recreational water treatment, devices are beginning to appear in the pool industry which deliver these benefits.
Table 5: Research related to advanced oxidation
|Disinfection of E. coli||Synergistic disinfection performance against E. coli using UV/O3 combination.|
|Oxidation of disinfection byproducts||Total trihalomethanes (THMs) and total organic halides reduced by 90 per cent and 98 per cent, respectively using UV/O3 combination.|
|Oxidation of organic carbon and disinfection byproducts||Total organic carbon reduced by 50 per cent over O3 or UV alone. THMs and haloacetic acids (HAAs) reduced by 80 and 70 per cent, respectively.|
|Drinking water treatment||Overall, the combination of ozonation and UV treatment leads to an important water quality with regards to disinfection, oxidation of micropollutants (atrazine, MTBE, 17-a-ethinylestradiol) and minimization of bromates.|
|Disinfection of poultry feed water||Fifty-fold improvement in disinfection against E.coli|
|Treatment of surface waters||Complete removal of MB (methylisoborneol) and geosmin and 40 per cent reduction in bromates with UV/O3.|
|Removal of disinfection byproducts||Increased reaction rate for the destruction of HAAs.|
It should come as no surprise that using UV and ozone (or a combination of the two) is gaining attention as a strategy for managing pool water contamination. Both contribute significantly to reducing the organic and inorganic contaminants that enter the pool from the source water, bathers, and the environment. In their role as supplementary oxidizers and sanitizers, UV and/or ozone help reduce the demand on the chlorine such that overall chlorine use is lowered. In addition to the benefit of reducing chlorine use, the pool water is more pure and healthy for bathers, a result that by any measure is a positive step for the pool industry.
This article was written by Ray Denkewicz and originally appeared on Pool & Spa Marketing [link].