Pool Water Chlorination Mitigates Potential for Recreational Waterborne Illness
The high potential for recreational water illnesses (RWIs) at public aquatic facilities these days must be making pool operators queasy. According to the most recent statistics from the U.S. Centers for Disease Control, 4,412 incidents of RWIs — including 116 hospitalizations and five deaths — were reported between January 2005 and December 2006.
Most commonly spread by swallowing, breathing, or having contact with contaminated water from swimming pools, spas, hot tubs, decorative water fountains, lakes, rivers or oceans, RWIs can cause a wide variety of respiratory, skin, ear, eye and wound infections. But the most commonly reported RWI is diarrhea, courtesy of such germs as cryptosporidium, giardia, shigella and E. coli.
With all the attention RWIs receive, particularly at the beginning of every summer swimming season, it's no wonder that pool operators are desperate to find a silver-bullet approach to sanitizing pool water. Filtration alone is not the answer, given the tiny size of RWI organisms. A recent report from the Professional Pool Operators Association suggested that high-rate sand filters can remove particulates in the range of 20 to 50 microns, and filters that utilize diatomaceous earth media can conservatively remove particulates in the range of 5 to 10 microns. However, since cryptosporidium measures 5 microns, and many bacteria are 2 microns or smaller, there is a strong probability that a majority of the millions of organisms associated with RWI infections will make it past even the most efficient filtration systems.
The use of flocculants and polymers — which help coagulate particulates — will likely help improve filtration, but those methods can be expensive and impractical for many pool operators. Clearly, sensible and informed water sanitization is the most cost-effective and user-friendly way to mitigate the potential for RWIs.
Most state and local health departments require the use of a chemical sanitizer (or active "oxidant") in public swimming pools. Sanitizers oxidize, destroy or burn out bacteria, viruses and other organic contaminants to keep pool water clean and safe. Traditional chemical sanitizers include chlorine, bromine and fluorine. Taking cost and availability into consideration, a majority of public pool operators — perhaps as many as 90 percent — utilize chlorine in one form or another. Chlorine for public swimming pools is typically purchased in three basic forms: liquid chlorine (sodium hypochlorite), tablet chlorine (calcium hypochlorite), and chlorine gas. In addition to traditional sanitizer delivery systems, saltwater chlorine generation systems that have been around for years are surging in popularity. Here are the pros and cons of each sanitizer delivery system: • Liquid chlorine is commercially available in sizes ranging from one-gallon bottles to 52-gallon drums to hundreds of gallons via tanker truck. It is delivered to a pool's recirculation system using a liquid feed pump that injects it into the pool-water supply line. Advantages of working with this type of chlorine include ready availability, the ability to ramp up the feed rate to meet increased demand due to changes in bather load, and a relatively low cost, depending on the geographic region.
Despite its designation as a corrosive material, liquid chlorine is fairly easy to handle. But because it is still considered hazardous, bulk delivery provides a distinct safety advantage by minimizing or completely eliminating maintenance staff exposure to the chemical. Similar to refilling gasoline storage tanks at a service station, employees simply need to provide access for the chemical vendor's tanker truck, which comes equipped with pump and hose to deliver the liquid chlorine to a storage tank.
The chemical tends to a have a limited shelf life, with inventory of 12 percent concentrate liquid chlorine losing eight to 10 percent of its strength every 30 days. Consequently, deliveries will need to be more frequent than that of other sanitization chemicals, and rotating stock every 60 days is recommended. Liquid chlorine also drives up total dissolved solids (TDS) and water salinity — sodium, after all, is in its name. Many pool operators annually end up draining two-thirds of a liquid-chlorinated pool, if not the entire thing, and then refilling it simply to keep TDS and salinity levels reasonable for public use.
• Tablet chlorine, which comes in briquette or hockey puck shapes, is commercially available in 50- or 100-pound plastic pails and can be ordered and delivered by the pallet. Tablet chlorine makes its way to a pool's recirculation system through an erosion-feed process, in which pool water is introduced into a pressure vessel that allows the dry chemical to erode into a solution that is then injected into the pool-water supply line.
Advantages of working with this type of chlorine include ready availability and a long shelf life. Tablet chlorine is easy to store, but local building codes often limit the quantity that can be kept on hand at any given time. Keep in mind that tablets should not be stored near petroleum products such as oil, gasoline or diesel fuel, and care must be taken by the maintenance staff to avoid lower-back injuries when handling the heavy containers.
The inability of automated tablet chlorine systems to respond to rapid changes in chemical demand may cause problems when bather load is high. Tablet chlorine typically operates at only one feed rate: the chlorinator is either on or off, so there also is a tendency to overshoot or undershoot the amount of chemical delivered to the pool water. Calcium hardness and TDS can build to potentially unacceptable levels, so (as with liquid chlorine) an annual draining of at least part of the pool may be required.
• Gas chlorine is pure elemental chlorine, not a compound like sodium hypochlorite or calcium hypochlorite. As such, it is the most effective sanitizer and protector against RWIs available in the commercial pool market. However, it also is highly toxic.
Gas chlorine is delivered to a pool's recirculation system using an anti-siphon venturi assembly that is similar to a hydrotherapy jet in a whirlpool. As water rushes past a restricted orifice within the assembly, the chlorine gas is drawn into the water stream by the vacuum force created by the venturi. The chemical is inexpensive, boasts a long shelf life and responds quickly to changes in chemical demand. All that said, gas chlorine is designated as an acutely toxic gas and therefore extremely dangerous. If, for example, an employee spills liquid chlorine (which, at 12 percent concentration, is a little stronger than household laundry bleach), he or she will have to purchase a new pair of pants. But if that person spills chlorine gas, a trip to the hospital is inevitable.
This is the same chemical that was used during World War I to kill tens of thousands of Allied and German soldiers. The only real difference between the type of gas chlorine available now and that once used in trench warfare is that the gas produced between 1915 and 1918 was yellow and called "mustard gas." Because of its high toxicity, local fire marshals and building officials seldom allow the use of gas chlorine, and in the rare cases when it is approved, the required safety measures (including chlorine gas scrubbers that help ventilate chemical storage rooms and backup generators to power them) can be cost prohibitive.
• Saltwater chlorine generation systems produce sodium hypochlorite on-site, but it is never stored. Nor is it delivered by a vendor, as are the other sanitizer systems. Non-iodized table salt (sodium chloride) is added directly to the swimming pool. Recommended sodium chloride dosages vary by vendor but typically range from 3,000 to 5,000 parts per million. By comparison, human tears have a salinity of 7,200 ppm and seawater has a salinity of 36,000 ppm, making the salt concentration in a pool relatively low.
As saline pool water passes through a chlorine-generating cell, an electrical current is passed from a negatively charged cathode to a positively charged anode. That electrochemical reaction disassociates the sodium and chlorine, thus producing hypochlorous acid along with a number of other compounds. Once the chlorine generated from the salt has killed bacteria, viruses and other organic compounds, it reverts back to salt, and the process begins all over again. Occasionally, more salt must be added to replace what is lost to splash-out and filter backwash.
A key advantage to saltwater chlorine generation is the reduction in or elimination of chlorine storage, resulting in less direct handling of chemicals and a "soft water" feel for swimmers. In fact, many patrons of pools utilizing saltwater chlorine generation report their experiences as very favorable and similar to those found in a European spa.
Disadvantages include significant capital outlay, maintenance expenses associated with cleaning the chlorine-generating cells every three months (or more frequently, depending on the metal quality of the cathodes and anodes), periodic replacement costs of the cathodes and anodes, and the reduced ability of the system to respond to changes in chemical demand. Similar to erosion-fed tablet chlorinators, saltwater chlorine generators have a preset production rate for generating liquid chlorine, so the ability to respond to fluctuating bather loads is limited by the on-off nature of the system. Some facility designers and pool operators have mitigated that concern by providing a backup or secondary system to meet the requirements associated with peak bather loads.
This involves the use of a moderately sophisticated water chemistry controller with dual-setpoint capability. The desired chlorine level setpoint (say, 2 ppm in the pool water) would control the primary sanitizer equipment (in this case, the saltwater chlorine generator). The minimum chlorine level setpoint (say, 1.5 ppm) would control the secondary sanitizer equipment (a tablet chlorinator, for example). If there is a light-to-moderate bather load, the saltwater chlorine generator can usually maintain the desired level of 2 ppm. However, a sudden increase in bather load can result in the water chemistry controller sensing a chlorine residual less than the minimum level of 1.5 ppm and activate the backup sanitizer to make up the difference.
Which sanitizer-delivery system should conscientious pool operators use to ward off the risk of RWIs? Unfortunately, the "best one" has yet to be invented. In most cases, pool operators find themselves having to choose the "best available" option. Gas chlorine is the most effective sanitizer, but the haz-mat issues associated with its use are daunting. The other sanitizers can all be effective when properly used, but each chemical has other impacts on pool-water chemistry and quality.
Giardia (pictured), cryptosporidium, shigella and E. coli are among the most common germs that cause recreational water illnesses.
Costs of liquid chlorine ranged from $1.80 per gallon in Arizona to $2.65 per gallon in Colorado, with an average cost per gallon among the 10 vendors of $2.16. Costs of tablet chlorine fluctuated between $1.70 per pound in Texas and $2.78 per pound in California, with an average per-pound price of $2.29 among the vendors. At first blush, the costs of the two chemicals look substantially similar. However, it takes 1.5 pounds of calcium hypochlorite to equal one gallon of 12 percent strength sodium hypochlorite, so the actual average cost of tablet chlorine is $3.44 for an equivalent $2.16 unit of liquid chlorine.
That means that an outdoor 25-yard-by-25-meter pool will require a daily average of 16 gallons of sodium hypochlorite or 24 pounds of calcium hypochlorite to maintain a setpoint chlorine residual of 1.5 ppm. Therefore, using the average cost for chemicals provided above, the chemical cost per day would be $34.56 for liquid chlorine and $54.96 for tablet chlorine.
When deciding what type of sanitizer system to use for a given pool, it is important to look at local availability of chemicals, the advantages and disadvantages to each one, their costs and the initial capital expenditures associated with the systems. Additionally, insist that the vendor provide detailed lifecycle cost analyses — including such factors as electricity usage, required monthly maintenance and replacement costs for major components.
Silver bullet? Sorry, but in the world of pool-water sanitizer systems, there is no such thing. Armed with the facts about each type of system, however, pool operators can effectivly and cost efficently turn their facilities into RWI-free zones.
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