PRB Articles


Pool Water Sanitizers

The potential for recreational water illnesses (RWIs) within aquatic facilities has pool operators scrambling. According to the Centers for Disease Control (CDC), between January 2005 and December 2006 (the most recent data available), there were 4,412 incidents of RWIs, including 116 hospitalizations and five deaths. According to the CDC, RWIs are 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 variety of skin, ear, respiratory, eye and wound infections. The most commonly reported RWI is diarrhea, which can be caused by germs, such as Cryptosporidium (Crypto), Giardia, Shigella and E.coli. Properly sanitizing pool water is thus of the utmost importance to pool operators. Given the small size of RWI organisms, filtration alone is probably not the answer. There are alternative methods to improve filtration--such as the use of flocculants and polymers--but they can be expensive and impractical. Good water chemistry is the most cost-effective and user-friendly approach to preventing RWIs.

Sanitizer Systems

Most public swimming pools require the use of a chemical sanitizer, or active “oxidant,” such as chlorine, bromine or fluorine to oxidize, destroy, or burn out bacteria, viruses and other organic contaminants. A majority of public-pool operators utilize chlorine because it’s cost-effective and readily available. Its three basic forms are:

1. Liquid chlorine (sodium hypochlorite)

2. Tablet chlorine (calcium hypochlorite)

3. Chlorine gas.

The following is a look at the pros and cons of traditional sanitizer delivery systems, as well as increasingly popular saltwater chlorine generation systems. Liquid Chlorine Liquid chlorine--which ranges from 10- to 12-percent sodium hypochlorite--is available in one-gallon bottles to 52-gallon drums to bulk delivery (hundreds of gallons) via tanker truck. Liquid chlorine is delivered to the pool recirculation system via a liquid feed pump.

Advantages: Relatively low cost and readily available; the ability to increase the feed rate to meet increased demand due to changes in bather load; and ability to prevent maintenance staff from handling the chemical since vendors’ tanker trucks come equipped with a hose and pump to deliver it directly to a storage tank.

Disadvantages: Shelf life--loses its effectiveness over time, so chemical deliveries are more frequent since the stock needs to be rotated. Liquid chlorine also tends to drive up pool water salinity and total dissolved solids (TDS), requiring pool operators to annually drain two-thirds to the full volume of a pool treated with liquid chlorine to keep proper salinity and TDS levels. Tablet Chlorine Tablet chlorine--with its familiar briquette or “hockey puck” shapes--is available in 50- or 100-pound plastic pails, and can be ordered by the pallet load. Tablet chlorine is delivered to the pool recirculation system via an erosion feed process.

Advantages: Long (if not indefinite) shelf life, readily available and easy to store. Local building codes may limit the quantity to be stored, and the tablets should not be stored in the vicinity of petroleum products. Disadvantages: Cost (it requires approximately 1-1/2 pounds of tablet chlorine to provide the same amount of chlorine as one gallon of 12-percent sodium hypochlorite); inability of tablet-chlorine systems to respond to rapid changes in bather load; tendency to overshoot or undershoot the amount of chemical being delivered because chlorinators typically only have two feed rates--it’s either on or off. Calcium hardness and TDS also can build up, requiring an annual draining of a portion of the pool volume. Gas Chlorine Gas chlorine--pure elemental chlorine--is the most effective sanitizer available. It is delivered to the pool recirculation system via an anti-siphon venturi assembly, which draws the gas under vacuum from 150-pound bottles or one-ton cylinders (similar to a municipal drinking water plant), and injects it into the pool water supply line.

Advantages: Low cost, long shelf life (gas loses little of its effectiveness over time) and ability to respond quickly to changes in demand for the chemical, assuming the chlorinator is properly sized. Disadvantages: It is designated as an acutely toxic gas, and is very dangerous to handle. Because of the high toxicity, it is difficult to get local fire marshals and building officials to sign off on using it. If it is approved for use, the safety measures required (i.e., chlorine gas scrubbers and backup generators), are extremely costly.

Saltwater Chlorine Generation Systems

A healthy, virtually chemical-free alternative to chlorine sanitizers is an on-site saltwater chlorine generation system. Non-iodized table salt (sodium chloride) is added directly into a swimming pool. Salt dosing levels typically range from 3,000 parts per million (ppm) to 5,000 ppm. Compared to the salinity of human tears (7,200 ppm) and sea water (36,000 ppm), the concentration in the pool is relatively low and does not leave a salty smell, taste or feel. As saline water passes through a chlorine-generating cell, low electrical currents transform salt into chlorine. Once the chlorine has killed bacteria and other organic compounds, it reverts back to salt, and the process begins again.

Advantages: Reduces or eliminates the need to store and handle chlorine and other chemicals; eliminates the cost of purchasing liquid chlorine; and produces water with a more natural, smooth and silky “soft water” feel, akin to a European spa treatment.

Disadvantages: Relatively high capital and maintenance costs associated with cleaning the chlorine-generating cells every one to three months; replacing the cathodes and anodes on a periodic basis; and the reduced ability of the system to respond to changes in chemical demand. Some pool operators have mitigated this issue by providing a back-up/secondary system, such as tablet chlorine.

Select A System

Many facilities look to the cost of purchasing chemicals as the primary factor in choosing a system. According to a survey of 10, large, commercial swimming-pool chemical suppliers, only one was able to provide a quote for gas chlorine: approximately $1.00 per pound. All of the vendors were able to provide quotes for 12-percent sodium hypochlorite and 50- to 100-pound pails of calcium hypochlorite:

· Liquid chlorine:

Lowest--$1.80 per gallon (Arizona);

Highest--$2.65 per gallon (Colorado).

Average cost per gallon--$2.16

· Tablet chlorine:

Lowest--$1.70 per pound (Texas);

Highest--$2.78 per pound (Northern California).

Average cost per pound--$2.29

At first glance, the costs for both chemicals appear similar. However, one must consider that it takes 1-1/2 pounds of calcium hypochlorite to equal 1 gallon of 12-percent sodium hypochlorite. The actual average cost for tablet chlorine is $3.44 per equivalent unit of sanitizer versus $2.16 for liquid chlorine. As an example, an outdoor 25-yard x 25-meter pool will require a daily average of 16 gallons of sodium hypochlorite and 24 pounds of calcium hypochlorite. Daily costs equal $34.56 for liquid chlorine and $54.96 for tablet chlorine. When deciding what type of sanitizer system to use, it is important to look at the advantages/disadvantages of each, availability of chemicals, initial capital costs and a detailed life-cycle cost analysis for each system. Life-cycle cost analysis should include factors like electricity usage, monthly maintenance and replacement costs for major components. Now that you’re armed with the facts, you can be an RWI “terminator” in no time.

Randy Mendioroz (rmendioroz@aquaticdesigngroup.com) is a Principal with Aquatic Design Group, a Carlsbad, Calif., consulting firm which specializes in the programming, planning, design and engineering of competitive, recreation and leisure-based aquatic facilities. For more information, call (760) 438-8400, or visit www.aquaticdesigngroup.com.

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