A Crash Course in Chlorine

By Kelly O'Toole

Even after decades of use and the introduction of alternative disinfectants, chlorine remains a king germ-slayer for commercial pool water due to its effectiveness, safety, low cost and convenience.

According to the U.S. Centers for Disease Control and Prevention, chlorine and pH are the first defense against germs that can make swimmers sick. Perhaps that's why a 2005 independent research study conducted by the Mullen Group found that more than 80 percent of commercial pool operators use some form of chlorine as their primary sanitizer.

"One of the main reasons chlorine continues to be so popular is its effectiveness against a wide range of pathogens," says Jeff Sloan, director of the Chlorine Chemistry Council's Disinfection Program. "Chlorine quickly and effectively disinfects pool water and kills most germs within minutes. Unlike some other disinfectants, it also provides a residual level that continues to disinfect long after it's applied. In addition, chlorine is easy to monitor, and you can adjust the levels based on the needs of the pool, such as increased bather load."

Birth of a disenfectant

Chlorine's history began in 1774 in the small experimental laboratory of Swedish pharmacist Carl Wilhem Scheele, who released a few drops of hydrochloric acid onto a piece of manganese dioxide, creating a greenish gas. In 1810, Sir Humphry Davy (pictured above), an English chemist, recognized the gas as an element and called it chlorine, based on the Greek word khloros for greenish-yellow.

Chlorine was first used as a germicide in 1846 to prevent the spread of "child bed fever" in the maternity wards of Vienna General Hospital in Austria. Continuous chlorination of drinking water began in the early 1900s in Great Britain, where its application sharply reduced typhoid deaths. This advancement in illness prevention quickly spread to the United States, first in Jersey City, N.J., in 1908 and then to other cities and towns across the country.

The first EPA-registered use of chlorine for sanitizing pools was in 1948. In the following decades, chlorine evolved into its four primary forms—gas, liquid, granular and tablet—used to shock and disinfect commercial pools today.

Choose the best form

For most pool operators, using chlorine is a given. Deciding on which form to use often depends on the trade-offs operators are willing to make.

Chlorine gas, for example, provides 100 percent available chlorine, meaning all of the volume is active product. The gas is contained inside a steel cylinder, maintained under pressure and applied via a chlorine injector.

Because it's a toxic and corrosive gas, strict safety guidelines exist to protect both the environment and personnel. On-site storage of 100 pounds or more requires emergency preplanning in cooperation with the state emergency response commission and the local emergency planning committee to comply with the Emergency Planning and Community Right-to-Know Act. In addition, personnel working with chlorine gas must be trained, certified and recertified on a regular basis to comply with various laws.

Safety information for pool operators using chlorine gas is available from the Chlorine Institute at www.chlorineinstitute.org.

Liquid chlorine, a common misnomer that refers to sodium hypochlorite or liquid bleach, eliminates many of the hazards inherent in the gas form of the chemical but offers just 10 percent to 15 percent available chlorine. Because of this dilution, more product is needed to get the same job done, resulting in increased storage space and higher shipping weights. In addition, a secondary containment must be in place for spill-prevention purposes.

The granular form of chlorine is typically calcium hypochlorite, which offers 65 percent to 75 percent available chlorine. Other options include lithium hypochlorite (35 percent available chlorine) and sodium dichloro-s-triazinetrione (typically around 55 percent available chlorine, but it could be higher or lower depending upon the formulation used).

Granular offers lower shipping weight, less storage space, minimal spill hazards and safer handling. Convenient for shock treatment, it can be broadcast over the surface of the water, added to the pool skimmer with the circulation system running, or pre-diluted in water and added to the pool.

For continuous chlorination, granular is not as convenient as the other forms.A few automatic feeders are available, but there is a tendency for water vapor to get to the granules in the hopper, causing clumping and unreliable feeding.

Issues with the other forms of chlorine led to the development of tablets. In commercial pools, the most common type is calcium hypochlorite (65 percent to 75 percent available chlorine). While trichloroisocyanurates (trichlor) tablets are the most common form used in residential pools, their presence in commercial pools is not as prevalent. This is because trichlor tablets have a built-in stabilizer that can cause a pool to become over-stabilized and inhibit chlorine's effectiveness. Most states have strict limits on stabilizer levels, and some even ban the use of trichlor for commercial pools.

Calcium hypochlorite tablet chlorination offers all the advantages of the granular form and has specially designed feeders that conveniently and consistently deliver continuous levels of chlorination with minimal effort and maintenance. To distinguish these tablets from white trichlor tablets, which could cause serious safety issues when mixed with calcium hypochlorite, one major manufacturer, for example, adds a blue colorant to its cal-hypo tablets.

Inconsistent chlorination can result in having your pool shut down by health inspectors due to either inadequate or excessive chlorine levels. A good way to avoid such problems is to install a properly sized calcium hypochlorite tablet chlorinator certified to NSF/ANSI Standard 50, follow the manufacturer's directions and always use the specified chemicals.

The NSF/ANSI Standard 50 is the globally recognized mark of a high-quality pool chlorinator that has passed rigorous independent testing and is certified by a third party for its chlorine delivery, reliability, durability and safety.

Evaluating chlorine alternatives

While chlorine is used in the vast majority of commercial pools, there are other disinfection methods currently available.

Bromine is less pH-dependent than chlorine (within the range of normal pool pH) and has less odor and irritation. However, the disinfectant cannot be effectively stabilized in sunlight, making it inefficient for outdoor use. BCDMH, the most common form of bromine used for spas and pools, has been associated with rapid onset skin rash in spas with insufficient water replacement. Bromine is also a fairly expensive alternative to chlorine.

Ozone and UV radiation are very aggressive disinfectants that are effective against Cryptosporidium, which has proven to be resistant to typical levels of chlorine.

Both require investment and maintenance costs for onsite generation, and they leave no residual in the pool, requiring them to be used in conjunction with a residual sanitizer like chlorine.

Polyhexamethylene biguanide, or PHMB, is effective as a bactericide, and neither sunlight or bather waste break it down at a high rate. The major disadvantages are that it is incompatible with a wide range of common pool chemicals and does not break down organic waste in the water.

Pool operators choosing PHMB must use a supplemental oxidizer to deal with the microbial nutrients that build up.

Long history

Having a safe and reliable system that maintains crystal-clear water and eliminates pathogens is but a small drop in a pool's overall costs. Of the disinfectant methods available, chlorine continues to rise to the surface.

"The efficacy of chlorine on certain pathogens has been extensively studied and is very well known," Sloan says. "It has such a long history—basically 100 years—of safe and effective use in water treatment, which makes it the best alternative for commercial pools."


Kelly O'Toole is a freelance writer in Wexford, Pa.

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