Breakpoint chlorination is the established method taught in operator certification courses for removing combined chlorine in recreational water. However, breakpoint chlorination was designed for inorganic ammonia. It does not always work with organic-based nitrogenous substances.
The reason for this is quite simple. The organic loading provides an excess of competing reactions between the chlorine and the carbon, sulfur, nitrogen and phosphorous that comprise the complex organics accumulated in the pool.
Many of these organics, such as enzymes found in saliva, contribute thousands of these atoms. This can require days of continuous oxidation from mainstream treatments to reach the desired end products. Initially, the chlorinated byproducts are large, complex compounds that have low volatility. As smaller fragments are produced, however, their volatility increases, inducing the fouling of the air and other commonly known effects, especially in enclosed or indoor pools.
Swimming pools are dynamic systems in the sense that the organic demand (load) is being added to the system all the time the pool is in use. To obtain and sustain excellent water and air quality, whatever quantity of demand is added today, by tomorrow morning we want that demand to be gone from the system. If it is not, over time the demand accumulates in the system, causing the water and air quality to be compromised.
If we want to sustain as close to 0.0 ppm combined chlorine as possible, the rate of decomposition must be very close to the rate of addition of the demand. Furthermore, we want to mineralize the organic demand. In other words, we want to decompose the organic demand into its inorganic end products before it forms combined chlorine. We do not want to form byproducts that linger.
Many factors compromise the performance of treatments in recreational water. A few include:
- Complex organic molecules have thousands of atoms requiring oxidation.
- Molecular structure inhibits access to the oxidation site, thereby preventing or severely limiting the rate of oxidation (steric hindrance).
- Bond strength can exceed the oxidation potential of the oxidizer.
- Competing reactions can drag out the time required to mineralize the chlorinated byproducts.
Furthermore, treatments used in the circulating system — even when properly sized — always are recovery procedures. That is to say, the combined chlorine has already formed in the pool, and the treatment is used to decompose it.
Advanced oxidation technology (AOT) is another method. It is used in potable water applications, primarily in Southern California, as well as wastewater treatment and groundwater remediation where oxidation-resistant compounds exist.
AOT incorporates the formation of extremely powerful “radicals” in-situ (produced in-use) to the application. “Hydroxyl” radicals (OH-) are common for potable water and wastewater. Groundwater remediation and wastewater with organic compounds extremely resistant to oxidation utilize “sulfate free radicals.”
The sulfate free radicals are the more effective of the two hydroxyl radicals. However, hydroxyl radicals decompose in the presence of alkalinity, and their exclusive generation is incompatible with chlorine. This would limit their recreational water-treatment use to a side stream application, similar to ozone. But sulfate free radicals are more powerful than ozone. They have a reported oxidation potential of 2,600 to 3,100 mV at pool-water pH values, compared with ozone at 2080mV.
Sulfate free radicals work differently than other oxidizers traditionally used in recreational water. Ozone, for example, is generally involved in “oxygen addition” reactions. This requires a suitable site on the organic compound for oxidation and oxygen addition to take place. Also, these types of oxidation reactions require excess amounts of oxidizer to drive the reactions forward to complete the mineralization process.
In comparison, sulfate free radicals consume electrons, but are not involved in oxygen-addition reactions. As a result, sulfate radicals are pure oxidizers (consume electrons) that induce radical formation on the organic compound. This is a strong and beneficial mechanism because the organic radical that was produced will undergo additional reactions independent of the presence of more sulfate radicals. The propagation steps may include hydroxylation from water; oxygen addition from dissolved oxygen; or autocatalytic decomposition, where organic radical strips an electron from its neighboring atom in the organic compound, destabilizing the compound and thus producing another radical.
Sulfate free radicals are extremely powerful and have been shown to decompose compounds far more resistant to oxidation than those involved in forming combined chlorine in pools, according to a 2002 study by the Chemical Society of Japan. But they have limited usefulness due to the conditions required to produce them. Sophisticated equipment and/or control systems and operational conditions have made this technology unsuitable for recreational water applications.
That may be about to change. The limitations preventing use of sulfate free radicals in recreational water have been overcome through development of a new-generation peroxygen chemistry, which uses proprietary designer oxidizer technology. A significant advantage of this technology is the ability to control the formation of the radicals in the bulk of the pool water, where the contaminants are added to the system. The radicals rapidly decompose combined chlorine and organic contaminants on contact, which prevents odors, irritation, algae blooms, hazy water and low ORP. It also can save money by reducing chlorine use and filter runs.
With more than two years of documented results on hundreds of high-use systems, this new-generation peroxygen chemistry has consistently provided combined chlorine levels from 0.0 to 0.2 ppm, even while the pool is in use. Furthermore, severely contaminated pools (more than 3 ppm combined chlorine) have recovered overnight, opening on time in the morning with no detectable levels of combined chlorine.
This new-generation peroxygen chemistry is a patent-pending AOT that comes in the form of a dry, stable powder. Once dissolved in water, it is designed to decompose within minutes while producing the desired sulfate free radicals. Stable, yet effective, it is registered with the California Department for Pesticide Regulation.