Not long ago [Spring 2009] we had a visit from Ed Lightcap and Dr. Roy Vore of DuPont Chemical Solutions. They came to share what they've been learning about chlorinated disinfection byproducts (DBPs) in recreational water, from their own research and others'. We'd already read Ed's Letter to the Editor of Aquatics International, wherein he stated emphatically that "breakpoint chlorination...does not eliminate chloramines...[it] actually leads to the formation of chlorinated organics and chloramines, collectively known as chlorine disinfection byproducts." We also knew that, acting on a similar belief, the National Swimming Pool Foundation® had decided to change the breakpoint chlorination formula in its 2009 edition of the Pool & Spa Operator Handbook.

To greatly simplify their presentation, what Ed and Roy believe is that a breakpoint dosage of chlorine, as calculated according to current teaching (i.e., 10 times the measured level of combined chlorine), results in an over-application of chlorine because the formula doesn't account for the chlorine content of the combined chlorine compounds themselves. These compounds, our visitors explained, are already more than halfway to breakpoint. Instead, they support subtracting the existing level of free chlorine from the "10 times calculation" to determine the treatment dosage. They showed us research indicating the lower dosage would be sufficient to drive inorganic chloramines to breakpoint and leave a free chlorine residual.

For example, if test results reveal 2 ppm of free chlorine and 0.5 ppm of combined chorine:

Current teaching for breakpoint dosage:
10 x 0.5 ppm = 5 ppm of FC needed, altogether

New NSPF teaching for breakpoint dosage:
5 ppm - 2 ppm = 3 ppm FC is the targeted increase

Beyond this, DuPont is advocating a multipronged strategy for DBP control:  (1) proactively reduce the formation of DBPs by preventing or eliminating their precursor compounds, through better education of swimmers and operators (shower before entering, don't pee in the pool, run the filter properly, etc.) and by regularly using non-chlorine oxidizers like ozone and potassium monopersulfate to remove organic contaminants; (2) use ozone or UV radiation as supplements to chlorine; (3) assure adequate ventilation of indoor aquatic attractions; and (4) replace water that has become problematic. The foregoing should make it easier for operators to maintain the free chlorine residual within the ideal range of 2–4 ppm and lessen the need to superchlorinate. Note, DuPont manufactures Oxone®, a potassium monopersulfate product used in the formulations of many non-chlorine oxidizing shocks.

Following the long-accepted practice of the pool/spa industry, Taylor's testing and treatment guides (items 2004B and 5695) advise using the "10 times calculation" to determine a breakpoint dose of chlorine. In the months ahead, we'll be monitoring reactions to this different way of thinking—particularly the real-world experience of pool operators with the new breakpoint formula in the CPO® manual. If industry consensus is "less really is more," we'll update our publications accordingly.

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