There are a lot of misunderstood concepts and myths in the aquatics industry. Unfortunately, many of these lead to operating unsafe facilities. Oxidation reduction potential (ORP) is one of those concepts in pool-water chemistry that is particularly poorly understood by many operators. A lack of understanding and application of ORP leads to potentially unsafe and unhealthy water conditions, whereas proper application of the concept is the only way to guarantee a sanitary pool.
Oxidation is a frequent topic of discussion in the industry, but it is persistently relegated to a position less important than sanitation (or disinfection). The truth, however, is that oxidation is much more important for overall water quality. In addition, oxidation is much more difficult and takes significantly more chlorine to achieve than sanitation. Naturally then, if there is sufficient oxidation, there is more than adequate sanitation. The goal is to optimize oxidation by measuring and controlling the effectiveness of the chlorine (or any sanitizer/oxidizer) added to the pool water, and sanitation will come along for the ride.
The method used to measure chlorine’s effectiveness is oxidation reduction potential. ORP is a qualitative measurement, significantly different than the quantitative DPD method of testing for chlorine residual in parts per million (ppm). It is measured electronically in millivolts (mV) and does not have a linear relationship with “free” chlorine (FC).
Based on a number of factors or “detractors,” the ORP can move independently of ppm residual. For example, a pool with 3.0 ppm “free” chlorine, at a pH of 7.4 and cyanuric acid (CYA) concentration of 40 ppm, has a lower ORP than a pool with 0.3 ppm “free” chlorine, at a pH of 7.2 and no cyanuric acid. Therefore, more chlorine does not necessarily mean more oxidation. ORP is reduced by the three detractors of chlorine’s effectiveness: rising pH, cyanuric acid and nitrogen introduction.
Due to its importance, all major pool operations textbooks discuss the relationship between hypochlorous acid (HOCl) and pH. When chlorine is added to water, it forms HOCl; this is the active form of chlorine that sanitizes and oxidizes. Based on pH, though, some of the HOCl ionizes into hydrogen (H+) and hypochlorite (OCl-) ions. The problem is that OCl- has less than 1 percent of the microbiological killing power of HOCl. As pH rises, the percentage of chlorine that is in its active, working form (HOCl) drops. The caveat is that DPD tests for ppm measure all the chlorine in the water, active and inactive (HOCl+OCl-). ORP, on the other hand, has been shown to be proportional to HOCl with all else being equal. This is proof of the fact that ORP is a direct measure of the efficacy of chlorine in the water.
Decades worth of scientific research studies show the relationship between ORP and bacterial inactivation. Some of the earliest significant studies from Germany and Sweden in the 1960s showed “that the inactivation or kill rate of bacteria by oxidizing agents was accurately predicted by ORP millivolt level” and that “it was not possible to formulate a relationship between bacterial kill rate and concentration of free chlorine.” It was also noted that “ORP millivolts, regardless of water quality, accurately predicts inactivation or kill rates of bacteria.” Continued and current research supports these findings. It is disappointing that current practice in the United States does not heed the science, but rather promotes the use of quantitative methods that are unreliable at determining whether water is sanitary.
The question of how to treat potentially infected pool water gives way to many opinions and little science-based advice. Indeed, even some prominent industry experts recommend using concentration time (CT) values to ensure the water is safe after potential contamination such as a fecal release. Though some studies have been conducted to establish CT values for various waterborne pathogens, they are unreliable and inconsistent. Crypto, for example, has been assigned CT values ranging from 7,200 to 15,300 in different studies. The fundamental problem with these studies is that they ignore basic chlorine chemistry, each using different pHs, temperatures and so forth. Not basing recommendations on valid, reliable research is irresponsible at best. Utilizing a proven, qualitative measurement of chlorine’s effectiveness that accounts for all its detractors is the only reliable way to ensure whether the water is biologically healthy and safe.
Another problem is that most health departments in the United States don’t regulate ORP. Therefore, many operators ignore its importance, preferring to control quantity and ensure regulatory compliance. Because of the lack of regulation, there is no established standard for swimming pools in the United States. Most research studies have shown that 650 mV is adequate to ensure virtually instantaneous disinfection; this is the standard for drinking water.
The World Health Organization, though, updated its guidelines for swimming pools and recreational water in 2006 and recommended 720 mV to assume the water is in good microbial condition. This ensures good disinfection and may provide adequate oxidation for many facilities. For high-use, commercial pools, ORPs in the range of 750 mV to 800 mV or more is considered best practice to achieve desirable oxidation and superior water quality. Even though it is generally not regulated, it is still to the benefit of the pool operator and the swimmer to measure and control ORP.
The only way to continuously measure and control oxidation reduction potential is with an automated water chemistry controller. All reputable controllers use pH and ORP sensors, not ppm. These rely on the ORP principle for the many reasons discussed. Even if a controller has a ppm readout, it is probably a calculated value from ORP and pH. Some new controllers actually do have a true ppm sensor and readout, but it is still critical to control based on ORP. After all, ORP is the important value when it comes to indicating what’s happening with the chlorine in the pool water.
A thorough understanding of the concept of ORP is imperative for a pool operator to effectively manage water quality. Science shows it to be the best predictor of bacterial disinfection because it is a measure of what the chlorine is doing in the water, not how much is there. Common misunderstandings have led even prominent agencies to recommend unreliable practices for treating water.
ORP control with an automation system is the only reliable way to ensure pool water is safe and healthy.
Matthew Griffith is the pool operator at the Georgia Institute of Technology in Atlanta as well as an AFO and CPO instructor. He is a frequent speaker at industry events and has published numerous aquatic-related articles.