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
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
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.