AOP Hydroxyl Radicals: A Sea Change for Aquatic Facility Operations
Why hydroxyl radicals are the future of commercial pool water treatment By Geoff McKenzie, Clear Comfort
water care professionals are switching from chlorine alternative disinfection
systems such as UV, ozone and saltwater systems to the advanced oxidation
process, or AOP sanitation, because of its oxidation power, ability to lower
chemical demand and disinfection byproducts, simple maintenance and lower
costs. However, many water care professionals are unfamiliar with how AOP
hydroxyl sanitation works and what it can achieve. Water care professionals
have long struggled with the complexity and costs of many chlorine
alternatives. AOP hydroxyl radicals may present the solution.
is AOP sanitation?
sanitation uses hydroxyl radicals to treat pool and spa water and destroy
unwanted organic and inorganic contaminants. As the name alludes to, it is an
advanced method to oxidize materials found in pools, spas, waterparks and more,
making the water clean, clear and sanitary.
recent years, AOP sanitation has become increasingly popular for residential
and commercial pool and spa treatment (see Figure 1) because it provides
powerful sanitation without harmful byproducts or residues. AOP systems even reduce
or eliminate disinfection byproducts such as chloramines, which can be
detrimental to swimmers’ health6. In addition, AOP doesn’t have some
of the limitations or downsides that pool owners and operators experience with
other sanitation systems, such as corrosion to pool surfaces.
UV systems can degrade chlorine in the pool, making it less effective, AOP
systems allow lower amounts of chlorine to work more efficiently because they
can reduce the organic load without degrading the chlorine. Some AOP systems
also operate at a fraction of the electrical consumption compared to many
alternatives, making them more economically and environmentally efficient.
are hydroxyl radicals?
radicals are highly reactive and oxidative molecules found throughout the
Earth’s atmosphere that destroy pollutants and impurities by breaking them down
into simpler molecules and decomposing them1. They attack organic
pollutants through four basic pathways: radical addition, hydrogen abstraction,
electron transfer and radical combination.
Hydroxyls are formed through a process: Oxygen molecules (O2) are broken down into excited atomic oxygen (O1) which react with water molecules (H2O) to form hydroxyl radicals (OH-). Hydroxyls are among the most potent oxidizing agents on the planet that are safe for water treatment. The oxidation potential of the hydroxyl radical (OH-) with available hydrogen (H+) ions far surpasses that of chlorine or molecular ozone [see Figure 1].
water treatment, the hydroxyls immediately oxidize and break down both organic
and inorganic contaminants, including chlorine-resistant Cryptosporidium parvum5.
Once they have reacted to the contaminants in the water, they dissipate without
leaving any harmful chemicals or residuals behind.
pools and spas, ‘work’ of advanced oxidation entirely takes place in the
plumbing and not in the pool or spa itself. Hydroxyls are highly reactive and
last for only a fraction of a second in the plumbing, which means they are
never exposed to vulnerable surfaces like tile, grout, coping, decking and
mechanical equipment. The addition of hydroxyl AOP sanitation to aquatics
facilities allows lower concentrations of corrosive chemicals to be used and
preserves surfaces and equipment.
pool sanitation systems make hydroxyls?
pool and spa sanitation, there are three common ways to produce hydroxyls:
ozone systems, ozone-UV combination AOP systems and hydroxyl-based (or direct
injection method) systems. While all three methods result in the formation of
hydroxyls, each one works differently and has varying levels of
Ozone Pool Systems
systems primarily leverage the oxidative value of O3 to disinfect
water. Indirectly, ozone produces hydroxyls in smaller amounts during the
stabilization or decay process. This process forms hydroxyls, although perhaps
not as efficiently as other AOP methods. In pool and spa treatment, only a
small amount of ozone actually ends up forming hydroxyl radicals because they
do not all break down into excited atomic oxygen (O1).To overcome
the inefficiencies of O2 to O3 conversion, effective
ozone systems utilize oxygen concentrators, desiccant air dryers, mixing
vessels, degas vessels and ozone destruct chambers. While effective, these
systems have proven to be complex and difficult to maintain for the mass-market.
Ozone-UV Combination AOP Pool Systems
effort to boost hydroxyl creation, recent systems blend UV and ozone. Ozone-UV
AOP systems work by injecting ozone gas in front of a UV system that treats the
ozone in the water and encourages hydroxyl formation. Certain wavelengths of UV
light break down the ozone molecules, creating atomic oxygen, which then reacts
to the water and forms hydroxyl radicals. Ozone-UV combination systems use two
separate systems, which increases the complexity with only a partial increase
of effectiveness of hydroxyl formation.
the UV and ozone systems must be maintained in regular intervals to assure
their peak performance. Usually this entails disassembly and reassembly of both
systems. Water care professionals have found this to be a time consuming and
laborious process. In addition to the complexity of a standalone ozone system
as mentioned above, a second system is now being added, which further increases
the potential points of failure.
biggest inhibitor to date for Ozone-UV AOP systems in aquatics has been the
inability to scale to larger water bodies. The majority of systems available
can only treat up to 50 gallons per minute or less. While newer systems are
being designed and launched, their limits appear to only treat up to 500 GPM.
Hydroxyl-based AOP Pool Systems
recent years, hydroxyl-based AOP systems have come to market and shown strong
results. The hydroxyl-based AOP works by directly injecting excited atomic
oxygen (O1) into the flow of water. Direct injection simplifies
installation while fast-acting hydroxyl reactions do not require complex
support infrastructure like the systems previously mentioned. In order to
create these unique direct-injection systems, ambient air is exposed to a
combination of proprietary UV wavelengths and an opposing magnetic field. The
advantage of these systems is their efficiency of power consumption and
effectiveness of oxygen conversion.
producing a steady stream of oxygen radicals in ambient air, direct-injection
systems require no degas, no oxygen concentrators, no desiccant air dryers or
other complex systems. Hydroxyl-based systems operate off a simple cartridge
exchange that takes less than a half-hour per year of total maintenance. These
newer technology systems have been shown to scale from spas to waterparks.
When O1 is directly injected into water it creates high concentrations of hydroxyl radicals. The effectiveness of these systems is evidenced in that they have been shown to reduce the chlorine consumed by 30 to 50% while maintaining similar residual levels of chlorine. The hydroxyls are directly replacing the oxidation workload that chlorine was performing previously. Figure 2 shows the chemical savings when direct hydroxyl systems are added to a pool over a nine-month study period.
systems are revolutionizing water
treatment for the aquatics industry by bringing accessible, affordable and easy
to maintain systems to the water care ecosystem. As water care professionals
look to improve their operations, these proven technologies will provide relief
from higher chemical costs, pump room complexity and long maintenance hours.
More importantly, patrons and facility owners will benefit from better air and
water quality and improved facility life. The significant improvements over
existing methods that AOP technology offers are enough to represent a complete
shift in the way water care professionals approach chlorine alternative