Another Look at Hydroxyl Radicals

The following is posted with permission of the author "Alan Lewis".

Though I had previously communicated via email, I had the pleasure of meeting Alan in person at the 2010 World Aquatic Health Conference. We had dinner together and spent an entire evening talking. He has much to share and I hope you will take a brief moment to read his article. I have found it both thought provoking and informative.

Hydroxyl Radicals (OH·)

The significance of HYDRODYNAMIC CAVITATION (HC) in Advanced Oxidation Processes (AOPs) and its bearing on the control of disinfection by-products (DBPs) in hotel swimming pools and spas
By ALAN LEWIS - Pool Consultant |

There is little or no evidence that hotel engineers are trying to keep pace with, employ, or take interest in, the very significant paradigm shift across the world, in the way that disinfectants are being applied in hotels.

The principal, most cost effective, and by far the outstanding ubiquitous form of disinfection, is still chlorination. Numerous and ever increasing attempts have been made at trying to apply otherwise non - compliant alternatives. A lot of hesitancy exists in trying out AOPs. This might be excused for lack of understanding of the chemistry involved. It is more likely to be from the fear of being found non-compliant and/or the liabilities that might flow from such audacity? My suspicion is that for the most part these restraints spring mainly from budgetary shackles.

If a brave engineer is ready to venture out into to the unknown – then it will soon become evident that AOPs offer new and exciting opportunities to save money and actually do more with less chemical than ever before. This is in fact the paradigm shift that is happening in front of our (cyberspace) eyes – but in other parts of the world. While most of the health regulations for swimming pools place the greatest emphasis on the art of killing bugs without killing the bathers – very few regulators are seriously concerned with the control of potentially toxic DBPs.

In Australia we are only required to test for total combined chlorine (chloramines) and even then the constraints are minimal compared with the parallel requirements in many European countries today. Neither are we expected to test for known carcinogens such as Trihalomethanes, let alone Nitrosamines, which of late have also become a real concern world wide. If this is the case, why not exploit the fact that discovery of the real nasties among the DBPs can only be achieved by employing expensive laboratory tests – well beyond the reach of the Council Environmental Health Officers who are the front line of defense in field checks meant to expose non-compliance with the Health regulations. Even more challenging is the idea of measuring the existence of the elusive Hydroxyl Radicals (OH·) which demand a high level of technical skill and very expensive laboratory equipment.
Hydroxyl radicals are important in disinfection because essentially (as described in a previous article) they are the most powerful available disinfectant we are aware of. They can be created by “splitting the water molecule”

H2O => H + OH·

Their action in oxidizing or inactivating bugs is exceedingly rapid. Even more enticing they mitigate the need for the omnipresent chlorine residuals in the pool. Their power in breaking down DBPs is considerable. They leave no significant residuals because they revert back to water eventually. Their action in reducing the DBPs - particularly the organic DBPs - usually results in separating complex molecules back to their components rather than joining them with other chemical elements found in the pool (such as happens in some applications of UV photolysis). They displace the need for chemicals replacing them with higher energy needs in many cases, but leave the water pristine, clear and pleasant to bathe in.

Most pool operators are familiar with the phenomenon of air bubbles appearing in the lint pot lid of a pool pump. In many cases this may not necessarily be “cavitation” in the strict sense of the word. However, if there are no leaks in seals; glued joints, or other fittings in the circulation system such as at a barrel unions or the like, there maybe true cavitation as it is technically defined (in Wikipedia) below:

The concept of Hydrodynamic Cavitation (HC) describes the process of vaporization, bubble generation, and bubble implosion, which occurs in a flowing liquid as a result of a decrease followed by a subsequent increase in pressure. Cavitation will only occur if the pressure declines to some point below the saturated vapor pressure of the liquid. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation.


It is well known that it is essential to design impellers and their housing with care and attention to reduce to a minimum the creation of the cavitation effect, because the implosion of the small vaporized bubbles is so powerful it destroys the housing and/or the impeller, by erosion caused by the friction and the heat generated by those implosions. In cases where the metal alloys used to cast the impeller housing are soft (such as brass) – cavitation can quickly damage the housing and the impeller and render the pump useless. Even if the materials used are resistant to heat and chemical (chlorine) corrosion, cavitation can destroy a badly designed pump within weeks or months of its commissioning. Hence the Hydrodynamic design of the housing and impeller itself should avoid or minimize friction with the aim of reducing energy losses and avoiding cavitation to retain the integrity of the pump’s efficiency.

If the reader wishes to see a good example of the creation of HC with a propeller on “You Tube” I recommend you visit:

The force of the minuscule implosions resulting from HC – and the speed with which they happen actually create very short lived Hydroxyl Radicals (OH˙). In the case of the propeller example, so long as the propeller is spinning it is continuously creating Hydroxyl Radicals from the water itself by “splitting the water molecule” with the force of the implosion of the vapor bubble.

In the last decade, much research has gone into the harnessing of HC for the purpose of water disinfection or breaking down unwanted organic compounds (in our case chloramines and THMs or Nitrosamines). Combined with Ozone; UV; and/or chlorine; Hydroxyl Radicals disinfect water more effectively than most know methods in existence today. Clearly the application of radicals outclass all other methods and guarantee both a rapid kill of bacteria and destruction of unwanted organic compounds in recreational and drinking water.

In public (hotel) swimming pools we cannot escape the need to prevent cross contamination between swimmers. That is why regulators commonly require a minimal residual of chlorine present in a pool at all times. As we have seen already, it is now viable to reduce the minimal residual of chlorine to well below the common regulatory requirements – so long as the ORP (the kill power of the water) is at a level of 740mV or more. The assistance given by Hydroxyl Radicals to the ORP is difficult to define because it is so difficult to actually measure them.

This technology does allow a continual feed of the short lived radicals to the point where the water is in such good condition, and the pH is appropriate, that there is only need for a supplementary free chlorine residual of 0.3 – 08 mg/l. This level of chlorine in the water is almost unnoticeable and would certainly pave the way towards “chemical free” pools in the not too distant future.
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  • Hope fully Dr Howard Dryden will comment soon ( I know he is busy getting ready for SPATEX expo) he is already producing ultrasonic resonators to cause nano bubbles to be produced which are attracted to the surface of particles such as viruses, bacteria, fungal spores and protozoa that may be in the water and causing their distruction. My only complaint is they only produce a large unit for commercial pools and not smaller cheaper units for the domestic market.

    Another piece of clever work is the zeta potential mixer (ZPM) which is a static mixer to agitate and probably shear the water to create hydroxyl radicals, the two units together could remove the need for chlorine or certainly running with very low chlorine residuals which in turn lowers the DBP's By not using cyanuric acid to stabilse the chlorine but using titanium oxide which protects the chlorine and works with the sun's rays to cause photo oxidisation of bacteria etc. I have been sucessfully running a pool (residential) for 2 season on 0.2ppm chlorine using his approach. Not an employee, a customer!

  • Alternative sanitizers, of all types, have a role to play and can greatly improve water sanitation and quality. However, the bottom line is that persistent sanitation, must always be present and oxidation is required to destroy organic contamination and waste products. Chlorine does both of these important functions. Touting that a system is 100% chlorine free may be good sales technique, but it may lose sight of the fact that the same device will perform better and more consistently with a low level of free chlorine.
  • Rex,
    One mechanism to produce Hydroxyl ++ free radicals (or AOP Advanced Oxidation Process) is by injecting/introducing Ozone upstream of a UV Germicidal system. It is also said that when using VUV lamps to produce ozone in ozonators that humidity/condensation across the lamp will draw out the 254nm in addition to the ozone creating wavelength of 185nm kicking the ozone up a notch creating Hydroxyl++ in a pool/water applications especially when introduced into a smaller side stream versus mass water. Our ozone lamps allow for both spectrums. Plus --Rex, you have many years of Ozone experience and I am sure you have seen where pools with ozone exceed the 740mv ORP in both indoor and outdoor pools.

    O3 + hν > O2 + O
    O + H2O > 2OH•
  • P.S. the electrolysis also produces some chlorine, the amount depending on the level of chloride in the water. Such systems do NOT need nor do they want the high chloride levels that saltwater chlorine generators use (if the salt level is too high, then they produce more chlorine and less radicals).
  • No, cavitation is not the only way to generate hydroxyl radicals and in fact they are usually produced through electrolysis of water using special materials that tend to emphasize production of radicals over oxygen (and hydrogen) gas. Such systems are often called "advanced oxidation processes" as noted near the start of the article Rex posted.
  • Is pump cavitation the only way to produce Hydroxyl Radicals ?
  • Where do I begin...

    Let's start out with ORP. Specifying an absolute ORP level as a regulatory requirement is ridiculous because different sensors measuring the exact same water will measure different ORP readings. Even if you adjust the "set point" to force them to match, different sensors from different manufacturers often have different "slopes" -- that is, mV ORP per doubling in chlorine concentration. As shown in the graphs in this post, ORP is a rough logarithmic measure of the hypochlorous acid concentration (NOT Free Chlorine alone since Cyanuric Acid significantly reduces hypochlorous acid concentration), but as I noted in the post, comparing ORP readings from a portable Oakton ORP measuring device vs. the built-in ORP controllers in many of the pools, 30 out of 130 (23% of those that had built-in ORP controllers) differed from each other by more than 100 mV. ORP manufacturers' own specifications show widely varying ORP at the same FC (with no CYA) level, but even worse, they show different slopes such that even a setpoint adjustment cannot make two sensors behave the same way.

    ORP is useful for process automation since you can explicitly measure FC (and CYA) and adjust an ORP setpoint. There will be variation with pH as well and it surprises me that ORP manufacturers don't automatically compensate for that, so you really need to have an independent pH controller to maintain pH. Between a constant pH and a stable ORP one can get a stable FC, IF the ORP isn't getting interfered from other oxidizers such as non-chlorine shock or ozone or ... hydroxyl radicals.

    Now on to hydroxyl radicals. First of all, they are VERY short-lived because they are very reactive, which means that they will NOT provide any significant sanitation in the bulk pool water. As radicals, they initiate chain reactions only terminated when two radicals meet again. When chlorine breaks down from UV in sunlight (or from broad-spectrum medium/high-pressure UV lamps) it produces free radicals. Hypochlorite ion breaks down more readily producing chlorine free radical, Cl•, and oxygen anion free radical, O[sup]-[/sup]•, but then this radical can react with water to produce hydroxyl radical, OH•, and hydroxyl ion, OH[sup]-[/sup]. When hypochlorous acid breaks down, it also produces chlorine free radical, Cl•, but also produces hydroxyl radical, OH•. So hydroxyl radicals are not new and in fact are created by sunlight in outdoor swimming pools. This is part of the reason why outdoor pools tend to do better regarding oxidation of bather waste than indoor pools, though this effect is small and really only helps low bather load pools (such as most residential pools) -- higher bather-load pools still need supplemental oxidation, even if outdoors.

    This paper on nitrosamines in swimming pools is consistent with the characterization I gave. The level of nitrosamines is far lower in outdoor pools as well as in the pool using UV than in indoor pools (indoor hot tubs were the worst). It is also lower under conditions of lower bather load though not as much as might be expected.

    As for having low active chlorine residuals, this is already possible today by using Cyanuric Acid (CYA) in the water and unfortunately the study on nitrosamines did not indicate whether there was any CYA in the water. One can probably assume that there was no CYA in the indoor pools, but it is not as clear for the outdoor pools -- high bather-load outdoor pools can sometimes not use CYA because the loss of chlorine from sunlight is much lower than the loss due to oxidation of bather waste. However, by not using CYA, even in indoor pools, the active chlorine level is much, much higher. Having 4 ppm FC with 20 ppm CYA is roughly equivalent to having 0.2 ppm FC with no CYA. The lower active chlorine level will slow down all elementray reactions for which hypochlorous acid participates, but whether it reduces nitrosamines depends on their mechanisms of formation -- in theory, dichloramine levels would be higher at lower chlorine levels so would result in higher nitrosamines, but that proposed mechanism in the paper may be wrong.

    It is absolutely amazing to me that study after study after study of DBPs in swimming pools measure the FC level and do not look at the CYA level, especially when looking at outdoor pools where CYA may be used. It's as if no one read the 1974 paper defining the chlorine/CYA relationship or the many subsequent papers (such as ) showing that it is active chlorine (hypochlorous acid) that is most relevant for chemical reactions, NOT Free Chlorine (FC).
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