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 | firstname.lastname@example.org
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.
NOTE: IN TRUE CAVITATION THERE ARE NO AIR BUBBLES – ONLY WATER VAPOR BUBBLES SUCH AS ONE COMMONLY SEES WHEN WATER IS BOILING IN A COOKING POT.
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.