UVc Irradiation  in swimming pools or chlorinated system

UVc has been gaining favour recently as a means of treating the water in chlorinated swimming pools. The reasons for this situation include,

1. Reduction of chlorine &  combined chlorine

2. Cryptosporidium protection

Reduction of Chlorine & combined Chlorine

UV is normally applied at an irradiation level of 20 to 60 mWs/cm² /sec at this irradiation level UV will certainly reduce the combined chlorine but it does this by chopping up the long chain organic molecules into smaller components.  Each one of the smaller organic molecules then reacts with chlorine.  When UV is used the chlorine demand of the system will  increase.  The graph opposite gives the chlorine and carbon dioxide consumption in a public swimming pool. The UVc was turned off on week 4, and the hypochlorite consumption dropped by 80 litres per week. The carbon dioxide consumption also dropped by 60 kg per week. In effect the use a medium pressure,  Ultra Violet disinfection doubled the chemical demand.

The combined chlorine level will also drop in the pool water,  however if you are using more chlorine you have to ask the question, what is happening to the chlorine and where is it going ?

In out-door pools triazine-s-trione  (cyanuric acid) is added to the water to stabilize the chlorine, this is not required for indoor pools without UVc.  However if the pool is using UVc irradiation of the water then hypochlorite undergoes rapid photolysis.

2 OCl ^-    + UV ----------> 2 Cl^- + O2 (g)

and 2 HOCl ^-      + UV --------> 2 HCl + O2 (g)

The photolysis reactions above are simplistic as the UV light  is also forming components such as hydroxyl radicals.  Organic molecules are also being chopped by UV light, this is how UV kills bacteria by breaking down the DNA molecule in the nucleus. UV light is therefore chopping up organic molecules into smaller and smaller bits,  and as the size of the molecule decreases it becomes volatile and more likely to escape through the surface of the water as a chlorinated THM ( tri-halomethane) gas such as chloroform CHCl_3.

The THMs are toxic and  carcinogenic, the use of UVc will dramatically increase the concentration of these chemicals in the atmosphere of the building,  but most especially in the layer of air just above the surface of the water. In Denmark the Danish Ministry of the Environment and the Danish Environmental Protection Agency, set a recommended THM limit of less than 25 ug/l and an absolute limit of 50 ug/l.  In drinking water the upper maximum value is 100 ug/l, however in swimming pools that use UVc without activated carbon absorption after the UVc unit will have levels well in excess of the recommend maximum.  Indeed in the pools examined by Dryden Aqua levels as high as 300 ug/l were recorded.  Chloroform is very volatile,  and is rapidly absorbed through your skin and lung tissue, concentration of chloroform in the blood of children can be 50 times higher after 30 minutes in a pool. While high levels of combined chlorine are not desirable, it is much better to have a relatively stable large organo-chlorine chemical in solution rather than turning it into a gaseous carcinogenic chemical which we can then breath in off the surface of the water.   It is the opinion of Dryden Aqua that the UK swimming pool industry is not being made aware of the serious atmospheric air quality and public health implications of using UVc in chlorinated water.   The Danish Technological Institute Department for Swimming Pool Technology recommend that activated carbon should always be used after UVc to remove the THM`s (click here to download report).

In drinking water systems,  UVc will also generate THM`s whether it is used before or after chlorination,  it is therefore essential that the water water is as clean as possible prior to treatment by UV.

Cryptosporidium reduction

UVc irradiation of the water will kill a high percentage of the viri and bacteria, the level of disinfection depends upon the irradiation dose received by the water.  If the water receives UV at 40 mwatts/cm2/sec then the bacterial kill rate will be in the order of 99.99%,  with regards to viri you need an irradiation dose of 200 mwatts/cm2/sec.  The UV unit will kill the organisms as they pass through the UV unit,  however when bacteria and viri are in the water  chlorine is also very effective at killing the organisms,  so in effect there is no need to use UV to kill viri and bacteria in chlorinated systems.  Cryptosporidium is neither a virus nor a bacteria but a small protozoan  that forms an egg like oocysts which measures approximately 4 microns in diameter. The oocysts has a very high level of tolerance to chlorine,  around 120mg/l of free chlorine is required to oxidise the oocysts.

If there is an out-break of crypto in a swimming pool,  then there is no mechanism to stop horizontal transmission of the parasite between bathers in the pool. However we can deal with the Crypto in the plant room. The standard approach is to use flocculation with PAC before sand filtration.  It is essential that the sand filters are operated at a flow below 20 cubm/hr/sqm, otherwise a higher percentage of solids including the Crypto oocysts could be pushed through the sand bed.  Also in many cases sand filters suffer from worm hole channeling which provides a conduit for the passage of the oocysts straight through the filter bed.

There has been an increase in the number of reports which are confusing with regards to the effectiveness of UVc in killing or de-activating Cryptosporidium.  The protozoan can be killed by a very low level of UV irradiation, 1 or 2  mWs/cm² is sufficient to give a high kill rate of 2 to 3 log¹⁰ reduction.  However the stage that is of concern is the cryptosporidium oocysts which is very resistant to chlorine, ozone and UVc.  According to the available literature in order to give a 100 fold kill rate you require at least 200 mWs/cm² of UV irradiation, and any value under 100 mWs/cm² does not have any impact on the oocysts whatsoever. In order to be effective the size of UVc units would need to be 3 to 10 times larger in swimming pools and 100% of the flow would need to be treated..  However some reports are now saying that the Crypto oocysts can be rendered non-infectious by low levels of UVc, and that unlike bacteria,  they do not recover from the exposure. The real situation probably lies some where between the two view points, in which case it is likely that UV will give a reduction of Crypto but that it will not completely eliminate the problem.

Summary

AFM (Active Filter Media) filtration in sand filters operating below 20 cubm/hr/sqm of filter bed surface area, with good flocculation and NoPhos will remove particles down to 1 micron including Crypto oocysts. There have been a number of incidents in which there have been accidents in public pools using AFM, however in all cases oocysts could not be found in the filtered water. If UVc was used with sand filters and there was an out break, infection would result from horizontal transmission and environmental health would detect Crypto in the water,  but there would be no way of knowing if it was non infectious, so it would have to be considered infectious.

Crypto is a problem that will always be with us in the swimming pool industry however with good hydraulics and and appropriate filtration system, the problem can be managed.  The question is,  is UVc irradiation the way forward ?,  in our opinion UV could reduce the crypto levels but the down sides in terms of running costs, chemicals costs and the health of the pool staff and public must also be considered. Certainly UVc should never be used in a swimming pool unless that water is treated by activated carbon after the UVc unit.  However if you use activated carbon, the carbon will absorb the free chlorine,  there will then be no chlorine residual in the base of the carbon filter  which will then act as an incubator and start to generate huge levels of bacteria which will be discharged into the pool water.

Our conclusion is that there is No Logical Reason  of benefit from using UV irradiation of swimming pool water,  indeed it could be potentially harmful and will increase running costs.

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Applied and Environmental Microbiology, November 2002, p. 5387-5393, Vol. 68, No. 11
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.11.5387-5393.2002
Copyright © 2002, . All Rights Reserved.
 

Efficacy of UV Irradiation in Inactivating Cryptosporidium parvum Oocysts

Shigemitsu Morita,¹ Atsushi Namikoshi,¹ Tsuyoshi Hirata,^1* Kumiko Oguma,² Hiroyuki Katayama,² Shinichiro Ohgaki,² Nobuyuki Motoyama,³ and Masahiro Fujiwara⁴

School of Environmental Health, Azabu University, Sagamihara, Kanagawa,¹ Department of Urban Engineering, The University of Tokyo, Bunkyo-ku,² Environmental Systems Engineering Department, Fuji Electric Co., Ltd., Shinagawa-ku,³ Japan Water Research Center, Minato-ku, Tokyo, Japan⁴

Received 16 April 2002/ Accepted 14 August 2002

To evaluate the effectiveness of UV irradiation in inactivating Cryptosporidium parvum oocysts, the animal infectivities and excystation abilities of oocysts that had been exposed to various UV doses were determined. Infectivity decreased exponentially as the UV dose increased, and the required dose for a 2-log₁₀ reduction in infectivity (99% inactivation) was approximately 1.0 mWs/cm² at 20°C. However, C. parvum oocysts exhibited high resistance to UV irradiation, requiring an extremely high dose of 230 mWs/cm² for a 2-log₁₀ reduction in excystation, which was used to assess viability. Moreover, the excystation ability exhibited only slight decreases at UV doses below 100 mWs/cm². Thus, UV treatment resulted in oocysts that were able to excyst but not infect. The effects of temperature and UV intensity on the UV dose requirement were also studied. The results showed that for every 10°C reduction in water temperature, the increase in the UV irradiation dose required for a 2-log₁₀ reduction in infectivity was only 7%, and for every 10-fold increase in intensity, the dose increase was only 8%. In addition, the potential of oocysts to recover infectivity and to repair UV-induced injury (pyrimidine dimers) in DNA by photoreactivation and dark repair was investigated. There was no recovery in infectivity following treatment by fluorescent-light irradiation or storage in darkness. In contrast, UV-induced pyrimidine dimers in the DNA were apparently repaired by both photoreactivation and dark repair, as determined by endonuclease-sensitive site assay. However, the recovery rate was different in each process. Given these results, the effects of UV irradiation on C. parvum oocysts as determined by animal infectivity can conclusively be considered irreversible.

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