OZONE FOR CRYPTOSPORIDIA ELIMINATION - A REVIEW OF OZONATION
BY-PRODUCTS AND OZONE APPLICATION Final report to the Department of
Report No DWI0269
To review information on by-products resulting from the application of ozone for drinking water disinfection; to consider and identify the best point in the treatment process to apply ozone; and to review recent practice in the design and installation of ozone facilities.
Cryptosporidium has been recognised as an important microbial contaminant of water. The oocysts of the parasite are resistant to most disinfectants, including chlorine, but may be effectively inactivated using ozone. Ozone is little used in the UK and there is little experience in the design, construction and operation of ozonation facilities. If ozone were to be used as a disinfectant to control Cryptosporidia, knowledge of the disinfection by-products resulting from its use, the best point to apply ozone, and how to engineer ozone plants into existing treatment plants would be required.
Ozone reacts with a broad range of organic compounds in water to produce a variety of by-products. Most published information relates to laboratory tests which may not be directly relevant to drinking water ozonation. Increased concentrations of bromate, formaldehyde and other aldehydes, and organobromine compounds, including bromoform, have been reported in water as a result of the use of ozone, whereas concentrations of certain raw water contaminants and by-products of final chlorination may be reduced. There is little information on the potential health risk to the consumer from ozonation by-products in drinking water but it is likely to be small compared with the benefits.
Ozone should be applied as late as possible in the treatment process to avoid unwanted side reactions. With surface waters, biological filtration after ozonation will be required. Terminal disinfection with chlorine or chloramine will be required to protect the distribution system. Installing ozone in an existing plant may involve significant engineering complexities.
Further studies should be undertaken to identify and quantify ozonation by-products in drinking water using pilot or full scale plant. The significance to health of ozonation by-products should be evaluated. However, any health risks posed by ozonation by-products in drinking water are probably low, and, consequently, there is no reason why ozone should not be used to eliminate Cryptosporidia.
With good quality lie low content of organic matter and low bromide concentration) groundwaters, there should be no problems with unwanted side-effects and ozone can be applied ahead of terminal chlorination. With surface waters, where coagulation is used, ideally ozone should be provided after the existing filters, and an additional, biological, filtration stage should be installed following ozonation. If it is not possible to install additional filtration plant, ozone should be dosed after sedimentation but before sand filtration. Ozone should be applied before slow sand filters. Terminal chlorination or chloramination should always be provided.
V RESUME OF CONTENTS
The reactions of ozone with organic compounds in water are reviewed, and published information from laboratory studies of by-products are summarised. Ozonation by-products in drinking water are reviewed. The applications of ozone for drinking water treatment are considered with a view to optimising the placement of ozone to minimise unwanted side reactions. Recommendations on the best place to install ozone are made and the practical aspects, including costs, of ozonation facilities are considered.Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.