HEALTH AND SANITATION
29
Ozone treatment : water quality and operational conditions for legionella
Ozone is used in drinking water treatment for disinfection and oxidation . It is generated on site as a gas using either air or liquid oxygen and is then transferred ( dissolved ) into the water phase .
By
Environmental Protection Agency , document EPA 810-R-16-001
When dissolved in water , molecular ozone ( O 3
) is unstable and decomposes to hydroxyl radical , which is a stronger and typically more reactive oxidising agent than molecular ozone . Ozone decomposes quickly during water treatment . Therefore , during a typical ozonation process , both molecular ozone and the hydroxyl radica may contribute to the oxidation of contaminants of concern .
POTENTIAL WATER QUALITY ISSUES Ozone decomposes in water relatively rapidly . The half-life of ozone in finished drinking water depends on temperature , pH , and alkalinity , and can vary from minutes to hours . This timescale is short relative to chlorine-based disinfectants , and as such , ozone is not generally considered to produce a disinfectant residual .
Therefore , water treated with ozone may , in some cases , be susceptible to contamination at downstream points . For this reason , more than one type of treatment or control measure may be necessary to protect the treated water .
Disinfection by-products formed from ozone disinfection include bromoform , monobromoacetic acid , di-bromoacetic acid , di-bromoacetone , cyanogen bromide , chlorate , iodate , bromate , hydrogen peroxide , hypobromous acid , epoxides , ozonates , aldehydes , ketoacids , ketones , and carboxylic acids .
Ozonation of water containing inorganic bromide can produce bromate , a regulated DBP with an MCL of 10 µ g / L . The disinfection process will likely have transformed any bromide in water to organically bound bromine or inorganic bromamines . In either case , these forms of bromine are less likely to contribute to bromate formation via an ozonation process in a premise plumbing system . As such , bromate formation may not be as relevant as in the water treatment plant .
Other ozonation by-products such as aldehydes and organic acids are more readily biodegradable and may contribute to assimilable organic carbon ( AOC ) and hence , biological growth in the distribution system . In addition , these ozonation by-products are more likely to form some types of DBPs upon chlorination or chloramination . However , these general concepts regarding ozonation pertain to treatment of water at the plant . Ozonation of water that has already undergone treatment , including exposure to a chlorine or chloramine residual in the distribution system en route to the building ( e . g . hospital ) has not been studied to a great extent . Therefore , impacts of ozonation on AOC or DBP formation in a premise plumbing system are still unclear .
Corrosion marks on mild and galvanised steel coupons installed in pipe loops for ozone treatment that were similar to corrosion effects caused by other disinfectants ( chlorine , chloramine , chlorine dioxide and CSI ) have been observed , except that the coupons exposed to CSI also had copper deposits .
OPERATIONAL CONDITIONS As water temperature increases , ozone disinfection efficiency increases . However , because ozone decomposes quickly in hot water , it is difficult to maintain an effective concentration throughout the system to control legionella .
Therefore , there is a need to balance the tradeoffs between potentially higher inactivation rates and lower available CT ( i . e . disinfectant residual concentration “ C ” multiplied by contact time
“ T ”) with increased water temperature . Due to the faster decomposition of ozone in warm water , water leaving the ozone contactor with a concentration of 1mg / L to 2mg / L may not have a concentration high enough to inactivate legionella when it reaches distal parts of the system .
In the range of 6 to 9 , pH will not impact the efficacy of ozone disinfection . However , ozone decomposes faster at higher pH , and as such , there is a lower available CT for a given ozone dose . Carbonate alkalinity also has a considerable impact on ozone decomposition , with increasing alkalinity slowing down ozone decay , and thus increasing the available CT for a given ozone dose .
One important aspect of ozone-based treatment in a building is the potential for ozone residual that reaches the tap to degas from the water and expose building occupants to ozone gas . It has been noted , ozone-related odours from the treated water and within the building where ozone treatment was being conducted may be present , but the researchers did not measure airborne ozone concentrations .
Ozone is a toxic gas ( i . e . it is a principal component of smog ). It can corrode steel pipes and fittings , concrete , rubber gaskets and other materials ( USEPA , 2007 ). Due to safety concerns and the corrosiveness of ozone , on-site generation of ozone gas requires containment or a separate structure . Ambient air monitoring may also be required for compliance with local regulations .
Ozone disinfection is a relatively complex process . Operational and maintenance demands are significantly greater than those for chlorine and chloramines . PA
www . plumbingafrica . co . za March 2018 Volume 24 I Number 1