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