1.6: Disinfection with ozone
Last updated
Last updated
The possible presence of microbial contaminants in drinking water poses a greater risk to public health than the possible presence of disinfection by-products (DBP). Therefore, disinfection should not be compromised in order to control DBP.
Where the concentrations of DBP consistently exceed associated health-based guideline values, the methods of water treatment, disinfection and distribution should be reviewed.
Ozone is generated on site by passing an electric discharge through clean dry air or oxygen. The resultant ozone is a very strong biocide and oxidising agent, and is effective in reducing colour, taste and odour, and oxidising iron and manganese.
The mechanism by which ozone inactivates microorganisms is not well understood. Ozone in aqueous solution may react with microorganisms either by direct reaction with molecular ozone or by indirect reaction with the radical species formed when ozone decomposes (Le Chevallier and Au 2004). Ozone is known to attack unsaturated bonds, forming aldehydes, ketones or carbonyl compounds (Langlais et al. 1991).
Free radicals formed by the decomposition of ozone are generally less effective for microbial inactivation than molecular ozone, because microbial cells contain a high concentration of bicarbonate ions that quench the free radical reaction, and many microbial cells also contain catalase, peroxidase, or superoxide dismutase to control free radicals produced by aerobic respiration. In addition, some bacteria contain carotenoid and flavonoid pigments that protect them from ozone. These factors can account for reports that heterotrophic bacteria may be less susceptible to ozone inactivation than Giardia (Wolfe et al. 1989).
Ozone can be used in medium to large treatment plants, although it has not been used in Australia to date for the primary disinfection of a sizeable drinking water supply. It reacts with natural organics to produce lower molecular weight compounds that are more biodegradable and promote the growth of bacteria in distribution systems, which may have significant consequences for many Australian distribution systems where elevated water temperatures create a predisposition for bacterial growth. To avoid fouling, a biological filtration step is advisable after ozonation of water containing a DOC concentration of >1 mg/L (von Gunten, 2003).
The production of lower molecular weight compounds has been used to advantage in biological filtration processes. Ozonation can break up high molecular weight organics before filtration through a bed of granular activated carbon. The resulting low molecular weight compounds increase the amount of assimilable organic carbon (AOC) that can be used by bacteria that grow on the carbon, thereby reducing organic concentrations in the water. Ozone has a long history of use for disinfection, and for the control of taste, odour and colour. Ozone is more expensive than chlorine and has low solubility in water.
Even though ozone systems are complex, using highly technical instruments, the process is highly automated and very reliable, requiring only a modest degree of operator skill and time to operate (USEPA 1999). Maintenance of ozone generators requires skilled technicians. If trained maintenance staff are not available at the plant, this work can be done by the equipment manufacturer.
Ozone is a toxic gas and the ozone production and application facilities should be designed to generate, apply, and control this gas, so as to protect plant personnel. Ambient ozone levels in plant facilities should be monitored continuously.
Table IS1.6.1 presents published C.t values for ozone that have been demonstrated as achieving a two and four log reduction in the target microorganism. These values are supplied for illustrative purposes only and are consistent with Table 5.6. For ozone C.t values that achieve a greater log reduction, the cited references should be consulted. The C.t value that is applied at a particular water treatment plant should be based on the microbial risk assessment for that particular water supply system.
Escherichia coli
0.02
0.04
LeChevallier and Au 2004
Enteric viruses
0.6
1.2
USEPA 1999
Giardia
0.5 - 0.6
1.0 - 1.2
Wickramamayake et al. 1984,
USEPA 1999
Cryptosporidium
32
64
USEPA 2010
Notes:
Water temperature is 5°C.
pH 7 for Giardia and within the range of pH 6-9 for the other organisms.
The values in the table are based on published values and should be viewed as the minimum values necessary to achieve effective disinfection.
The important conclusion to draw from Table IS1.6.1 is that ozone is more effective than chlorine, chloramines and chlorine dioxide for the inactivation of viruses and protozoa.
Ozone is highly sensitive to turbidity. Turbidity should be less than or equal to 1 NTU at the time of ozonation. The pH should be less than 9 for effective disinfection because ozone is unstable above pH 9 (note that at pH 8, half of the ozone is lost in less than 30 minutes).
Due to its low solubility in water and instability above pH 9, an ozone residual cannot be maintained in a distribution system, particularly as temperature increases.
Ozone is a powerful oxidant and can convert naturally-occurring bromide to bromine, and this can lead to the formation of brominated trihalomethanes (THMs), brominated acetic acids, bromopicrin, brominated acetonitriles, as well as the formation of bromate (USEPA 1999). However, the brominated THMs produced in ozonation usually occur in lower concentrations than chlorinated THMs produced by chlorination. The NHMRC health-based guideline value for bromate is 0.02 mg/L, and bromate formation can become a serious problem for waters containing bromide levels above 0.1 mg/L (von Gunten, 2003). Bromate formation can be reduced to a certain extent by ammonia addition and pH depression, but bromate is very difficult to remove once formed (von Gunten, 2003). An alternative method of disinfection should be used with high bromide waters.
Low molecular weight aldehydes, such as formaldehyde and acetaldehyde, have also been detected as by-products of ozonation.
Given that where ozonation is used as a primary disinfectant it will be a critical control point (CCP) important operational considerations to ensure the effectiveness of the process are:
establishing target criteria and critical limits for the ozonation process (section 3.4.2);
preparing and implementing operational procedures (section 3.4.1) and operational monitoring (section 3.4.2) for the process;
preparing corrective action procedures (section 3.4.3) in the event that there are excursions in the operational parameters; and
undertaking employee training (section 3.7.2) to ensure that the ozonation process operates to the established target criteria and critical limits.
The table below summarises the operational monitoring that should be undertaken for ozone, based on recommendations from the New Zealand Ministry of Health (NZ MoH 2008).
pH
Online monitoring
Turbidity
Online monitoring
Ozone concentration
Online monitoring
Residual Concentration⁽¹⁾
Online monitoring
⁽¹⁾ measured at a point representing the end of the contact period
Ozone has been used in NSW for removal of algal toxins and for taste and odour control by Orange Council for over ten years. Similarly, ozone has been used for such control by MidCoast Water, Rous Water and Tweed Council for over four years.
Langlais B, Reckhow DA, Brink DR. (1991). Ozone in Water Treatment, Applications and Engineering, Lewis Publishers, Inc., Chelsea, MI
LeChevallier MW and Au K-K. (2004). Water treatment and pathogen control. World Health Organization, Geneva.
New Zealand Ministry of Health (2008). Drinking-water Standards for New Zealand 2005 (Revised 2008).
United States Environmental Protection Agency (USEPA) (1999). Alternative disinfectants and oxidants guidance manual. Washington DC.
United States Environmental Protection Agency USEPA (2010). Long term 2 enhanced surface water treatment rule toolbox guidance manual. Washington DC.
von Gunten U. (2003). Ozonation of drinking water: Part II. Disinfection and by-product formation in presence of bromide, iodide or chlorine. Water Research 37: 1469-1487.
Wickramamayake GB, Rubin AJ, Sproul OJ. (1984). Inactivation of Naegleria and Giardia cysts in water by ozonation. Journal of the Water Pollution Control Federation, 56:983–988.
Wolfe RL, Stewart MH, Scott KN, McGuire MJ. (1989). Inactivation of Giardia muris and indicator organisms seeded in surface water supplies by peroxone and ozone. Environmental Science and Technology, 23:744–745.