The Use of Ozone for Disinfection and Oxidation
With rising concerns about disinfection byproducts (DBPs) from chlorination, many drinking water utilities are considering ozone as an alternative primary disinfectant. Ozone is a powerful oxidant and disinfectant that can inactivate pathogens, degrade contaminants, and improve water quality without forming regulated DBPs. The use of ozone for disinfection and oxidation, its advantages and challenges, and key considerations for implementation.
Ozone Inactivation Mechanisms
Unlike chlorine, which works through oxidation, ozone inactivates microorganisms through direct oxidative damage to cells. The high reactivity of ozone disrupts vital cellular components like proteins, lipids, and nucleic acids. This nonspecific, multi targeted action makes it difficult for pathogens to develop resistance to ozone. Studies show ozone can achieve a 3-4 log inactivation of Giardia and Cryptosporidium and a 2-6 log inactivation of viruses at typical dosages.
Ozone Oxidation Capabilities
In addition to disinfection, ozone is a powerful oxidant of inorganic and organic compounds like iron, manganese, taste and odour-causing compounds, and trace organic chemicals. Ozone readily degrades pesticides, pharmaceuticals, endocrine disruptors, and other emerging water contaminants through oxidation. The presence of a hydroxyl radical makes ozone 1.5 times stronger as an oxidant than chlorine.
Advantages Over Chlorine
Ozone offers several advantages over chlorine:
1- No formation of trihalomethanes, haloacetic acids, and other regulated DBPs
2- More effective viral and protozoan inactivation
3- Improved removal of taste, odor, color compounds
4- Enhanced oxidation of metals, pesticides, and trace organics
5- Reduced formation of brominated and chlorinated DBPs
6- However, ozone also has unique challenges.
Limitations and Challenges
Implementing ozone requires significant capital investment and skilled operation. Key limitations include:
1- High equipment and energy costs
2- Requires complex generation and contact systems
3- Short half-life requiring on-site generation
4- Produces biodegradable organic DBPs like aldehydes, ketones, and carboxylic acids
5- Increases assimilable organic carbon and biodegradable organic matter
6- Possible formation of bromate at high doses
Systems Requirements
Incorporating ozone into treatment requires:
Ozone generators using air or oxygen feed gas
Ozone contractors like bubble diffusers or sidestream injectors
Off-gas destruction of residual ozone
Monitoring and control systems
Because ozone decomposes quickly, contactors must provide sufficient time for disinfection and oxidation reactions to occur.
Ozone Production Methods
Two main methods exist for producing ozone:
Ultraviolet Light: Exposing air or oxygen to UV light at wavelengths below 240 nm splits oxygen molecules into single oxygen atoms, which then recombine into ozone.
Corona Discharge: Drying and filtering feed gas before passing it through a high-voltage corona discharge produces an oxygen plasma that generates ozone.
Corona discharge is more energy efficient and produces higher ozone concentrations. However, UV systems avoid potential nitrogen oxide byproducts.
Application Points
Ozone systems can be designed for pre-treatment oxidation, intermediate disinfection, or final disinfection prior to distribution. Pre-treatment can enhance coagulation, reduce DBP formation potential, and improve water disinfection. Later addition provides a residual disinfecting capacity but requires a separate primary disinfectant.
Integration With Other Processes
Ozone works well with other treatment processes like granular activated carbon filtration for contaminant removal and biological filtration for ozone byproduct reduction. A dual media filter containing manganese oxide and GAC can help remove ozone byproducts. Ozone also enhances the formation of floc particles when paired with biofiltration processes.
Regulatory Requirements
The Stage 2 Disinfectants and Disinfection Byproducts Rule regulates bromate at an MCL of 10 ppb if ozonation is used. Bromide levels above 30 ppb can lead to bromate formation. The rule also sets limits on haloacetic acids and trihalomethanes that ozone can help utilities meet.
Conclusion
As concerns over chlorination DBPs grow, drinking water facilities are turning to ozone as an alternative primary disinfectant and powerful oxidant. With proper design and operation, ozone can inactivate pathogens and degrade a wide array of water contaminants without forming regulated chlorinated or brominated DBPs. However, ozone does produce its own suite of DBPs that must be controlled. Meeting the capital and operational demands of ozone requires careful planning, but the benefits for water quality merit consideration of ozone as part of a multi-barrier treatment strategy.
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