Disinfection is crucial at many points of contact in our lives. The water in our taps and bottles, the fresh meat and produce we buy, the meals we serve our families and an overwhelming amount of the things we rely on must have proper disinfection in order to prevent the spread of infectious diseases and contaminants from one environment to another.
Recent infectious disease outbreaks ranging from E. coli in spinach and lettuce to norovirus on cruise ships and in medical facilities are regrettable and, with the proper consideration for disinfection infrastructure and application, also preventable. Unfortunately, there is no single solution for all these problems, as there are innumerable biological threats, and each treatment site is unique in application. However, we know certain things about what is needed from the disinfectants we choose.
Disinfection
Obviously, the primary feature of any disinfectant is antimicrobial effectiveness. It doesn’t matter how benign the byproducts or cheap the application—a disinfectant has to work well every time, even in the worst-case scenario. Ozone is a very strong oxidant and virucide, formed when oxygen is disassociated by an energy source (normally electricity or UV radiation) and the oxygen atoms recombine to form ozone, an unstable gas. Because of ozone’s chemical structure, it has a very high oxidizing potential—the ability to contribute electrons in a chemical reaction (see Table 1).
The mechanisms of ozone disinfection include:
- Direct oxidation/destruction of the cell wall with leakage of cellular constituents outside the cell.
- Reactions with radical byproducts of ozone decomposition.
- Damage to the constituents of the nucleic acids (purines and pyrimidines).
- Breakage of carbon-nitrogen bonds leading to depolymerization.
With ozone’s powerful oxidation process, the product of Concentration x Time (CT value) will allow for the physical rupture, or lysis, of a bacterium’s cell wall. This not only kills the bacteria, but also exposes the cellular components to further oxidation, where ozone can actually denature the proteins of bacterial DNA, leaving it wholly incapable of reproducing.
More specifically, Gram-negative bacteria like E. coli have lipoproteins and lipopolysaccharides in their outer cell membranes. Ozone is a highly reactive oxidant with these cellular constituents, allowing ozone to penetrate the phospholipid bilayer and destroy the bacteria.
Some bacteria cells have spore protection, which allows them to resist adverse conditions; however, ozone will continue to oxidize even these cell membrane constituents and eventually destroy the cell structure. Consequently, ozone has been shown to be highly effective against bacteria like E. coli (including O157:H7), Salmonella, Giardia and Cryptosporidium.
Viruses are even simpler than bacteria, often with a single layer of protein (capsid) protecting the nucleic acid, the transfer and reproduction of which is the sole purpose of a virus. It has been shown that ozone oxidation of viral RNA is the key in the deactivation of polio virus type I, which means that ozone is effective in both overwhelming the protein capsid layer and destroying the nucleic acid that is the critical focus of a virus’ existence.
Benefits of Ozone
One benefit of ozone is its synergistic reactions with other disinfectant technologies, such as UV and hydrogen peroxide. Ozone decomposition in water often creates free radicals: hydroperoxyl and hydroxyl. Because ozone in combination with UV or hydrogen peroxide will create the hydroxyl radical, the oxidizing potential of that tandem is extremely high. Hydroxyl is so reactive as to be all but undetectable—it reacts and is reduced almost immediately. Both ozone and UV are on the U.S. Environmental Protection Agency’s list of Best Available Technologies for meeting the Final Long Term 1 Enhanced Surface Water Treatment Rule.
The half-life of ozone in water is about 20 minutes at 20°C, as it decomposes naturally back to diatomic molecular oxygen. So the chance of ozone continuing downstream and having any effect on people is virtually nil. Similarly, ozone’s reactivity requires it to be produced on site. This eliminates any concern for the handling, shipping or storage associated with chemical treatments.
Application Design & Development
Many water treatment professionals across a variety of industries are successfully applying ozone as a primary disinfectant, especially in conjunction with complementary disinfectants, and leaving chlorine to maintain a residual disinfection. One of the biggest benefits of ozone as a primary disinfectant is the dramatic reduction in disinfection byproducts such as trihalomethanes and haloacetic acids.
As with any water treatment application, the efficacy of an ozone disinfection system is only as strong as the system design that executes your treatment. Proper ozone systems require high quality feed gas, whether it is dry air or oxygen, in order to fuel the ozone generator with sufficient input to maximize production of your disinfectant.
Ozone has become a more economical choice for water treatment professionals because of several developing trends. The evolution of improved preliminary treatment steps such as filtration and membrane bioreactors have resulted in effluents and water flows with less ozone demand, requiring less ozone dosage (and therefore generator sizing). Improvements in ozone systems, including air-cooled generators and maximized mass transfer, have also lowered the cost of achieving a given disinfection goal.
Sequentially, the next crucial step in ozone system design is the contacting, the transfer of ozone gas into aqueous solution for delivery to the disinfection site. Gas injectors and bubble diffusers, as well as tall contact tanks, are key to both maximum retention of ozone gas and prolonged contact time for disinfection. The more ozone gas that is captured for disinfection, the less gas has to be decomposed by ozone destructs, an important safety feature for any ozone system. Finally, ozone delivery and consumption must be monitored and controlled by reliable and integrated instruments to ensure full oxidation use by the target water stream.
The field of disinfection will continue to be a critical area for many industries around the world as they encounter unforeseen dangers of microbial infection. With ozone, solutions are available. With good ozone system design, those solutions are practical and beneficial.
