Ozone (O3) is an unstable gas composed of three oxygen atoms that possesses an extremely high oxidative potential. This inherent instability makes it a powerful and highly reactive agent used globally as a strong neutralizer of biological threats. Ozone’s capability to successfully destroy hardy waterborne parasites, which often survive traditional disinfection methods, positions it as a significant tool in water treatment.
How Ozone Attacks Pathogens
Ozone acts as a biocide by utilizing its strong oxidizing power. The O3 molecule readily seeks to shed its third, loosely bound oxygen atom, which creates an oxidative burst upon contact with a pathogen. This process is significantly more aggressive than that of many other disinfectants.
The released oxygen atom, often in the form of a free radical, initiates a chemical attack on the microorganism’s cell structure. Ozone targets and oxidizes the fatty acids, proteins, and enzymes that make up the cell wall or membrane. This oxidation damages the cell’s integrity, leading to a process called lysis, where the internal cellular components spill out, killing the organism almost instantly. The mechanism of physical destruction makes it virtually impossible for pathogens to develop resistance to ozone treatment.
Ozone’s Effectiveness Against Waterborne Parasites
Ozone is particularly valued in water treatment for its ability to inactivate protozoan parasites. Two of the most challenging waterborne parasites are Giardia lamblia and Cryptosporidium parvum, which cause giardiasis and cryptosporidiosis, respectively. These organisms protect themselves with thick, multi-layered outer shells known as cysts (Giardia) or oocysts (Cryptosporidium).
The protective shells of these parasites make them highly resistant to common disinfectants, such as chlorine. Ozone overcomes this barrier because its strong oxidizing power enables it to penetrate the protective cyst wall rapidly. Once the ozone passes through the shell, it quickly oxidizes the internal structures, rendering the parasite non-infectious.
Effective parasite inactivation depends on a combination of the ozone concentration and the contact time. Studies have shown that relatively low doses of ozone, between 0.3 to 1 milligram per liter, can achieve a high level of inactivation for Giardia cysts and Cryptosporidium oocysts in a short period, often within one minute. Ozone has been demonstrated to be the most efficient disinfectant for Giardia and Cryptosporidium inactivation when compared to other common disinfectants.
Comparing Ozone to Other Disinfectants
Ozone’s performance against waterborne parasites stands out when compared to other widely used disinfectants, primarily chlorine and ultraviolet (UV) light. Chlorine neutralizes most bacteria and viruses effectively but struggles significantly against the resistant cysts of Cryptosporidium and Giardia. Ozone’s rapid oxidation mechanism destroys the physical structure of these cysts, achieving inactivation in seconds or minutes where chlorine may take hours.
Ozone decomposes back into oxygen after the oxidation process, leaving no chemical residue in the water. Chlorine, on the other hand, reacts with natural organic matter to form potentially harmful disinfection byproducts, such as trihalomethanes.
UV light is also highly effective against these parasites, working by scrambling their DNA to prevent reproduction. UV light, however, does not provide any residual disinfection, meaning its effect is only present while the water is actively passing through the treatment unit. Ozone also does not leave a long-lasting residual, which is why it is often used in combination with UV or a low-level chlorine residual is maintained in the distribution system.
Safety and Practical Considerations
Using ozone in water treatment involves practical challenges and safety requirements due to its unstable nature. Ozone cannot be stored or shipped; it must be generated on-site at the treatment facility by passing oxygen or air through a high-voltage electrical discharge. This on-site generation adds complexity and cost to the treatment infrastructure.
The molecule’s inherent instability means it has a very short half-life and decays quickly. This is a benefit because it leaves no residue, but a disadvantage because it does not provide sustained protection in the water distribution pipes. Treatment plant operators must ensure the ozone is thoroughly mixed and that enough contact time is provided before the ozone decays or reacts with other compounds.
A major safety concern is that ozone is highly toxic and a potent respiratory irritant to humans and animals, even at low concentrations. Facilities using ozonation require stringent safety protocols, including specialized monitoring equipment and ventilation systems. These systems use ozone destruct units to convert any off-gassing ozone back into harmless oxygen before it is released into the air.