Ozonation is a powerful environmental technology that employs ozone gas to disinfect and purify various substances. This technique relies on the potent chemical reactivity of ozone, a naturally occurring allotrope of oxygen. By forcing this highly reactive gas into contact with water or air, ozonation serves as an advanced method for oxidation and sterilization across numerous industries.
The Chemistry of Ozone
Ozone is a molecule composed of three oxygen atoms, designated as triatomic oxygen. Unlike the stable, diatomic oxygen we breathe, the extra oxygen atom makes ozone highly unstable and eager to react with other compounds. This instability is the source of its powerful oxidizing capacity, making it the third strongest known oxidant after fluorine and persulfate.
Because of its rapid decomposition, ozone cannot be stored or transported and must be generated on-site immediately before use. The primary method for industrial generation is corona discharge, which involves passing dry, oxygen-rich air through a high-voltage electrical field. This electrical energy splits oxygen molecules, allowing them to recombine into ozone. A secondary method uses ultraviolet light at a specific wavelength, often employed for smaller-scale applications.
Ozone’s inherent instability means it has a short half-life, ranging from minutes to hours depending on environmental factors like temperature and pH. This characteristic is advantageous because, after reacting with a contaminant, it quickly reverts back to harmless, breathable oxygen. The powerful oxidation process works by rupturing the cell walls of microorganisms and breaking down complex organic molecules into simpler, less harmful compounds.
Primary Applications of Ozonation
Ozonation is widely recognized for its application in municipal water treatment, used to disinfect drinking water and treat wastewater. Its superior germicidal properties make it highly effective against pathogens resistant to traditional disinfectants, such as the protozoan parasite Cryptosporidium. Ozone’s high reactivity allows for rapid inactivation, often requiring a significantly lower concentration and shorter contact time than chlorine.
The technology is also employed to improve water quality by addressing taste and odor issues. Ozone effectively oxidizes compounds like geosmin and methylisoborneol (MIB), which are responsible for earthy or musty tastes. It can also oxidize dissolved metals like iron and manganese, converting them into solid particles easily removed through subsequent filtration steps.
Beyond water treatment, ozonation plays a significant role in surface sanitation within the food and beverage industry. Ozonated water is used to wash produce, sterilize equipment, and preserve food products by deactivating bacteria and viruses on surfaces. It is also applied in air purification and deodorization systems to break down volatile organic compounds and neutralize strong odors in industrial settings.
The Ozonation Process in Practice
Implementing ozonation involves a precise, multi-step engineering process that begins with on-site ozone production. Once generated, the ozone gas must be efficiently transferred into the liquid stream, a process known as mass transfer. This is typically achieved using contact chambers where the gas is introduced via fine-bubble diffusers or venturi injectors, which use a pressure differential to rapidly dissolve the gas into the water.
The effectiveness of the treatment is governed by the concentration-time (CT) value, a metric that quantifies the total exposure of the water to the disinfectant. The CT value is calculated by multiplying the residual dissolved ozone concentration (C, measured in milligrams per liter) by the effective contact time (T, measured in minutes). Operators must carefully control the flow rate and ozone dose to achieve the target CT value necessary for microbial inactivation.
For instance, a facility may require a CT value of 1.6 mg-min/L for a specific pathogen, met either by a high ozone concentration for a short time or a lower concentration over a longer duration. Monitoring the dissolved ozone residual at the end of the contact chamber ensures the required exposure is maintained. This precise control over concentration and time is crucial for both effective disinfection and managing operational costs.
Safety and Residuals
A significant advantage of ozonation is that the ozone rapidly decomposes back into stable oxygen molecules once its work is complete, leaving behind no stable chemical residual requiring further neutralization. Because ozone is a powerful respiratory irritant, safety protocols require that any undissolved gas, or off-gas, be treated before venting into the atmosphere. This off-gas destruction is typically accomplished using a thermal or catalytic unit that converts the excess ozone back to oxygen.
While the primary residual is oxygen, the ozonation process can lead to the formation of regulated byproducts if the source water contains bromide ions. The oxidation of bromide by ozone or its decomposition products, such as the hydroxyl radical, can create bromate. Bromate is a potential human carcinogen, and regulatory agencies have established a strict maximum contaminant level of 10 micrograms per liter for drinking water.
To minimize bromate formation, operators manage factors such as the water’s pH and the ozone dose, often balancing disinfection requirements against byproduct generation. Maintaining a lower pH or reducing the ozone dose, for example, can inhibit bromate formation pathways. The management of bromate represents a primary operational consideration in implementing ozonation technology.