Ozone has quickly become the disinfectant of choice among water bottlers worldwide. Although ozone was 'discovered' in the mid-19th century, it is only during the last several decades that its numerous applications and benefits have become evident. This has occurred because of:
1) Continued research in ozone technology
2) The dramatic increase in environmental pollution
3) Recognition of byproducts and the biological hazards of historically used oxidants/disinfectants
4) The development of reliable commercial ozone generators
5) Increased health awareness.
Growing international concern for air and water quality has called for 'cleaner' chemicals to be used in treatment technologies, and ozone has quickly become a major ingredient. In the bottled water market, ozone disinfection is driven primarily by the increased health awareness of the consumer.
A Distinctive Smell
Ozone (O3) is a triatomic allotrope - that is, a different form - of oxygen (O2). It is a natural ingredient of the Earth's atmosphere, generated by solar energy and exists as a colorless gas at ambient conditions. Its molecular weight is 48 and has a weight density of 2.144 grams per liter (g/L) at 32°F (0oC).
Ozone gas has a distinctive smell that is most commonly noticed when standing near an operating copier machine. It is also the 'tangy' odor you smell after a lightning storm. Ozone can occur naturally wherever static electrical discharges occur.
In the upper portion of the Earth's atmosphere, the ozone layer shields life forms from the sun's harmful ultraviolet (UV) rays. In the lower atmosphere, however, increased concentrations of ozone are harmful for humans and animals. The U.S. Environmental Protection Agency (EPA) guideline for maximum human exposure is set at 0.1 parts per million (ppm) for a cumulative eight-hour period per day.
Producing Ozone Commercially
Because ozone is an extremely effective oxidant, it has a short-lived residual (converting quickly back to oxygen), and the byproducts of ozone reactions are ecologically benign. It is both reactive and unstable. Ozone rapidly decays back into molecular oxygen, with a half-life of about 20 minutes in air. As such, it is not practical to store ozone in containers.
Unlike other chemicals, there is no natural resource to tap for ozone. It is created using an ozone generator - referred to by many as an ozonator, although an ozonator is actually an ozone generator with accessory equipment and controls.
Ozone is formed by energetic excitation of molecular oxygen, causing some of it to disassociate into oxygen atoms, which can recombine with oxygen. The most practical way to do this in an ozone generator is via a method called 'silent-arc' or 'corona' or 'brush' discharge. The discharge occurs when charged electrons flow through a gas containing oxygen.
Two electrodes are separated by an air gap. A dielectric material (usually gas or ceramic) is inserted in the gap with sufficient voltage potential existing between the two electrodes to cause current to flow through the dielectric material and the gas.
Other methods to produce ozone exist, but they are not commercially viable. For example, electrolytic dissociation of hydrogen peroxide or water can create ozone. Chemonuclear and thermal methods can also be used, but they are not energy-efficient. Ultraviolet (UV) light at a wavelength of 185 nanometers (nm) produces ozone at a concentration of 0.1 percent by weight, which may be too low for effective application to water treatment.
Ozone for Bottled Water
Ozone is used in water treatment for its oxidative qualities. Second only to fluorine in electronegative oxidation potential, ozone will oxidize both organic and inorganic substances; remove unwanted taste, odor and color; and provide effective disinfection. Ozone is extremely efficient as a bactericide, fungicide and virucide, killing even chlorine-resistant Cryptosporidium. It is also used for oxidation and removal of heavy metals such as iron and manganese.
Another benefit is that ozone will not lead to the formation of trihalomethanes (THMs), which are formed when chlorine is added to raw water containing humic materials. Once a THM is formed, it is difficult to oxidize, even with ozone.
As with any unstable gas, ozone needs minimum pressures, temperatures and agitation to ensure that the ozone gas maintains its maximum life expectancy and work potential.
Ozone is normally produced at a positive pressure, zero to 15 pounds per square inch (psi), for contacting purposes. Contacting of the ozone gas into the water is the most crucial element in designing an efficient ozonation system. The objective is to get as much of the ozone gas as possible dissolved into the water. This can be accomplished in a variety of ways.
By industry standards, tall-tower diffusion is the most widely used contacting method. It introduces the ozone into the bottom of a column of water, or contact tank. The incoming (raw) water enters the top of the contact tank and leaves by the bottom. The ozone gas bubbles from the bottom of the tank upward in a counter-current flow arrangement.
The gas is distributed from the bottom of the tank with the aid of porous, ozone-resistant diffusion stones, similar to the bubble-rods found in home fish tanks. The rise velocity of the bubble should ideally fall within a range of 0.5 to 1 foot per second in order to achieve maximum ozone transfer to the water. Typical water height for this method should be between 10 and 15 feet.
Venturi injection uses an eductor - a device similar to ejectors for mixing two fluids. Eductors work on the Venturi Principle: Water is channeled through a short tube with a constriction in the middle which causes an increase in the velocity of the water flow and a corresponding decrease in the fluid pressure. The resulting suction draws ozone into the flow stream.
In-line static mixers have ozone gas at a higher pressure than the liquid flow introduced upstream of baffles in the pipe. The ozone enters through a diffusion rod and the turbulence of the water passing the baffles mixes the liquid and gas.
The International Bottled Water Association (IBWA) recommends that ozone be applied in the 1.0 to 2.0 milligram per liter (mg/L) range for a period of 4 to 10 minutes contact time to safely ensure disinfection. Maintaining a 0.1 to 0.4 ppm residual ozone level is recommended at the time of bottling. This provides an additional safety factor because the bottles can be disinfected, inactivating bacteria or viruses present after the bottles are washed and run down the conveyor.
Because ozone can be added at the point of water treatment and does not need to be removed at the point-of-use (POU), it can keep water sanitized throughout a facility. This reduces the potential for bacterial growth in unchlorinated water found within the distribution system. It also reduces the amount of Clean-in-Place (CIP) required to keep the operation disinfected. It can also be used during CIP operations to disinfect the equipment.
Ozone is even more effective when used in conjunction with other water treatment processes. By using reverse osmosis (RO) and nanofiltration (NF) or ultrafiltration (UF), organic precursors can be removed from waters before ozone is used. With this configuration, 99 percent of naturally occurring organic materials (such as lignin, humic and fulvic acids) can be removed, reducing the amount of ozone necessary to disinfect the water while providing superior product taste and clarity.
As more people turn to bottled water for a clean alternative to drinking tap water, bottled-water suppliers will have to look harder for sources of water free from external contamination. Ozone can help treat source water on its way to becoming a quality bottled water product.