By Diann Gleason
Water-borne diseases are estimated to cause 2.2 million deaths each year, according to the World Health Organization (WHO). These deaths are attributable to water-borne pathogens due to inadequate treatment in public water treatment and sanitation systems. In a paper titled Risk of waterborne illness via drinking water in the United States, (Reynolds, K. A., Mena, K. D., Gerba, C.P., University of Arizona, Mel and Enid Zuckerman College of Public Health) it is reported that, “The total estimated number of waterborne illnesses/yr in the US is therefore estimated to be 19.5 M/yr. Others have recently estimated waterborne illness rates of 12M cases/yr (Colford, et al. 2006) and 16 M cases/yr (Messner, et al. 2006), yet our estimate considers all health outcomes associated with exposure to pathogens in drinking water rather than only gastrointestinal illness.” The study goes on to state, “Drinking water outbreaks exemplify known breaches in municipal water treatment and distribution processes and the failure of regulatory requirements to ensure water that is free of human pathogens. Water purification technologies applied at the point of use can be effective for limiting the effects of source water contamination, treatment plant inadequacies, minor intrusions in the distribution system, or deliberate post-treatment acts (i.e., bioterrorism).” Of course, treatment of private drinking water supplies such as wells and springs have the most to gain from testing and treatment to ensure a safe supply.
There is no general solution or selection of treatment processes for any particular water supply, and it is difficult to standardize the solution for water from different sources. Treatability studies for each source of water need to be carried out to determine the most appropriate treatment. For immune-compromised populations, POU water treatment devices are recommended by the Centers for Disease Control and Prevention (CDC) and US EPA as one treatment option for reducing risks of Cryptosporidium and other types of infectious agents transmitted by drinking water. Other populations, including those experiencing normal life stages such as pregnancy, or those very young or very old, might also benefit from the utilization of additional water treatment options beyond the current multi-barrier approach of municipal water treatment. Ozone can be a part of one of those treatment options.
Water treatment and purification
While purification is treatment, treatment is not necessarily purification. Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment processes is to remove existing contaminants in the water, or reduce the concentration of such contaminants so the water becomes fit for its desired end-use. Water purification is the removal of contaminants to produce drinking water that is pure enough for the most critical of its intended uses, usually for human consumption. Substances that are removed during the process of drinking water treatment include suspended solids, bacteria, algae, viruses, fungi, minerals (such as iron, manganese and sulphur, listed as contaminants in US EPA’s Secondary Drinking Water Regulations), gases such as sulphur and carbon dioxide and other chemical pollutants such as fertilizers. Measures taken to ensure water quality relate to the treatment of the water and also to its conveyance and distribution after treatment. It is a common practice to add a residual disinfectant to the treated water in order to kill any microbacteriological contamination that might occur during distribution, and this method itself can actually produce more toxic substances, such as halogenated organics. Three processes involved in treating water for domestic purposes are physical, chemical, and biological. Physical processes include settling and filtration, and biological processes may include aeration or activated sludge. Ozone treatment is one of the chemical processes and is used for disinfection, sanitization and sterilization. By itself, ozone will not produce chlorinated organic byproducts. In most cases, when ozone is used for purification, no toxic substances are produced; ozone will revert back to oxygen. In some applications, however, when water containing bromide is purified using ozone, the carcinogen bromatecan be formed. This may not occur at a significant extent at low O3/TOC ratios; however when higher doses of ozone are used, oxidation can cause this reaction. Reducing bromate formation may be achieved by lowering pH and limiting ozone dosage.
About ozone
Ozone is one of the best disinfectants known to science and ozonation is one of the many methods now used for water treatment. Produced by nature, ozone has been around forever. While its discovery as a powerful disinfectant came in the late 1800s, it has been slow to gain understanding and acceptance as a viable method for water treatment. With increased concern for finding environmentally sound treatment solutions, ozone is getting a closer look as an energy saving alternative for purification and disinfection applications.
Occurring in nature, ozone (O3) is formed whenever lightning strikes, or whenever an electrical spark occurs in air. It is also the result of direct ultraviolet radiation from the sun reacting with our planet’s atmosphere. In a natural setting, UV rays in the upper atmosphere strike oxygen (O2) molecules in the air, splitting them into separate, individual oxygen atoms. Some of these individual atoms combine with other molecules of O2, creating O3. Ozone is often referred to as tri-atomic oxygen; it is an unstable pale-blue gas having a soluble half-life in water of about 20 minutes, under the right conditions. The O3 molecule is always trying to go back to its stable state of O2 and it is more than willing to give up the extra oxygen atom. This free oxygen atom in turn wants to combine with something else, basically any molecule that will accept it. When this bond breaks, the free oxygen atom is available to neutralize unwanted substances in a process called oxidation.
How it works
An accepting molecule may be a virus, bacteria, cyst, pollutant (such as organic molecules), or even dissolved metals. When the free oxygen molecule combines with these substances, they are inactivated, changed into another less harmful substance, or literally split apart. Because if its instability, ozone cannot be generated and stored for future use; it must be generated and used onsite immediately. We no longer have to wait for lightning to strike to have ozone available, however. What Mother Nature provides, science has duplicated. Originally available only through natural phenomena, ozone-generating equipment now supplies ozone gas on demand via UV radiation, corona discharge or cold plasma technologies; all have their appropriate applications.
Due to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. To protect workers potentially exposed to ozone, US Occupational Safety and Health Administration (OSHA) has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 Table Z-1), calculated as an eight-hour time weighted average. Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL.
Inside an ozone generator
The generation of ozone is a relatively simple process; as air or oxygen is drawn through an ozone reaction chamber, energy is applied, splitting oxygen molecules into oxygen atoms. Some of these O1 atoms quickly recombine with oxygen molecules (O2) to form ozone (O3).
Ultraviolet ozone generation. Vacuum-ultraviolet (VUV) ozone generators employ a light source that generates a narrow-band of UV light, a subset of that produced by the sun. The sun’s UV sustains the ozone layer in the stratosphere of Earth.
Ultraviolet spectrum
While standard UV ozone generators tend to be less expensive, they usually produce ozone with a concentration of about 0.5 percent or lower. Most often used for small applications, they are relatively simple but lack the output capacity required to maintain adequate concentrations of ozone for many disinfection purposes. This method requires the air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated, making it impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization, for example). UV generators or germicidal generators are often used, however, for low-flow, low-contaminant applications. UV lamp output degrades over time and eventually burns out, typically needing replacement every year.
VUV ozone generators, unlike corona discharge generators, do not produce nitrogen byproducts and also unlike corona discharge systems, VUV ozone generators work well in humid air environments. There normally is not a need for off-gas mechanisms, or for air driers and oxygen concentrators.
Corona discharge ozone generation. Corona discharge (CD) is the most popular type of ozone generator for residential and light industrial applications due to ozone output capabilities, level of ozone gas concentration (typically to about five percent) and low maintenance costs. CD cells do not degrade over time but care must be taken to prevent water and moisture from entering the cells, which will lead to arcing and cell burnout. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen. Corona discharge generators utilize a potent electrical field through which processed, moisture-free air or oxygen passes through a reaction chamber where the molecules are split and the ozone is formed. While variations of the hot spark and cold spark coronal discharge method of ozone production exist, these units usually work by means of a discharge tube or cell.” Corona discharge is usually preferred over UV for most applications due to its ability to provide higher ozone output and concentration for the removal of complex impurities. Typically very cost-effective, they do not require an oxygen source other than the ambient air; however, an air dryer or oxygen concentrator is recommended.
Cold plasma ozone generation. In the cold plasma method, pure oxygen gas is exposed to a plasma field created by dielectric barrier discharge. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone. Cold plasma generators utilize pure oxygen as the input source and produce a high ozone output. They produce far greater quantities of ozone per unit of time compared to ultraviolet or corona discharge ozone production with typical concentrations of five percent or more. Because cold plasma ozone generators are very expensive, they are found less frequently than either the UV or corona discharge types, but definitely have their place in applications where high ozone output is needed. Some cold plasma units also have the capability of producing short-lived allotropes of oxygen. These species are even more reactive than ordinary O3.
Types of water treated
Ozone water purifiers for POU/POE and small system applications need to address three types of waters that may contain undesirable contaminants: surface water, ground water and municipal water supplies. The first two types involve the same kinds of contaminants as are treated by all municipalities, and experience with ozone purification in these settings translates directly to installation of small scale systems and POU/POE systems.
Municipal water treatment processes actually produce some pollutants due to the use of chlorine (see above). These pollutants can be removed at the point of use by specific treatment methods. Ozone can help, but ozone alone is not effective in removing these substances. Two common water treatment tools, activated carbon and chlorine, deserve special mention.
Activated carbon filters are the principal and most effective way to remove impurities from water; however, used by themselves, they have serious disadvantages. Activated carbon will not inactivate virus, bacteria, or cysts, such as those that cause dysentery. Carbon filters must be serviced on a regular basis, or they can become saturated with bacteria and other contaminants. Depending on the severity of the application, this can be a costly and ongoing expense. If water has been purified by ozonation prior to passage through an activated carbon filter, however, the only remaining task of the filter is to remove non-toxic matter from the water. Ozone pretreatment can significantly extend carbon bed life and such filters will need to be serviced less frequently.
Ozone is rapidly replacing chlorine in many applications as a chemical reducing or chemical-free alternative. Chlorine produces DBPs such as chloroform and other trihalomethanes, and haloacetic acids, all of which are listed as cancer-causing chemicals under the Safe Drinking Water Act. Ozone has equally been realized as a major cost advantage over chlorine when used in conjunction with filtration, reducing or eliminating chemical dependency.
Home water purifiers
In today’s world of strict drinking water standards, it is challenging to find a disinfection solution that can meet these standards in a cost-efficient and safe manner. Ozone disinfection has the potential not only to meet but also to exceed the quality requirements for pure drinking water. Along with disinfection, the use of ozone for drinking water treatment can also have many other benefits such as taste and color removal, iron and manganese removal, and insecticide removal. Ozone’s high oxidation potential gives it excellent germicidal properties. Ozone has an oxidation potential 52 percent greater than chlorine and acts over 3,000 times faster.
Today, many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine. Since all the available ozone will quickly react, it does not remain in the water after treatment. Therefore, the Safe Drinking Water Act mandates that these systems introduce a small amount of chlorine to maintain a minimum of 0.2 ppm residual of free chlorine in the pipes, to protect water as it flows through the municipal system. Unfortunately, the chlorine thus introduced can combine with other materials in the water and produce additional toxic substances. All the treatment methods available for POE/POU water treatment, including filtration, ion exchange, activated carbon adsorption, and RO can be used effectively with ozone as either a pre- or post-treatment to solve specific water treatment problems.
Some benefits of ozone water purification include:
- Disinfection: bacterial disinfection (disinfection by ozone is a direct result of bacterial cell wall disintegration, also known as lysis), viral and cyst inactivation, biofouling control
- Oxidation of inorganics: iron, manganese, organically bound heavy metals, cyanides, sulfides, arsenic
- Oxidation of organics: color, taste and odors, detergents (some), pesticides (some), phenols, algae control, turbidity control, microflocculation (of soluble organics), pretreatment of organics for biological oxidation, trihalomethane (THM) precursor control
Conclusion
As the properties of ozone, its application and results continue to be better understood, use of this powerful disinfectant will continue to develop. With the need for safe drinking water and demand for treated water for industrial use, more attention will be paid to ozone application for disinfection and DBP control. Ozone should be considered for its value as a primary disinfectant and as part of multi-stage treatment processes.
About the author
Diann Gleason is the Sales and Marketing Manager of Ozotech, Inc. Her expertise encompasses working in manufacturing companies involved with specialized process technologies, which includes chemical treatment, ion beam technology and ozone generation. She has a Bachelor’s Degree in business from Brooklyn College and a Bachelor of Fine Arts from the Academy of Art College. Gleason can be reached at (800) 795-9671 or via email, [email protected]
About the company
For 25 years, Ozotech, Inc. has manufactured ozone-generating equipment for non-chemical water purification, wastewater treatment and other applications requiring the oxidation power of ozone. The Northern California based company sells its products domestically and internationally to water treatment specialists and engineering companies, and includes ‘private label’ products to OEM water treatment product manufacturers and marketers. Ozotech’s equipment is based on their patented cold spark corona discharge technology. Every CD cell is hand blown on-site in the company’s own glass-blowing facility. The company’s success is based on constant improvement of technology and manufacturing processes, and dedication to quality.