Ozone vs. Chlorine for Water Disinfection

Across the U.S., municipal water treatment agencies are faced with evaluating ozone systems for water treatment to help safeguard against bacteria, viruses, and pathogens. According to the EPA, “Disinfection is considered to be the primary mechanism for the inactivation of pathogenic organisms to prevent the spread of waterborne diseases to downstream users and the environment.” (See Table 1 for some common micro-organisms found in domestic wastewater and the diseases associated with them.)

TABLE 1: Infectious Agents Potentially Present in Untreated Domestic Wastewater

Source: Adapted from Crites and Tchobanoglous, 1998

Both ozone and chlorine can be used to treat these organisms. So, the question when it comes to comparing ozone vs. chlorine for water disinfection is, “Which is better?” Chlorine has long been the default choice. It is a powerful oxidizer and is very effective at treating many pathogens. However, factors such as environmental issues, long-term cost, particulate removal effectiveness, and health issues have made ozone the preferred choice for many municipalities.

Ozonated Water Disinfects Better than Chlorine

Ozone, like chlorine, is an oxidizing agent effective at eliminating bacteria in water. It is recognized as among the strongest and fastest commercially available disinfectants and oxidants for water treatment. While chlorine does kill many microorganisms, it cannot treat all water-borne pathogens if used at EPA approved doses. On the contrary, ozone can while remaining well within EPA regulations. When ozone decomposes in water, the free radicals that are formed, hydroperoxyl (HO2) and hydroxyl (OH), have great oxidizing capacity and play an active role in the disinfection process. Bacteria are destroyed by protoplasmic oxidation, which results in cell wall disintegration (cell lysis).

The Ozonation Process

Ozone is created using an ozone generator. This exposes a stream of air to either UV light or to a high voltage electrical discharge, the method known as corona discharge. The corona discharge method is typically preferred and takes place by breaking apart the two oxygen atoms of an oxygen molecule (O₂), then charging one of those atoms to another oxygen molecule, thus creating ozone (O₃) molecules. Unfortunately, ozone cannot be stored or packaged because of its instability. Its effectiveness will depend on CT, which will be explained later.

In a water treatment application, the raw water passes through a venturi injector, which creates a vacuum and pulls the ozone gas into the water. An alternate method is bubble diffusion, where air bubbles up through the water being treated. Since the ozone will react with other contaminants or metals to create insoluble metal oxides, post filtration is sometimes required.

Ozone CT Explained

CT is the product of residual disinfectant concentration (C) in mg/l and the corresponding disinfectant contact time (T) in minutes. In other words, ozone CT is the dissolved ozone concentration multiplied by the contact time.

Some sanitizing treatments with ozone can be accomplished very quickly, but other treatments will require higher levels of ozone or longer contact time in the water. This contact time is required for the dissolved ozone to oxidize organic contaminants and disinfect the water. This CT value is assumed to be unitless. To assure a given level of disinfection is obtained, either the ozone concentration can be held constant while the time is varied or vice-versa. For example, a CT value the bottled water industry generally uses is 1.6. This means the dosage rate is 1.6 mg/l minutes. Ozonation can happen at 0.2 ppm for 8 minutes or 0.4 ppm for 4 minutes. Either approach yields a final CT of 1.6.

The CT value is roughly 3000x higher when using chlorine compared to ozone. For example, if a dissolved ozone level of 0.2 ppm for 1 minute (CT=0.2) is needed to inactivate a specific microorganism, you will need 200 ppm of chlorine for 3 minutes (CT=600).

How Bad Is Chlorine?

Chlorine is a powerful, cost-effective disinfecting agent for many applications. It has played a transformative role in bringing clean, safe drinking water to many areas throughout the world. But we are just beginning to assess the overall health impact of chlorine as a disinfectant in our water.

There is an abundance of well-founded concern about chlorine. Studies have shown that when chlorine is added to our water, it combines with other natural compounds to form Trihalomethanes (chlorination by-products), or THMs. These chlorine by-products trigger the production of free radicals in the body causing cell damage, and they are highly carcinogenic. “Although concentrations of these carcinogens (THMs) are low, it is precisely these low levels that cancer scientists believe are responsible for the majority of human cancers. Breast cancer, which now effects one in every eight women in North America, has recently been linked to the accumulation of chlorine compounds in the breast tissue". A study carried out in Hartford Connecticut found that, “women with breast cancer have 50% to 60% higher levels of organochlorines (chlorination by-products) in their breast tissue than women without breast cancer.”

This increased link to cancer-causing compounds is corroborated in another recent study. According to Johns Hopkins University, “Toxic and carcinogenic compounds are produced when phenols in drinking water mix with chlorine.”

However, it’s not just the threat of cancer that is problematic when evaluating chlorine. A study by the American Chemical Society showed that chlorine use in water and sewage treatment may be contributing to antibiotic resistance. Beyond direct human impact, there are potential environmental impacts to microorganisms that may pose little health threat to humans but play a vital role in various ecosystems.

When evaluating this data as a whole, it is clear that chlorine comes with meaningful risks. But what are some specific advantages and disadvantages of ozone.

Advantages of Choosing Ozone vs. Chlorine

1. Chloroforms in chlorine have been linked to cancer.
2. Unlike chlorine, ozone leaves no harmful by-products in the water or the environment. Ozone quickly reverts back to pure oxygen if unused.
3. Ozone is one of the strongest and fastest commercially available disinfectant and oxidant for water treatment, purifying water 3000 time faster than chlorine.
4. In approved doses, ozone can treat all water borne pathogens (bacteria, viruses, yeast, molds, cysts, mildew), while chlorine cannot kill protozoa. Additionally, E. coli and Giardia can build resistance to chlorine over time.
5. Ozone is generated on site and does not require storage or transportation.
6. Ozone aids in the removal of minerals such as iron, manganese, and sulfur.
7. Ozone leaves no unpleasant chemical taste or smell.
8. Ozone dissolved in water will not irritate skin, nose, or ears.
9. No rinse steps are needed with ozone which saves on cleaning time and water usage costs and provides more productive hours.
10. Ozone is less corrosive than chlorine in water, especially compared to salt chlorination.
11. Bonus #1: Ozone is effective over a wide pH range and disinfection qualities are not dependent on pH, nor does the addition of ozone affect the pH of water.

Disadvantages of Choosing Ozone vs. Chlorine

1. Initial Cost. Start-up equipment and operational costs are higher with ozone. However, ozone systems are designed for long-term disinfection. A typical ozone system will generally become more cost effective than chlorine in 12-24 months.
2. No Residuals. Ozonation provides no germicidal or disinfection residual to inhibit or prevent regrowth. However, chemical residue is not always positive.
3. Bromide.Ozone should never be used if there are traces of bromide ion in the water, otherwise potentially harmful bromate can be formed.
4. Solubility. Ozone is less soluble in water compared to chlorine. Therefore, special mixing techniques may be needed.
5. Equipment Specifications.According to the “Wastewater Technology Fact Sheet: Ozone Disinfection” provided by the EPA, ozone’s effectiveness in disinfecting water will depend on the concentration of ozone. A common mistake is using an incorrectly sized ozone generator. Consult with your Ozone Solutions expert to make sure you can get the concentration you need.

For long-term waterborne disinfection requirements, ozone is clearly the safer, more environmentally-friendly, lower-cost option. If you have any questions about how ozone can provide better results for your needs, please contact us today.

Applications: Dairy; Water Treatment/Drinking Water; Bottled Water

Sources: WaterTech, water-research.net, USEPA, Competitive data