Drinking Water Treatment – Ozone

EFFECTIVE AGAINST: Pathogenic (disease-causing) organisms including bacteria and viruses, phenols, some color, taste and odor problems, iron and manganese, and turbidity.

NOT EFFECTIVE AGAINST: Large cysts and some other large organisms, inorganic chemicals, and heavy metals.


How ozone treatment works

Ozone is an unstable form of pure oxygen. Like chlorine, ozone is a strong oxidizing agent and is used in much the same way – to kill disease-causing bacteria and viruses. Ozone may not kill large cysts and some other large organisms, so these should be eliminated by filtering or other procedures prior to ozone treatment. Ozone is also effective in eliminating or controlling color, taste, and odor problems. It oxidizes iron and manganese into solid particles that can be filtered out. Some research has shown that ozone is more effective than chlorine or ultra-violet disinfection for treating Giardia lamblia or Cryptosporidium parvum. Ozone may be recommended as a pretreatment to control biofouling of reverse osmosis membranes and ion exchange resin, as certain membranes and resins are degraded by chlorine.

Ozone treatment units are installed as a point-of-entry treatment system. At the point where the ozone mixes with the water, turbulence and bubbles are created; these ensure that the ozone contacts as much of the untreated water as possible. The greater the water flow rate, the greater the pressure differential and turbulence, and the more effective the treatment.

The effectiveness of ozonation depends in part on the contact time. In general, ozone requires a shorter contact time than chlorine. The contact time required varies with the water treatment. The solubility of ozone in the untreated water also affects treatment. In general, the lower the water temperature, the more ozone is dissolved in the water. Ozone has been found to be effective over a wide range of pH, but a pH slightly above 7 increases treatment efficiency. The ozone demand is related to the level of contamination in the water. When substances in the untreated water react with ozone, part of the ozone is used up, which may leave less ozone available to treat the targeted contaminants.

Ozone treatment can produce harmful by-products in drinking water. For example, if bromide is present in the raw water, ozone reacts with it to form bromate, shown to cause cancer in rats.

Capacity of ozone treatment units

The ozone dose required will vary according to water quality, but a typical ozone dose is 1.0 to 2.0 milligrams per liter, which is sufficient to kill most bacteria and control tastes and odors. The capacity of the storage tank determines how much water is available for use. The amount of storage required depends on household water use.

Maintenance of ozone systems

Most ozone systems do not require extensive maintenance. Some systems use an air-drying material, which needs to be replaced periodically. It is also necessary to periodically clean the water storage tank and check pumps, fans and valves for damage and wear. If UV radiation generates the ozone, the lamp must be replaced periodically. Any pre-treatment or post-treatment devices may require additional maintenance.

In addition, the entire system should be routinely inspected by a water treatment specialist for any ozone leaks. Some ozone systems have monitoring devices that alert the owner to a malfunction or failure by shutting down the entire system, sounding an alarm, or activating a warning light.

Always follow the manufacturer’s instructions for maintenance, cleaning, and part replacement. Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless maintained in accordance with the manufacturer’s recommendations. Keep a log book to record water test results, equipment maintenance, and repairs.

Other considerations for ozone treatment

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the raw and treated water annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water. Frequent testing will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

Like chlorine, ozone is a toxic gas and ozone generators may leak and could create an ozone hazard within your home. A leak could cause illness, although not much is known about the chronic health effects of ozone. Health effects resulting from exposure to 0.1 to 1 mg/l ozone include headache, dry throat, and irritation and burning of the eyes. The greatest drawback with ozone treatment is its lack of an ozone residual time. Ozone can corrode some pipes and fixtures, so all surfaces coming in contact with ozone should be made of ozone-resistant materials, such as stainless steel.

Most ozone systems require a storage tank for the treated water. With ozone treatment, disinfection occurs primarily at the point of contact between the ozone and the water. The disinfection process does not occur beyond the treatment unit. This contrasts with chlorination treatment where the residual chlorine remains in the water and continues the disinfection process for some time. Because ozone is so unstable, it does not produce a reliable residual. Ozone has an active residual time measured in minutes, whereas the active residual time for chlorine is measured in hours.

Bacteria and microorganisms can grow in the storage tank and in the water distribution system and recontaminate the water after treatment. Some systems store treated water in a contact chamber to assure that water is continuously treated until used.

Ozone treatment systems frequently have pre-treatment and post-treatment devices. Filtration may be required as pre-treatment to remove excessive turbidity or suspended solids that may hide microorganisms from disinfection or foul the ozone device. Filtration may be necessary as post-treatment to remove any oxidized particles. Activated carbon is a common post-treatment device for removing partially oxidized organic material.

Most people can smell ozone at a concentration of 0.01 milligrams per liter (well below the level for general comfort) and breathing traces of ozone for a few minutes is of little health concern. To prevent ozone from entering the home, any residual ozone in the vent gas should be vented to the outside of the home.

Ozone can corrode some pipes and fixtures, so all surfaces coming in contact with ozone should be made of ozone-resistant materials, such as stainless steel or Teflon. Also, combustible materials such as gasoline, oil, or grease should not be stored near the ozone system, as ozone may cause fire if it comes in contact with them.

Ozone equipment is one of the most expensive home water treatment technologies, and chlorination may still be desirable because of the low residual time for ozone. The water treatment industry is examining ways to make ozone more affordable. Check with water treatment dealers for information on the cost per gallon for treatment.

Questions to ask before you buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if ozone is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.

Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.