Ozonation
Ozonation in Water Treatment

by Mr. Brian Oram, PG

Ozone was first used in water treatment in the late 1800’s.

It is more widely used in Europe and Asia than the United States.

Ozone is an unstable gas comprising of three oxygen atoms. The gas will readily degrade back to oxygen, and during this transition a free oxygen atom, or free radical form is released.

The free oxygen radical is highly reactive and short lived. Under normal conditions it will only survive for milliseconds.

Ozone is a colorless gas that has an odor similar to the smell of the air after a major thunderstorm.

Ozone has a greater disinfection effectiveness against bacteria and viruses compared to chlorination.

In addition, the oxidizing properties of ozone can also reduce the concentration of iron, manganese, sulfur.

It can also reduce or eliminate taste and odor problems.

Ozone oxides the iron, manganese, and sulfur in the water to form insoluble metal oxides or elemental sulfur. These insoluble particles are then removed by post-filtration.

Organic particles and chemicals will be eliminated through either coagulation or chemical oxidation.

Ozone is unstable, and will degrade over a time frame ranging from a few seconds to 30 minutes.

The rate of degradation is a function of water chemistry, pH and water temperature.

Ozonation Process

The formation of oxygen into ozone occurs with the use of energy.

This process is carried out by an electric discharge field as in the CD-type ozone generators (corona discharge simulation of the lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultraviolet rays from the sun).

In addition to these commercial methods, ozone may also be made through electrolytic and chemical reactions.

In general, an ozonation system includes passing dry, clean air through a high voltage electric discharge, i.e., corona discharge, which creates an ozone concentration of approximately 1% or 10,000 mg/L.

In treating small quantities of waste, the UV ozonation is the most common while large-scale systems use either corona discharge or other bulk ozone-producing methods.

The raw water is then passed through a venturi throat which creates a vacuum and pulls the ozone gas into the water or the air is then bubbled up through the water being treated.

Since the ozone will react with metals to create insoluble metal oxides, post filtration is required.

Primary Advantages to Ozone

1. Ozone is effective over a wide pH range and rapidly reacts with bacteria, viruses, and protozoans. It has stronger germicidal properties than chlorination, and has a very strong oxidizing power with a short reaction time.

2. The treatment process does not add chemicals to the water.

3. Ozone can eliminate a wide variety of inorganic, organic and microbiological problems and taste and odorproblems. The microbiological agents include bacteria, viruses, and protozoans (such as Giardia and Cryptosporidium).

Disadvantages to Ozone

1. There are higher equipment and operational costs and it may be more difficult to find professionals proficient in ozone treatment and system maintenance.

2. Ozonation provides no germicidal or disinfection residual to inhibit or prevent regrowth.

3. Ozonation by-products are still being evaluated and it is possible that some by-products may be carcinogenic. These may include brominated by-products, aldehydes, ketones, and carboxylic acids. This is one reason that the post-filtration system may include an activated carbon filter.

4. The system may require pretreatment for hardness reduction or the addition of polyphosphate to prevent the formation of carbonate scale.

5. Ozone is less soluble in water, compared to chlorine, and therefore, special mixing techniques are needed.

6. Potential fire hazards and toxicity issues associated with ozone generation.
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http://www.water-research.net/index.php/ozonation

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