INDUSTRIAL APPLICATION OF OZONE

WHAT IS OZONE?

Ozone is active oxygen, a three-atom form of oxygen, and a normal trace element in the Earth's atmosphere.  Ozone is the strongest commercially available oxidizing agent.  Because gaseous ozone is highly reactive, it readily oxidizes organic matter and has a variety of uses as a bactericide.

Natural Ozone Generation

Nature produces ozone by solar radiation ionizing oxygen in the atmosphere at high altitudes, in the arctic and over snow covered terrain.  The air we breathe contains a small amount of ozone.  Ozone may also be produced by electrical discharges, air to air lightening and air to surface lightening, which purify the air.  Ozone is also formed when hydrocarbons and nitrogen oxides react with each other in the presence of sunlight.  Contributing to this method of ozone production are the following:  automobiles; industrial emissions from smokestacks, oil wells, gasoline stations and refineries; dry cleaning and chemical plants.  When these chemicals form ozone by photosynthesis, the result is often a major component of smog.  Thus, ozone can be both an oxidant and an irritant, depending upon its chemical makeup or its quantity and quality.

Artificial Generation

Ozone can be manufactured by ultraviolet and corona discharge generators.  Ultraviolet light systems utilize a special frequency of UV-light to dissociate the oxygen contained in air to form ozone.  These systems are relatively low cost and they do not require dry air for ozone production.  Corona discharge ozone generators consist of two electrodes separated by a dielectric, or non-conducting material.  A narrow discharge gap is provided where electrical current is passed, producing a blue glow, called a "corona".  When air is passed through this gap, it is partially ionized by the sparkling electrical current.   During this process, ozone is produced through the dissociation of the oxygen molecule.  The yield of ozone obtained from corona discharge generators is significantly greater than with UV-light systems, as long as dry air is utilized.   For this reason, air dryers are required with corona discharge systems.   Otherwise, the yield of ozone will be significantly diminished.  More importantly, nitric acid will be formed from the nitric oxide by-products which are produced when moist air is used with corona discharge systems.  This nitric acid can be destructive to the generator and it can damage equipment downstream from the corona discharge system.

Ozone and Water Treatment

As environmental concerns and the new discharge regulations shift the burden of compliance to the operators of industrial water treatment, industries are being forced to practice new methods of water treatment to comply with ever tightening discharge limits.  In many cases, ozone-oxidation will allow water that is being currently discharged to be recirculated and reused in the process.  That not only eliminates the environmental exposure and employee chemical exposure, but often results in a tremendous savings for the industry.  From decontaminating wastewater to providing high-quality water for product processing, ozone is being widely accepted for its diversity and functionality.

Ozone kills bacteria by oxidizing the organic molecules that form the cell surface.  As a result, the cell bursts (a process called lyzing) allowing its contents to leak out where they, too, are destroyed.  Ozone treatment effectively kills cells of all known organisms.  Similarly, viruses are also killed by oxidation when their protein covering is broken down and their contents exposed to ozone.  Chlorine works the same way, but is a much weaker oxidant than ozone.  As a result, effective concentrations of chlorine are much higher than those required for ozone, and the results are not always satisfactory.  For these reasons, chlorine treatment is often accompanied by additional treatment with toxic organic biocides to kill organisms that chlorine may miss.  Ozone is highly effective at killing all cells quickly, at low concentrations with no need for accompanying biocides.

Ozonation solves the problem of calcium build-up.  In chemically treated water systems, there are two main sources of calcium scaling:  one being the calcium dissolved in the water added to the system (a major part of the total dissolved solids, TDS), and the other from some of the biocides used to chemically treat the system.  These sources contribute tot he scale build-up on surfaces.  The calcium carbonate bonds with algae residue and forms deposits on the heat transfer surfaces and decreases efficiencies.  Ozonation accomplishes a scale free environment in two ways.  First, ozone oxidizes dissolved calcium, magnesium and similar scale-forming elements into insoluble mineral oxides.  When the system reaches chemical equilibrium, these oxides precipitate into microscopic crystals.  At this point, the total dissolved solids (TDS) can increase no further.  Through a process called microflocculation, ozone causes these suspended microcrystals to join together forming larger, heavier particles that settle to the sump where they are easily removed by filtration.  Second, through effective biological control, ozone eliminates the organic slime that encourages scale to bind to surfaces.  Existing scale is broken down by ozone when the organic matrix holding it together is destroyed by oxidation.

Another advantage of ozonation is the ability of ozone to passivate the corrosion process.  As a strong oxidant, ozone reacts with exposed metal surfaces creating a tough, inert layer of oxidized metal which prevents further corrosion.

Ozone is produced on site and requires no storage.  Ebbco ozone systems pass air through a generator which dissociates the oxygen molecules in the air and produces between 1% and 2% ozone.  The air/ozone mixture is then aspirated into the system by way of a venturi injector and allowed to mix throughout the systems.  Unreacted ozone is unstable and rapidly converts back into harmless oxygen, resulting in no fugitive ozone.  The process is environmentally safe and leaves behind no toxic residues or bi-products.

Recirculating Water Systems

Circulating water in a closed system forms an ecosystem in which contaminants in the water become highly concentrated.  As the water in the system is evaporated during the process, all contaminants are concentrated in the remaining water.  This requires that the water be treated chemically to prevent this high concentration of dissolved solids from combining with biological contamination to form scale.  Ozone treated water will precipitate out these scale producing contaminants and eliminate the biomass that acts as a cementing agent to form scale.   Chemicals added to the water in a conventionally treated system contribute to the build-up of suspended solids.  The chemical residue as well as the mass from the oxidation of biological contamination remain in suspension and are too small (in size) at this stage to be removed by filtration or settling.  As with other conventional water treatment systems, you end up using a chemical to treat a problem created by another chemical.  Ozone will not contribute to the suspended solids concentration, but in fact, acts as a microflocculant to bind solids together, bringing them to a point that they can be filtered out or will precipitate (settle).  The goal of ozone oxidation is to accomplish the treatment of the water without the addition of chemicals.  Any chemical added to the water will react with ozone which creates an added demand on the ozone and, in turn, raises the amount of ozone required to accomplish the treatment objectives.

In a cooling tower system the air is continually being washed by water it is cooling.  This provides an ideal breeding ground for bacterial contamination.  The legionella bacterium is an especially dangerous problem for the cooling tower operator.  This bacteria can multiply several times a second and can be transferred to the air system very easily.  Bacteria such as legionella can develop a resistance, or in some cases, even an immunity to chemical treatment, so the operator must continually rotate the chemicals to try to control this problem.

Ozone as a biocide is the most powerful sterilizing agent we are able to generate and use commercially.  Ozone will destroy any and all bacteria and viruses that it comes in contact with.  Due to the complete oxidation of the contaminant, there is no chance that the bacteria or virus will develop an immunity or a resistance.

Table 1:  Relative Oxidation Power of Various Oxidizing Species