OZONE USE IN AGRICULTURE, THE FOOD INDUSTRY, AND RELATED SECTORS

1. Introduction

Ozone is a familiar substance in our modern industrial society. Stories about destruction of the ozone layer in the upper atmosphere are common, as are the periodic “ozone alerts” in large metropolitan areas throughout the world. Ozone occurs naturally in the atmosphere, but it is also produced synthetically, and is an important industrial chemical with broad applications, including agriculture and food processing.

Ozone (O3) is a molecule composed of three oxygen atoms while molecular (ordinary) oxygen (O2) has only two oxygen atoms. Ozone is an unstable gas that quickly decomposes into ordinary oxygen, especially in water. It is this tendency to naturally decompose that makes ozone an important chemical. As ozone decomposes into oxygen, its extra oxygen atom splits off from the ozone molecule. These free oxygen atoms have two important characteristics: they are toxic to bacteria, and they oxidize many chemical compounds, changing them into less toxic or harmful substances.

Ozone has several industrial uses. Although ozone technology has only recently been employed in the food industry, it has long been used in water treatment and other industrial areas.

Physical Characteristics of Ozone

Ozone has a characteristic pungent odor that is noticeable, for instance, after an electrical storm. At ordinary temperatures and at high concentrations, ozone is a bluish gas. At concentrations at which it is ordinarily generated for commercial purposes, however, this color is not noticeable unless the gas is viewed through a considerable depth.

At –112 degrees Celsius, ozone condenses to a dark blue liquid that explodes easily. Concentrated oxygen/ozone mixtures (above about 20 percent ozone) also explode easily, either in the liquid or vapor state. Under conditions in which ozone is generated commercially, however, concentrations of ozone in oxygen above 20 percent cannot be obtained easily, and no instances of ozone explosions have been reported during its long history of use.

Ozone gas is sparingly soluble in water, although it is about 13 times more soluble than oxygen at standard temperature and pressure. It readily decomposes back to oxygen, from which it is formed—rapidly in aqueous solutions containing xygen-attracting impurities, but more slowly in high purity water or in the gaseous phase. The rate of ozone decomposition in water is greatly affected by the purity of the water and by the cleanliness of the glassware or other equipment with which it comes in contact during use.

Advantages and Disadvantages of Ozone

Ozone is a useful chemical because of its ability to oxidize and sterilize. It does, however, have characteristics that make it inappropriate for some applications. Following is a list of advantages and disadvantages.

Advantages

  • Ozone is the strongest oxidant and disinfectant available commercially for the treatment of aqueous solutions and gaseous mixtures contaminated with oxidizable pollutants or microorganisms.
  • Although ozone is only partially soluble in water, it is sufficiently soluble and sufficiently stable such that its oxidation or disinfectant properties can be utilized to full advantage.
  • When ozone oxidizes or disinfects, or when ozone auto decomposes, the stable end product is oxygen. The reaction of ozone with organic molecules is called “ozonation”. Ozone reacts with a large variety of organic compounds, although at varying rates, resulting in oxygencontaining organic byproducts. Halogenated organic compounds (i.e., containing one of the halogenic elements: fluorine, chlorine, bromine, etc.) cannot be produced during ozonation, unless a bromide ion is present. The ability of ozone to produce “free bromine” by oxidation of bromide ion is an advantage of ozone in treatment of swimming pools and cooling towers.
  • Although ozone is the strongest oxidizing agent available, it is reasonable safe to handle. The primary reason for this is that it cannot be stored and, therefore, must be generated and used on-site.
  • Although reactions of ozone in its gaseous phase are significantly slower than in the aqueous phase, ozone in its gaseous phase is a proven deodorizer for a variety of odorous materials.
  • In treating potable water, some wastewaters, and landfill leachate, ozone has the proven ability to convert biorefractory organic materials to biodegradable materials. As a result, combining ozone oxidation with subsequent biological treatment can produce water or wastewater with lower concentrations of problematic organic compounds more costeffectively than either process used individually.

Disadvantages

  • Producing and using ozone requires a higher capital cost compared with other oxidation/disinfection techniques. This is because ozone must be generated on-site and cannot be bottled, shipped, or stored, thus eliminating the usual economies of scale encountered with centrally produced chemicals.
  • The most common and economical method to generate ozone in commercially significant quantities (corona discharge) is an electrically inefficient process: more than 75 percent of the electrical power sent to a corona discharge ozone generator is converted into waste heat and unusable light rather than the production of ozone. The major cost of producing ozone is for electrical energy. Even with this economic disadvantage, however, ozone can be and often is cost-effective compared with alternative treatment techniques.
  • Ozone equipment is complex and expensive.
  • Because ozone is the most powerful oxidizing agent available, it is also potentially the most dangerous, particularly with respect to potential damage to human lungs exposed to the gas. Fortunately, techniques have been developed to minimize the chance of such accidents.

Advantages and disadvantages are not always clear and often depend on the particular application. For example, if ozone is compared to gaseous chlorine for a water treatment application, ozone always loses if costs are the only consideration. This is because the cost of a pound of ozone is about three times the cost of a pound of chlorine (disregarding the fact that the pound of ozone has a greater oxidizing/disinfection capability than a pound of chlorine). In locations where transportation of gaseous chlorine to the site is inordinately expensive, however, ozone has an advantage because it can be generated onsite (assuming that electrical energy is available).

Advantages and disadvantages are not always clear and often depend on the particular application. For example, if ozone is compared to gaseous chlorine for a water treatment application, ozone always loses if costs are the only consideration. This is because the cost of a pound of ozone is about three times the cost of a pound of chlorine (disregarding the fact that the pound of ozone has a greater oxidizing/disinfection capability than a pound of chlorine). In locations where transportation of gaseous chlorine to the site is inordinately expensive, however, ozone has an advantage because it can be generated onsite(assuming that electrical energy is available).

2. Ozone in the Food, Beverage, and Agriculture Industries

Although ozone has long been recognized as a powerful disinfectant, it has had only limited application in industries relating to food and beverages. This situation is changing as these industries seek effective, safe and economical alternatives to traditional disinfection technologies, which are usually based on chlorine. Because ozone is much more effective then conventional disinfectants against such notorious contaminating organisms as Salmonella, Giardia, E.coli, and Cryptosporidium, the motivation of these industries is even greater.

Although ozone has long been recognized as a powerful disinfectant, it has had only limited application in industries relating to food and beverages. This situation is changing as these industries seek effective, safe and economical alternatives to traditional disinfection technologies, which are usually based on chlorine. Because ozone is much more effective then conventional disinfectants against such notorious contaminating organisms as Salmonella, Giardia, E.coli, and Cryptosporidium, the motivation of these industries is even greater.

Ozone in the Food Industry

Because ozone is a safe, powerful disinfectant, it can be used to control biological growth of unwanted organisms in products and equipment used in the food processing industries. Ozone is particularly suited to the food industry because of its ability to disinfect microorganisms without adding chemical by products to the food being treated, or to the food processing water or atmosphere in which foods are stored.

In its aqueous form, ozone can be used to disinfect equipment and process water. In gaseous form, ozone can act as a preservative for certain food products and can also sanitize food-packaging materials. Some products currently being preserved with ozone include eggs during cold storage, fresh fruits and vegetables, and fresh fish.

Until recently, the food processing industry limited its use of ozone mainly to the treatment of wash water and wastewater, because ozone was not approved by the FDA for food preservation. Recent changes in FDA regulations, however, have cleared away some major barriers to wider applications of ozone. In 1997, through the efforts of the Electric Power Research Institute (EPRI) in promulgating changes in the following FDA guidelines, ozone was conferred with the status of “Generally Recognized As Safe” (FRAS) as a sanitizer and disinfectant for foods. EP RI accomplished this by assembling a panel of experts on food, toxicology and ozone. After evaluating scientific and historic information on the use of ozone in food processing, the panel affirmed GRAS classification for ozone “as a sanitizer or disinfectant for foods when used at levels and by methods of application consistent with Good Manufacturing Practices”. The FDA does not have to reaffirm the GRAS classification, and food processors now are free to use ozone for sanitation or disinfection.

The GRAS classification for ozone was announced within a few months of the passage of a new Federal law, which, for the first time, limits the presence of E.coli and Salmonella on meat and poultry. The timing of the GRAS classification is propitious because ozone is particularly efficacious in neutralizing these infectious agents.

Food processors and beverage manufacturers consume billions of gallons of water daily for food handling, washing, processing, and cooking, and for cleaning equipment. All of this water must be free of contaminants. Even before ozone received GRAS status, the food and beverage industry had begun to recognize its potential as a disinfectant and as an alternative to chlorine, which traditionally has been used to treat food-processing water. This is because ozone eliminates a problem associated with chlorine disinfection—the potential for the build- up of toxic chlorine residues in water that has been treated more than once.

Ozone in Agriculture

The agriculture industry is less advanced in applying ozone for disinfection, although there are several promising opportunities. One with high potential is the use of ozone as a safe, economic and environmentally benign alternative to the conventional practice of fumigation with methyl bromide. Methyl bromide is being phased out as a fumigant and will be banned by the year 2001 because its toxicity poses a threat to human health. As a replacement for methyl bromide, there is interest in applying ozone for the control of insect infestation in stored food, grains and other agricultural products as a general soil fumigant/sterilant in drip irrigation systems. Ozone is also being investigated as a preprocessing wash water treatment for fruits and vegetables harvested in Mexico and moved to the United States. Although ozone shows much promise for these and other applications, additional R&D will be required before it will be ready for widespread use.

Ozone has found application in the emerging area of aquaculture. Ozonation systems are currently being used to reduce bacterial levels in freshwater fish growing facilities. These systems are effective and economical.

Ozone in the Beverage Industry

The bottled water and soft drink industries are moving toward the use of ozone for disinfection. The sanitizing properties of ozone are obtained nearly instantly, with no unwanted by products. Many European mineral water and soft drink bottling plants have adopted this technique. The FDA has recently approved the use of ozone for disinfection in bottled water treatment and as a sanitizer in bottled water production lines. There are several plants across the country using ozone to treat wash water and wastewater. Several of the larger soft drink firms are studying wider us of ozonation for bottle sterilization, and at least one American bottler has a bottle sterilization unit in operation.

Ozonation is also being considered for other applications within the beverage industry, which are discussed below:

Taste uniformity and improvement
Ozone offers a solution to a problem common among beverage manufacturers. Soft drinks are produced regionally throughout the USA, using local water (termed raw water), which has widely differing qualities. When a standard syrup formulation is added to these local raw waters, the resulting soft drinks may not taste the same from bottling plant to bottling plant. To address this problem and produce water of a similar standard quality, the beverage industry has adopted the practice of treating raw water with a single process.

Ozone potentially has a second taste-related application in another sector of the beverage industry. Fruit juice-based soft drinks introduced in recent years have a pH of approximately 6-7 when they are bottled. In this pH range, bacteria can thrive and proliferate if the process water is not adequately disinfected after GAC filtration. While chlorine is effective for disinfection, it imparts a disagreeable taste. Ozone could solve this problem. One concern, however, is that the residual ozone may be present when ozonated water contacts the organic materials contained in the fruit juices. If this occurs, oxidation products may be formed that also impart tastes to the final drink. This concern could be addressed by destroying the residual ozone with UV radiation.

Removal of chlorinated organic compounds
Ozone could eliminate a potential health problem caused by chlorination. Soft drink bottlers in the US are becoming increasingly aware of health threats posed by chlorinated organic by-products from chlorination disinfection that cannot be completely removed by the GAC filters. Consequently, the bottling industry is considering ozone as a potential replacement for the prechlorination step. Not only would ozonation eliminate the formation of chlorinated organic compounds, but GAC losses would be only a fraction of those currently experienced because preoxidation with chlorine would be replaced by preoxidation with ozone.

For current super chlorination treatment, chlorine dosages of 10-25 mg/L are applied to incoming waters. Since the oxidizing capability of ozone is higher than that of chlorine, and since some of the added chlorine is converted to chlorinated organic byproducts, it is reasonable to assume that the amount of ozone required would be less than the amount of chlorine (5 to 20 mg/L) currently added—perhaps 50-75 percent less.

There is an intriguing additional advantage to incorporating ozone to replace super chlorination—prolonging the useful life of the GAC columns, which today must be steam cleaned monthly, and replaced and supplemented at least annually. Based on industry experience with full- scale drinking water treatment technology, ozonation prior to GAC filtration extends the useful life of the initially charged GAC five to ten fold.

“Rinse and Hold”
Another application for ozone in the beverage industry is in the process of preparing waters to accept soft drink syrups. This application requires smaller amounts of ozone than conventional ozonation. First the water is “superozonized”, treated with lime and ferric chloride, and then filtered through sand and GAC. Next, a small dosage of ozone (0.5-2 mg/L) is added to assure the absence of microorganisms in the treated water during storage and hold time before it is sent to the bottling production line. This technique already is in use in pharmaceutical and high purity water installations (e.g. electronic chip manufacturing).

Ozone in the Brewing Industry

In breweries, ozone has several potential applications:

  1. Treatment of process waters (similar to soft drink bottlers)
  2. Assistance in wastewater treatment by lowering BOD (biochemical oxygen demand, i.e. bacterial levels prior to discharge)
  3. Air treatment in cellars and beer storage areas to control mold, yeast and spore growths, and for odor control
  4. Sterilization of bottles (bottle washing with ozone-containing water or spraying of water mists and ozone-containing gas simultaneously into the bottles.

In European breweries, the first, third and fourth applications are common, particularly air treatment and bottle washing. In these European facilities, process water is treated with ozone when raw water sources warrant. Many, if not most, German breweries reuse bottles directly, e.g. without re-melting. This means very thorough washing and sterilization, for which ozone is the selected procedure.

In addition to large national breweries, the growing number of microbreweries offers a potential market for small-scale ozonation equipment.

Waste water Treatment
For the brewing industry, ozo ne offers savings in sewer charges for wastewater treatment. Sewer charges are based to a large extent on the level of BOD concentration in the waste stream from the industrial user. If a brewery can lower its BOD level significantly by treatment with ozone at its plant, the savings in sewer charges could cancel out the costs of ozone treatment.

If a brewery has installed ozone generation capacity for the purposes of treating process water over a 16-hour period daily, it can use the generator to add ozone to its wastewaters during the additional 8- hour period for only the electrical costs associated with ozone generation. The amount of sewer charge savings from this arrangement is dependent mainly on the volume of wastewater at the plant, local costs of electrical energy, and local sewer charges. For brewery treatment, doses of ozone should be in the same range as for process water treatment, i.e. 5-15 mg/L. If higher ozone capacities are available for addition during the third 8-hour daily shift when process water is not treated, so much the better, since the more applied ozone results in greater destruction of BOD. No US brewery using ozone is known to be treating its wastewaters in this fashion.

Water treatment and bottle sterilization
Ozone is used for process water treatment at several breweries (Coors, Schmidt, Schlitz, Genesee, and Molson in Canada). The largest North American ozone installation for this application is at the Coors brewery in Golden, CO, for treating process water. One California winery is known to be using ozone for bottle sterilization, and is using a 1-3 lb/hour ozone generating system.

For bottle washing/sterilization, perhaps 5-10 lb/day of ozone capacity should suffice for an average sized brewery. This will provide dosages of ozone sufficient to ensure bacterial disinfection, when applied as a fine, fog- like spray.

3. Basic Components of an Ozonation System

A. Introduction

Ozone is produced synthetically by exposing oxygen molecules to high energy, usually an electrical discharge or ultraviolet radiation. An ozonation system comprises components to produce, control, and apply this useful but volatile gas. Because ozone is unstable and cannot be stored, it must be generated where it is to be used. By far, the most common type of ozonation system for industrial applications uses a corona discharge to generate ozone. The following system descriptions are oriented toward this type of system. CD is used as an abbreviation for corona discharge.

An ozonation system consists of the following six major components:

  1. Power supply—Provides and controls the energy to convert oxygen into ozone. The power source must be commensurate with the voltage, amperage, and wattage requirements of the ozone generation equipment, as well as with other components such as feed gas preparation, contacting, off-gas destruction, controls, instrumentation, and monitoring.
  2. Feed gas treatment—Supplies specially treated, oxygen-laden input gas to the ozone generator. The input gas can be air, high purity oxygen, or oxygenenriched air. The treatment process produces dry, particulate- free gas through which electrical energy is passed to generate ozone.
  3. Ozone generation—Uses electrical energy to produce ozone. The generator also produces heat and light as byproducts.
  4. Ozone contactor—Brings ozone from the generator into contact with the water or air to be treated. 5. Ozone off-gas destruction subsystem—Removes and destroys ozone not dissolved or used in the contacting apparatus. The destruction system converts remaining ozone to oxygen and benign compounds so that the resulting products may be safely released to the environment.
  5. Monitor and control subsystem—Ensures that the equipment is operating effectively and safely.

Each of these subsystems is described in more detail later in this discussion.


The five basic components of the ozonation system.

B. Relative sizes of Ozonation Systems Required for Food, Beverage, and Agriculture

The most common application of ozonation today is in municipal water treatment plants. Those systems are much larger, in terms of both equipment and ozone capacity, than would be found in most food, beverage and agricultural applications. This section describes the ozonation capacity required for these three industries.

The ozone generator is matched to a single power supply because ozone generators form a capacitance load on the power source, varying with the size, configuration, and type of dielectric. The power system must be designed to furnish the necessary voltage at the required frequency and must e adjustable to allow for output control. All systems must be safe, provide sufficient power for maximum ozone production needs, operate within a predetermined and specified power factor and efficiency range, and allow for any expected variations in load, input voltage or frequency.

When a constant low frequency of 60 Hz is used, the ozone production rate can be controlled readily by varying the voltage applied to the ozone generator. Ozone production requirements also may be met by the use of variable frequency controllers. The use of a frequency converter to increase the 60 Hz line frequency to levels up to 2,00 Hz at the primary side of the transformer is becoming more common because of the availability of solid-state electronic control systems. Ozone generation at 600 Hz (medium frequency) produces nearly double the amount of ozone from the same generator operated at 60 Hz, but at a higher power consumption (10-15 percent) per unit weight of ozone produced.

For medium frequency ozone generator, the quality of power supplied depends o several factors: converter switching speed, how the converter is connected to the transformer and the transformer to the ozone generator, current and voltage wave forms, and materials and construction techniques used to minimize electrical losses. A header rectifier allows the type of current selected by the manufacturer, particularly a single-phase supply, to be produced easily from three-phase mains.

For optimum operation of the power supply, the current and voltage must be in phase and the waveform must be as close to a sine wave as possible. Careful selection of the ozone generator capacitance can provide combined correction (power factor and harmonic) to optimize the complete power supply system.

Electric utilities usually require power factor correction on all ozone generating installations. Manufacturers of ozone generators generally include filters in their equipment to avoid interfering with the electrical supply. Power factor usually is corrected at the power supply unit; however, it also can be corrected at the source (ozone generator), or at both points.

C. Feed Gas Treatment

Corona discharge is the primary technique available to produce commercial quantities of ozone from small (grams per hour) to large (tons per day) applications. Corona discharge generators require oxygen to produce ozone. Oxygen can come either from oxygen in ambient air or from a high purity oxygen source. Ambient air has impurities such as particulates, moisture, and hydrocarbons that are detrimental to the ozone generation process and must be removed. Pure oxygen can be purchased from a supplier or generated at the point of use with sir separation equipment.

Factors which influence the feed gas system design include:

  • Quantity of ozone required
  • Type of feed gas used
  • Ozone system turndown required
  • Maintenance requirements
  • System automation
  • Expected system design life
  • Power costs
  • Ozone contactor design
  • Ozone concentration requirements
  • Operating cycle

Air-based feed gas system
The principal goals of an air feed gas preparation system are to

  • Remove feed gas impurities
  • Provide adequate gas flow and flow turndown capability
  • Provide adequate gas pressurization
  • Limit the gas temperature

An air feed gas system for large-scale applications involves compression, filtration, and drying of the ambient air. The equipment required to do this includes a compressor with an after cooler, filters that remove contaminants and particles, and a desiccant dryer, which removes moisture. Smaller systems (<25 lb/day) often consist solely of filters and desiccant dryers, with no need for compressors or refrigerant coolers.

Particulates must be removed because their presence in the gas flow can lead to shortcircuiting within the ozone generator. The feed gas must be dry (dew point of less than -65 degrees Celsius) for proper operation of the ozone generators and to avoid harmful nitric acid formation inside the generator. Incomplete drying will lead to excessive power usage and fuse or dielectric failure due to the interference caused by moisture in the ozone formation reaction.

Under-compression, resulting in low outlet pressures, can prevent the ozone gas from reaching its point of contact with the process fluid. High gas temperatures may cause the decomposition of ozone to oxygen before the ozone reaches its contact point with the process fluid.

Oxygen-based feed gas system
For some applications, high purity oxygen, 95 percent oxygen, or oxygen-enriched air are the more attractive feed gases than air. There are several reasons for this:

  • Higher ozone production densities
  • Lower specific energy consumption
  • Higher gas-phase ozone concentrations (3 percent to 16 percent by weight for the same applied electrical energy than when using dried air)
  • Approximately double the quantity of zone generated per unit time
  • Smaller gas volumes that must be handled, reducing costs for ancillary gas handling equipment
  • Higher ozone transfer efficiencies into water, due to the higher concentrations of ozone in oxygen
  • Elimination of air preparation equipment for once-through systems

Typically, oxygen is provided for the generation of ozone by one of two methods: (a) onsite generation, or (b) liquid oxygen (LOX) stored on-site but produced elsewhere.

On-site generation involves separation of oxygen from the other constituents in air through a cryogenic or molecular sieve absorption process. Cryogenic oxygen production will provide 90-99 percent pure oxygen and appears to be best suited for demands exceeding 4,000 lb per day of ozone.

Absorption technologies such as Pressure Swing Absorption (PSA) or Vacuum Swing Absorption (VSA) are best suited to producing 3 to 30 tons of oxygen capacity per day. This correlates to ozone production levels of approximately 350 to 4,000 lb per day. PSA and VSA technologies provide 90-95 percent purity level product.

For smaller ozone generation rates, down to a few pounds per day, “oxygen concentrators” are now commercially available. They can produce oxygen-enriched air with oxygen concentrations between 80 and 95 percent at a rate ranging from 6 to 42,000 scfh of oxygen.

Cost considerations
The capital costs for generating ozone from air are significantly higher than from oxygen. As a rule of thumb, capital costs for sir preparation equipment are 25 percent to 33 percent of the total capital cost for ozone generation equipment, with the lower percentage applying to the equipment with larger ozone generation capabilities.

Generating ozone in oxygen requires about half the volume of gas because of the much greater concentration of ozone in oxygen. The size of upstream and downstream gas handling equipment, as well as ozone contactors, is, therefore, much lower, resulting in larger capital cost savings.

D. Ozone Generation

Ozone can be generated by a number of techniques:

  • Corona discharge
  • Photochemical formation (ultraviolet radiation)
  • Electrolysis of water
  • Electrolytic reduc tion of concentrated sulfuric acid
  • Nuclear radiation (using 60Co)
  • Passing moist air over elemental phosphorus

Only the first three of these procedures have any commercial significance, and by far the most commercially important procedure is corona discharge. Consequently, this section emphasizes corona discharge and only briefly describes ultraviolet radiation and electrolysis. The last three are theoretically practical but have not advanced beyond the laboratory.

a. Ozone Generation by Corona Discharge

The principal elements of a corona discharge generator are an electrical source, a discharge gap through which the oxygen containing feed gas flows, a dielectric material to prevent short-circuiting, and a heat-removal mechanism to dissipate waste heat that is a byproduct of the exothermic reactions, which produce ozone.

A corona is characterized by a low-current electrical discharge across a gas- filled gap at a voltage gradient on the order of the electrical breakdown potential of the gap. During breakdown, the gas becomes partially ionized and a characteristic, diffused bluish- violet glow results. Kilovolt voltages and milliampere-to-ampere currents are typical within a corona. In contrast, an arc discharge is characterized by a high current density, producing an ionized gas and a low voltage gradient in the discharge gap. In general, an arc is a localized discharge producing high temperatures and a bright white light.

Figure 2 depicts a typical corona cell consisting of two metallic electrodes separated by gas- filled gap and a dielectric material. An oxygen-bearing gas flows through the discharge gap while high voltage is applied to the electrodes. Most of the electrical energy input to the corona (approximately 85 percent) is dissipated primarily as heat, with smaller portions going to light, sound and chemical reactions. Therefore, the electrical efficiency of generating ozone is rather low.


Typical corona discharge cell configuration

Many generators in operation today are powered at 60 Hz, although medium- frequency (up to 1,000 Hz) and high- frequency (above 1,000 Hz) equipment is available. Dielectric materials commonly used are glass or ceramic, their thickness ranging from about 0.5 to 3 mm. Driving peak voltages commonly range from about 8 to 30 kV. Electrically, a corona cell presents a capacitance load to the power supply due to both the gas- filled gap and the dielectric materials present. Ozone is produced in the corona as a direct result of power dissipation in the corona.

Because of the influence of heat removal on ozone yield, the yield from essentially all generators is sensitive to changes in the temperature of the coolant used. For air-cooled generators, the coolant usually is ambient air. For water-cooled or water/oil-cooled units, the excess heat ultimately is rejected to water.

Air-cooled, plate type ozone generators generally are used for small-scale applications of ozone. Less heat is produced in smaller ozone generators, so small changes in ambient air temperatures do not have as large an impact on the efficiency of cooling. This is in contrast with large-scale corona discharge generators, which produce a much greater amount of heat, which must be removed so as to maximize ozone yield. Additionally, cooling of heat-containing water is more energy efficient than cooling of heat-containing air.

Types Of Commercially Available Corona Discharge Ozone Generators Corona discharge generator designs differ with respect to configuration, corona cell geometry, operating frequency and power supply, cooling systems and typical applications.

There are two basic types of corona cell geometries: concentric tube and flat plate. Among the more common configurations is a horizontal concentric tube design that is water-cooled. This type is frequently used for commercial ozone applications, particularly for large-scale operations. The tube and two cross-sectional views are shown in Figure 3. It is representative of how corona discharge generators operate. The corona cell consists of an outer-grounded stainless steel tube and a concentric glass dielectric tube that has its inner surface coated with metallic material. The two metal surfaces make up the two electrodes. Typically, a 3-mm gas space is maintained between the glass dielectric and the outer steel tube with electrode spacers. Power is supplied to the inner metallic coating through an axial bus bar containing electrode brushes. Each corona tube can be fused individually to allow the generator to maintain continuous operation in the event of a single dielectric failure within the unit. The corona tube is cooled by passing water of potable or heat exchanger quality along the outside of the outer stainless steel tube.


Horizontal tube, water-cooled ozone generator

Operating frequency

Corona generator designs are classified by the frequency range in which they operate:
Low Frequency 50-50 Hertz/TD>
Medium Frequency >60 Hz up to 1,000 Hz
High Frequency >1,000 Hz, nominally ca 2,000 Hz

Generator characteristics vary according to operating frequency range. For example, when comparing the three frequency types, there is a relationship between component simplicity, ozone concentration, and “turndown” efficiency. Low frequency systems still are prevalent in the 1,000 lb per day or less ozone production applications. Over the past ten years, however, medium frequency systems have been installed in larger applications. High frequency generators have found most of their application in less than 100 lb/day (air feed gas) applications. Both low and high frequency equipment continue to dominate the under 100 lb. Per day applications, which are typical in the food, beverage and brewing industries.

Operating characteristics of the three frequency ranges of corona discharge ozone generators are summarized in the accompanying table.

Table 1: Comparison of corona discharge generators for three frequency ranges.
Characteristic Low Frequency (50-60 Hertz) Medium Frequency (up to 1,00 Hz) High Frequency (.1,000 Hz)
Power Density (kW/ft2) 0.1 to 1.7 0.3 to 3.8 0.4 to 4.2
Degree of Electronics Sophistication Low High High
Peak Voltages (kV) 19.5 11.5 10
Phase Balancing No Yes Yes
Operating Reliability High High High
Turndown Ratio 5:1 10:1 10:1
Cooling Water Required (gal/lb of ozone produced) 0.5 to 1.0 0.5 to 1.5 0.25 to 1
Typical Application Range <500 lb/day To 2,000 lb/day To 2,000 lb/day
Operating Concentrations
Wt-% in air
Wt-% in oxygen
0.5 – 1.5%
1.0 – 3.0%
1.0-2.5%+
2.0-6.0%
1.0-2.5%+
2.0-6.0%
Power Required to Generate 1lb of O3 (kWh) Air feed: 8-12
O2 feed: 4-6
Air feed: 8-12
O2 feed: 4-6
Air feed: 8-12
O2 feed: 4-6

b. Ozone generation by ultraviolet radiation
Photochemical generation of ozone occurs naturally in the Earth’s troposphere as highenergy ultraviolet (UV) radiation from the sun breaks oxygen molecules down into oxygen atoms. The subsequent combination of an oxygen atom with an oxygen molecule produces a molecule of ozone, just as in the generation of ozone by corona discharge. Manufacturers of ultraviolet lamps duplicate this natural generation of ozone, but on a much smaller scale using low-pressure mercury lamps optimized for 185- nm radiation, the most effective wavelength.

much smaller scale using low-pressure mercury lamps optimized for 185- nm radiation, the most effective wavelength.

Ozone generated by UV radiation is produced in low concentrations (less than 0.2 percent by weight) at low volume, and at higher energy expenditure than by corona discharge. The maximum rate at which ozone can be generated per kWh of energy applied is 1.94 g/h. The primary commercial application of UV generators is treating swimming pool and spa water.

In general, the concentration of ozone in gas exiting a 185-nm ozone generator can be as high as 0.2 percent by weight (2,400 ppm). The higher the concentration of ozone in the gas phase being applied to water, the higher the concentration of ozone that will dissolve in the water.

Generating ozone by UV radiation has two primary advantages: (a) drying the feed gas air is not necessary, and (b) equipment costs are much lower than for corona discharge generators. In addition, a new 172-nm vacuum UV lamp shows promise for producing higher ozone concentration, thus broadening the use of this technology in small-scale ozone applications.

The major disadvantages of generating ozone by UV radiation include the following:

  • Maximum ozone production rate is two grams per hour.
  • Highest concentration of ozone which can be produced by 185-nm UV lamps is 0.2 percent by weight, approximately 10 percent of the average concentration available by corona discharge.
  • Considerable more electrical energy is required to produce a given quantity of ozone by UV radiation than by corona discharge.
  • Lower gas phase concentrations of ozone generated by UV radiation translates into the handling of much higher gas volumes than with CDgenerated ozone.
  • UV lamps solarize over time, requiring periodic replacement.

c. Ozone generation by electrolysis
This technique for generating ozone produces very high concentrations of ozone (up to 50 percent by weight in air) without needing to dry the air. Offsetting this advantage is the fact that ozone and air form an explosive mixture above about 20 percent ozone by weight. Also, since concentrations of about 16 percent can be attained by corona discharge techniques using high purity oxygen feed gas, the capability of achieving a higher concentration is not a strong advantage.

Currently, electrolytic generation of ozone is used only for laboratory studies and very small-scale application in Japan to provide ultra-high purity water for the electronics and pharmaceutical industries. Considerable research and development must be done before this technique could have wide commercial applications. All economic projections for electrolytic production of ozone currently appear to be most unfavorable. Although ozone generation by electrolysis is less efficient than by corona discharge, the cleanliness of the produced ozone/oxygen (free of nitrogen oxides) and the high concentrations (>20 percent) continue to encourage research efforts to bring a commercial unit to market.

d. Summary

  • Although there are several methods for generating ozone, only the corona discharge method is in commercial use for small (g/h) to large (>2,000 lb/day) ozone applications.
  • By corona discharge techniques, ozone can be generated using dry air, high purity oxygen, or oxygen-enriched air.
  • As a rule of thumb, generation of one pound of ozone by corona discharge techniques requires 8-12 kWh using dried air as the feed gas. Power costs for air preparation equipment requires an additional 5-7 KWh per pound of ozone generation capacity.
  • In corona discharge generators, essentially double the amount of ozone can be produced from pure oxygen as from dried air, at the same power expenditure and in the same size ozone generator. In addition to halving the size of the ozone generator, air preparation equipment is eliminated, and gas handling and some downstream equipment also can be reduced in size, thus providing additional capital cost savings.
  • Use of air as feed gas requires significant operation and maintenance; use of high purity oxygen on a once-through basis eliminates this requirement.
  • Ozone generation by UV radiation is used commercially for very smallscale applications (primarily swimming pool water treatment), in North America.
  • Technology of generation of ozone by electrolysis has evolved from the laboratory stage to initial market applications, but also on a very small scale.

E. Ozone Contacting Subsystem
Ozone contacting is the process of placing the ozone-rich gases in contact with the water requiring treatment so that the mass ozone is transferred to the water efficiently. The contactor design must allow for a suitable retention time so that the required reactions can occur. Contactors vary according to ozone transfer efficiencies, application parameters (including energy requirements), and the need for follow-up reactors. Following are some common configurations:

  • Bubble diffusion contactors
  • Mechanical mixing contactors (including submerged and surface turbines)
  • Injection type contactors (total and partial injection)
  • Spray towers
  • Packed column contactors
  • Static mixing

Other varieties of contactors and reactors also exist (or are in development) that show promise for increasing transfer efficiencies or providing unique advantages in specific applications. Some major types of contactors using these configurations are discussed below.

The purpose of a contactor is to provide sufficient amounts of ozone in solution to meet the requirements of the application, whether in an oxidation or disinfection role. The transfer of ozone gas to the solution phase should be accomplished with minimal losses, providing at least 85 percent ozone transfer efficiency, and preferably at least 95 percent. The contactor should be designed for thorough distribution of the ozone throughout the water volume and should provide sufficient ozone at the design concentrations required for the reaction. Some important parameters in contactor effectiveness are bubble size, pattern and method of dispersion, mixing efficiency, contact time, and liquid/gas volume mass ratio. Ozone transfer is controlled primarily by oxidation/disinfection reaction rates and the mass transfer rates of ozone into water so the contacting method must take these factors into account. The main goal is to maximize water-to-ozone contact.

Dissolving ozone in water is much more involved than simply adding gas to liquid. Ozone is generated on-site, and it must be applied as soon as practicable after generation. Another factor affecting the choice of contactor for ozone is the potential for oxidized solutes to form insoluble products. In such cases, the precipitating product(s) can plug or otherwise foul some types of contactors. For example, if a porous ceramic tube or disc is used and the water contains soluble iron and manganese, ferric and manganese, hydroxides will be produced during ozonation, which coagulate and precipitate. The effectiveness of the porous disk or tube in producing fine bubbles of ozone soon will be reduced, and the contactor may fail, requiring shutdown and cleaning.

In some water treatment applications where iron and manganese are present in the raw water, oxidation of these two elements may be an objective of ozonation in addition to disinfection. Providing iron and manganese oxidation as well as primary disinfection in the same ozone contactor, however, is difficult because the precipitating iron/manganese hydroxides can foul the residual ozone probe and render it useless. Consequently, although disinfection is likely being attained, there is no practical method of monitoring residual ozone levels to confirm it. To address this problem, ozonation is carried out in two stages. First iron and manganese are oxidized using an ozone contactor, which cannot be plugged by the precipitating solid. Next, after settling/filtration, the water undergoes a second stage of ozonation, this time for disinfection, using a porous diffuser contacting system.

Following are common types of contactors:

Diffuser-Based contactors are the most common, primarily due to their simplicity, high transfer rates, zero energy costs (over those required for the generation of ozone) and the process flexibility inherent in the design. With no moving parts, transfer efficiencies in the 85-98 percent range, and single-step contactor/reactor capabilities, bubble diffusers can be a cost-effective option. These contactors, however, have some disadvantages. They require water levels of at least 16 feet to maximize mass transfer during bubble rise. They also need a relatively large space, and they can suffer from diffuser plugging under certain circumstances.

Injection Diffusers entrain ozone directly into a water flow. Injectors are simple, relatively low cost, and easy to maintain. Injectors have serious drawbacks, however. Ozone transfer efficiency is only about 70 percent, and a constant air/water volume ratio is essential. The technique has limited flexibility and may exhibit high head [?] losses. A secondary reactor, such as a static mixer, may also be required to extend ozone reaction times.

Static mixers are devices placed in the pipe carrying the flowing water to distribute the ozone gas. The process is simple, inexpensive to buy and operate, and offers transfer efficiencies of 70-90 percent. Its disadvantages are similar to those of direct injection, including a requirement of constant air/water volume ratios, limited flexibility, and potentially high head losses. As with direct injection, it is poorly suited to disinfection in potable water treatment without a secondary reactor.

Packed column reactors are basically vessels containing a stationary medium through which ozone is diffused, typically in a counter current flow pattern with the inlet water. Packed columns show 80-95 percent transfer efficiencies, have plug flow characteristics (excellent mixing), and provide simple, low-cost operation. There is, however, potential for high head losses due to scale buildup on the medium.

Spray chambers involve spraying water into an ozone-rich gaseous environment. Typically used for iron/manganese removal in some German drinking water treatment plants, this simple, low maintenance process provides transfer rates of 60-80 percent, but has limited applicability in water and waste water treatment because of the high energy required to pump the water to be treated through a very narrow orifice, or to a height which will allow gravity flow through the narrow orifice.

Mechanical mixers utilize mechanical energy to mix the ozone and water, and to develop fine bubbles. Used mostly in raw water applications when particulates or iron/manganese may be a problem, the technique is capable of providing 90-98 percent ozone transfer rates, the short primary disinfection or oxidation of refractory organic compounds. The technique also has a high operating cost (energy), limited process flexibility, and may have a high maintenance demand.

Submerged turbines are a class of mechanical mixers which are in less common use to provide primary disinfection because of their energy requirements, particularly on large scale. On small scale, however, this type of contactor can be cost-effective. The size of the contact chamber required is much smaller, compared with bubble diffusers. Additionally, the turbine action creates a partial vacuum, which can be used to draw the ozone-containing gas from the ozo ne generator into the submerged turbine.

Surface Turbines (Surface Aerators) are similar to submerged turbines in principles of operation, but are placed atop the water to be ozonated. The primary advantage of this type of contactor is its accessibility, but its disadvantage is that efficient contacting depends on large energy expenditures to ensure sending the ozone-containing gas downward into and throughout consequential escape of at least small amounts of ozone to the atmosphere.

F. Ozone off-gas destruction subsystem
When ozone is used to treat water or wastewater the ozone never totally dissolves in the water if sufficient ozone is being applied to accomplish its intended purpose. Also, the consumption of ozone is rarely 100 percent. As a result, some ozone always can be anticipated to escape the ozone contactor.

Ozone is hazardous and corrosive, even in low concentrations, and no ozone-containing gas should be allowed to escape into the ambient environment. Consequently, ozone contactors and reactors are designed to collect all residual gases and vent these gases to an ozone destruction system.

Typically, vent gas is collected from the contactor through a stainless steel vent gas line and passed through a pad or woven mesh-type de-mister. The de- mister removes large particles of moisture, primarily droplets. After de-misting, a blower or vacuum pump forces the gases through a destruction unit, which neutralizes the residual ozone. Four major types of destruction units are used in commercial practice at full- scale ozonation plants:

  • Thermal
  • Catalytic
  • Thermal-Catalytic
  • Granular Activated Carbon

Thermal Ozone Destruction
These units decompose ozone to a residual of 0.1 ppm by heating it to 350 degrees Celsius for a contact time of 3 seconds. The heat can be transferred to the off- gas stream in several ways, e.g., by flowing over electric resistances with insulated wires or by combustion of oil or natural gas inside the heat exchanger with direct heating. A heat exchanger is sometimes included in the cold and hot line loop, thus recovering a significant fraction of the heat generated by ozone decomposition.

Catalytic Ozone Destruction Units
In these units, a catalyst reacts readily with ozone to remove it from the exhaust stream. Capital costs for the ozone catalytic destruction unit are about the same as for the thermal destruction system. From an operating cost point of view, however, the catalytic system is lower in cost, since the amount of preheating is lower than the amount of energy consumed by a thermal destruction system.

Thermal-Catalytic Ozone destruction Systems

Hybrid thermal-catalytic systems combine features of the thermal and catalytic systems to achieve ozone destruction. Less common than the other two methods, this process relies on heat levels in excess of 50o – 150o C to reduce the level of initial ozone in the feed gas by converting a portion of the ozone to oxygen. A catalyst step immediately follows the thermal step, so that all remaining ozone can be converted to oxygen.

Granular Activated Carbon Destruction
In these units, activated carbon reacts with ozone to produce CO2. GAC units have been used successfully for the energy- free destruction of residual ozone levels in small system applications generating less than 10 pounds of ozone per day. This process may not be as cost-effective as other alternative, particularly on large scale, because of the gradual loss of the carbon. GAC is used routinely to destroy excess ozone in contactor off- gases in European (some North American) ozonation systems designed to treat municipal and residential swimming pools, in bottled water treatment plants, in cooling water treatment systems, and for small- scale and residential potable water applications.

Table 2. Comparison of Ozone Off-Gas Destruction Techniques
System Operating Cost W-h/Nm Major Advantage Major Disadvantage
Preozonation 80-150 Ozone is reused Only partial
destruction
Thermal

W/ Heat recovery
130-170

85
Easy monitoring

Good yield
Hot off-gases

Difficult to
automate
GAC absorption 10-15 Static operation;
low cost
Small system
applicability only;
fire danger
Catalytic 5 Small equipment Catalyst poisoning

G. Monitoring and Controls
Monitoring and control of an ozone system are vital for proper operation. These functions involve four distinct areas:

  • Monitor and control equipment operation, feed gas preparation, and ozone production
  • Monitor and control reactor or contactor ozone concentrations
  • Monitor and control efficiency of ozone contactor off- gas destruction
  • Monitor and control ambient air ozone levels

The control and monitoring systems can be based on manual, semi-automatic or fully automatic control philosophies with feedback and feed- forward control logic.

  • Manual Methods In this category of system, the operator is responsible for the startup, monitoring, control and shutdown of all system components through direct interaction. All equipment, ozone dosage setting, and checks of operating parameters are monitored and recorded directly by the operator. Alarm functions may require the operator to initiate shutdown procedures manually.
  • Semi-Automatic In semi-automatic systems, initiation of an automatic function is accomplished manually. The operator is responsible for the initial startup of the system, while the control system assumes responsibility for the operation of the equipment at a pre-specified operating point. Monitoring and alarm functions may be the responsibility of the control system as well. In this case, sequential equipment startup and shutdown as well as emergency shutdown may become an independent automated procedure, although selection of on- line versus off- line equipment remains a manual procedure. An automatic selfregulating feedback or feed-forward control loop may be included to regulate ozone production and application at the contactors or reactors.
  • Automatic Automatic control is an approach which relies on programmable controllers to sequentially start up, operate and shut down equipment, maintain ozone production at predetermined levels based on water flow or ozone residual requirements, monitor critical process parameters and, if necessary, bring equipment on-line and off- line as required. Automatic systems also are capable of modifying gas flows, ozone production levels, and liquid flows to ozone contactors (or sections of the contactors) on a predetermined basis without direct operator involvement.

Semi-automatic or fully automatic control systems usually are justified on the basis of improved process control and lower operating costs. In general, the reduction in system energy use, combined with accurate and consistent process control with limited operator attention, has led design engineers to provide fully automated controls for ozone systems.

For disinfection applications, automatic and semi-automatic control loops provide constant on- line monitoring of ozone produced and applied, as well as residual levels in the various contactors or compartments of the contactors.

4. Energy Aspects of Ozonation Systems

Ozone systems are complex. They must generate ozone, apply it, destroy excess ozone, control the process, and monitor many parameters. Some of these subsystems require more electrical energy than others. Effective and efficient operation of an ozone plant requires a thorough understanding of energy flows in the process.

Power Needs for Ozone Generation and Application
In ozone production the four processes which consume the most power are:

  • Ozone generation
  • Gas preparation
  • Ozone generator cooling
  • Ozone off- gas destruction

Ozone generation has by far the largest power requirement. The remaining processes on ozone production also require energy, though in much smaller amounts.

Ozone Generation
In corona discharge ozone generators, electrical power is applied directly across the discharge gap, i.e., between the high voltage and the ground electrode. Free electrons in the current flowing between dielectric and ground are energetic enough to split oxygen molecules. The recombination of atomic molecular oxyge n creates the ozone molecules. Some energy losses occur when the incoming voltage (and in many cases the frequency) is increased to provide optimal conditions for ozone generation. Only a small fraction of the supplied energy is actually used to change the oxygen into ozone. Most of the incoming energy ultimately goes to produce waste heat and, to a lesser extent, light.

Ozone generation accounts for about 75 percent of the electrical power required by an ozonation system. About 8 KWh is required to ge nerate one pound of ozone from air at 1-4 percent (by weight) concentration, while about 7-9.5 kWh is required to generate one pound of ozone from oxygen.

Gas Preparation
Efficient ozone generation requires a dry, oxygen containing feed gas because the presence of moisture in the feed gas attenuates the ozone production field. Most ozonation systems, especially larger ones, specify a dew point of –60oC (-76oF) or lower, measured at one atmosphere pressure. This is equivalent to approximately 11 ppm of water by volume in the dried feed gas.

Feed gas preparation—either to generate oxygen on-site or to properly dry a feed air supply—uses approximately 20 percent of the total power required by an ozonation system. This amounts to about 3 kWh for each pound of ozone produced. The exact amount of energy used for feed gas preparation depends on the nature of the feed gas (air, high purity oxygen, or air-oxygen mixtures) and on the specific moisture removal process used.

Ozone Generation
In corona discharge ozone generators, electrical power is applied directly across the discharge gap, i.e., between the high voltage and the ground electrode. Free electrons in the current flowing between dielectric and ground are energetic enough to split oxygen molecules. The recombination of atomic molecular oxyge n creates the ozone molecules. Some energy losses occur when the incoming voltage (and in many cases the frequency) is increased to provide optimal conditions for ozone generation. Only a small fraction of the supplied energy is actually used to change the oxygen into ozone. Most of the incoming energy ultimately goes to produce waste heat and, to a lesser extent, light. Ozone generation accounts for about 75 percent of the electrical power required by an ozonation system. About 8 KWh is required to ge nerate one pound of ozone from air at 1-4 percent (by weight) concentration, while about 7-9.5 kWh is required to generate one pound of ozone from oxygen.

Gas Preparation
Efficient ozone generation requires a dry, oxygen containing feed gas because the presence of moisture in the feed gas attenuates the ozone production field. Most ozonation systems, especially larger ones, specify a dew point of –60oC (-76oF) or lower, measured at one atmosphere pressure. This is equivalent to approximately 11 ppm of water by volume in the dried feed gas.

Feed gas preparation—either to generate oxygen on-site or to properly dry a feed air supply—uses approximately 20 percent of the total power required by an ozonation system. This amounts to about 3 kWh for each pound of ozone produced. The exact amount of energy used for feed gas preparation depends on the nature of the feed gas (air, high purity oxygen, or air-oxygen mixtures) and on the specific moisture removal process used.

Ozone Generator Cooling
Because most of the incoming electrical energy is converted into heat, almost all ozone generators have some type of liquid cooling system to minimize the thermal decomposition of the ozone being produced. The exceptions are smaller corona discharge units that use air-cooling small ultraviolet radiation lamps, and electrolytic units that produce ozone directly in aqueous solutions. Although some small corona discharge generators employ forced air-cooling with a fan to remove the excess heat, systems generating more than about 10 lb/day normally are water-cooled.

Energy requirements for water-cooling of ozone generators are about 1-5 percent for that required to generate ozone. Medium frequency generators require higher amounts of cooling than do low frequency generators.

Ozone Contacting
Energy requirements for ozone contacting vary according to the type of gas/liquid contacting employed. Bubble diffusers do not require energy, relying on the gravity flow of water through a treatment plant and on the fact that ozone in air or oxygen exits the generator at about 15 psi—an amount sufficient to overcome a 15-foot head of water. Conversely, injectors, static mixers, and turbines all require electrical energy. Although the power requirement for each type of contactor varies widely, it is only a small fraction of the total system.

Ozone Off-Gas Destruction
The major methods for destroying ozone are thermal, catalytic, and thermal/catalytic, each of which requires input energy. Small amounts of ozone or low concentrations of the gas also can be destroyed by another method that does not require input energy: passing the gas through granular activated carbon GAC.

Off- gas Destruction requires on the order of 2 KWh per pound of ozone to be destroyed. This is about 25 percent of the power cost to generate ozone from dried air.

Instrumentation and Controls
Only minor amounts of energy are required to operate ozonation system controls.

In Summary

  • 8kWh/lb. To generate ozone from air at optimum efficiency
  • 7-9.5 kWh/lb. To generate ozone from oxygen (6 percent O3)
  • 3 kWh/lb. For air preparation
  • ~0 kWh for diffuser contacting
  • 2 kWh/lb. for off- gas destruction
The actual total energy requirements of any ozonation system will depend entirely upon the composition of the feed gas, the type of feed gas pretreatment, the type and rated output of the ozone generator(s), the actual output of the generators during use (e.g., full capacity versus maximum turndown), the type of ozone contacting, and the type of offgas destruction employed.

5. Economic Consideration

Costs for ozone systems are difficult to assess because of the new and changing nature of the technology. This is especially true for small and medium applications such as would be common in the food, beverage, and agriculture industries. Most of the design, construction, and operational experience have been in large municipal water treatment facilities, which could differ in configuration from industrial applications. As a result, rules of thumb for cost estimation in these large municipal applications do not apply in all cases.

Despite these difficulties cost estimates can be made. The techniques common in the engineering and construction industry, e.g., general construction estimating guides, can be adapted to determine approximate capital costs. Estimating operating and maintenance (O & M) costs is more difficult, however, because of the relatively small number of ozonation facilities in industrial use.

Following are some general guidelines for estimating costs for ozonation systems.

Considerations For Assessing Capital Costs
Capital costs include the cost of the ozonation plant, construction costs, installation cost, and contingencies. These costs depend on a number of factors. For a new plant these factors include:

  • Ozone requirement, including standby capacity and redundancy equipment
  • System size (depending on gas preparation and ozone production technologies) and housing
  • Availability of power to the site
  • Contractors and associated diffusion systems
  • Degree of automation

For retrofitting an existing plant, additional cost factors become important. These include:

  • The compatibility of existing materials of construction in facilities immediately downstream of the ozonation step (possible corrosion due to ozone)
  • Process interconnections to existing channels, conduits, and control system
  • Ability to maintain operations and to continue producing potable water during construction

As in all large construction projects estimating capital cost requires knowing the rate and period of depreciation for buildings, contactors, and equipment as well as financing costs. Other costs, like “site work”, design and construction management services, owners’ administration, and legal and other expertise, need to be considered as well.

Considerations For Assessing Operating and Maintenance Costs
Fixed operating and maintenance costs consist of costs that do not vary with the hourly effective production of ozone. These fixed O&M costs include:

  • Labor for supervising the plant during operation, if this has not been entered as overhead charges
  • Amortization cost, if not included as a capital cost.

Variable costs include:

  • Utilities (electricity, water, liquid oxygen [if used]) and
  • Maintenance (cleaning), spare parts, maintenance personnel.

For large ozonation systems, the most important O&M cost is electricity. Therefore, it is important to develop a realistic cost estimate for energy. The specific electricity consumption of an ozone generator changes at different production levels. These production requirements, in turn, change throughout the year, and sometimes within a given day. The operating cost estimate must consider both of these factors.

Other operating and maintenance costs will depend primarily upon:

  • The technologies selected for feed gas preparation and ozone generation (air, enriched air, O2), which may affect the maintenance and the consumable material
  • Ozone dosage rate
  • Ozone dissolution and destruction systems and
  • Degree of automation.

Considerations For Assessing Secondary Benefits and Costs
There are many attractive aspects of ozone for several applications. In drinking water treatment for example, improvements in later stages of treatment or a saving sin the amount of reagents consumed may result. Following are some of the benefits of using ozone in treating drinking water, many of which are similar for other applications:

  • The possibility of decreasing the amount of flocculant needed in certain cases, or according to the season
  • Longer filtration cycles
  • Transformation of a conventional filtration process into a biological one for removal of biodegradable organic compounds, which in the case of activated carbon beds, results in a much longer useful life of the carbon and
  • Decrease in trihalomethanes and/or haloacetic acid formation potentials.

Although these benefits are often very difficult to interpret in terms of cost, they can have a significant affect on operations and, therefore, should be considered as part of any economic analysis.

Other Considerations

  • Parameters, which are important for the estimation of capital costs of ozonation systems, include ozone dosage, ozone production capability, ozone standby production capacity, type of ozone contactor (including hydraulic detention time if a liquid is being treated), and size of building to house the ozonation system.
  • Parameters, which are important for the estimation of operating costs of ozonation, include ozone dosage and the system-specific energy of the total ozonation system (feed gas preparation, ozone generation, contacting and off- gas destruction).

6. Other Resources

A list of equipment suppliers and other resources, e.g., suggested reference materials, consultants, will be added in later ve rsions.

» GLC Fishing
     Products
» GLC Product
     Outsourcing
» Corporate
     Alliance Building
» About Vietnam
» Tours
» Tips

Links

www.is-tours.com
www.lacquerworld-fact.com
www.thachban.com
www.lamsonvn.com
www.aacorporation.com
www.namhoatoys.com
www.nhabe.com
www.o7paint.com
www.yp.com.vn
www.cheye.com.vn

US Technology Exports

ø Dip N Grow
ø Gulf Atlantic
ø Trio3
  • Trio3 Ox Fresh system
  • Technology for Water
  • Ozone use in Agriculture
  • Ozone Technology for Fruit
  • Ozone Technology for Fish
  • Ozone for fruit and vegetables