In the United States, Whiting, Indiana and Strasburg, Pennsylvania have used ozone in their drinking water treatment process. Other cities have run pilot studies. Ozone is used as a bleaching agent for miscellaneous items: petroleum, clays, wood products, and chemical baths. It has been proposed as a bleaching agent for hair and as a disinfectant for oils and emulsions. Ozone is used to modify tryptophan and indigo plant juice. It is an important factor in colorfastness. The desulfurization of flue gases by ozone has been considered an application where it promotes liquidphase oxidation. The operations are carried out with vanadium catalysts, and the oxidation step is performed in gasfluidized beds. The desulfurizing effect of ozone on light petroleum distillates has also been reported.
The use of ozone has been proposed in special ore-flotation processes. Two widely different applications involve hydraulic cement and the fabrication of coating on insulators.
The metallurgical applications include steel refining, electrochemical processes, and gold recovery. The aggressive reactivity of ozone is evident in the corrosion of stainless steel and in chemical etching. The inhibition of ozone decomposition is accomplished in the presence of SF6, CC12172, or CF3Metal coatings, paints, and lacquers have been evaluated with respect to ozone resistivity. Ozone has been examined as a potential source of high-energy oxidation and for combustion and propulsion applications.
Ozonation systems are comprised of four main parts, including a gas-preparation unit, an electrical power unit, an ozone generator, and a contactor which includes an off-gas treatment stage. Ancillary equipment includes instruments and controls, safety equipment and equipment housing, and structural supports. The four major components of the ozonation process are illustrated in Figure 8.
Figure 8. Components of ozonation.
A high level of gas preparation (usually air) is needed before ozone generation. The air must be dried to retard the formation of nitric acid and to increase the efficiency of the generation. Moisture accelerates the decomposition of ozone. Nitric acid is formed when nitrogen combines with moisture in the corona discharge. Since nitric acid will chemically attack the equipment, introduction of moist air into the unit must be avoided. Selection of the air-preparation system depends on the type of contact system chosen. The gas-preparation system will, however, normally include refrigerant gas cooling and desiccant drying to a minimum dew point of -40° C. A dew-point monitor or hygrometer is an essential part of any air preparation unit.
Conversion efficiencies can be greatly increased with the use of oxygen. However, the use of high-purity oxygen for ozone generation for disinfection is, cost effective. The Duisburg plant and the Tailfen plant of Brussels, Belgium, are the only operational municipal water treatment plants known which use high-purity oxygen instead of air as the ozone generator feedgas.
Electrical power supply units vary considerably among manufacturers. Power consumption and ozone-generation capacity are proportional to both voltage and frequency. There are two methods to control the output of an ozone generator: vary voltage or vary frequency. Three common electrical power supply configurations are used in commercial equipment:
• Low frequency (60 Hz), variable voltage.
• Medium frequency (600 Hz), variable voltage.
• Fixed voltage, variable frequency.
The most frequently used is the constant low-frequency, variable-voltage configuration. For larger systems, the 600-Hz fixed frequency is often employed as it provides double ozone production with no increase in ozone generator size. The electrical (corona) discharge method is considered to be the only practical technique for generating ozone in plant-scale quantities. In principle, an ozone generator consists of a pair of electrodes separated by a gas space and a layer of glass insulator. An oxygen-rich gas is passed through the empty space and a highvoltage alternating current is applied. A corona discharge takes place across the gas space and ozone is generated when a portion of the oxygen is ionized and then becomes associated with nonionized oxygen molecules.
Figure 9 shows the details of a typical horizontal tube-type ozone generator. This unit is preferred for larger systems. Water-cooled plate units are often used in smaller operations. However, these require considerably more floor space per unit of output than the tube-type units. The air-cooled Lowther plate type is a relatively new design. It has the potential for simplifying the use of ozone-generating equipment. However, it has had only limited operating experience in water treatment facilities.
After the ozone has been generated, it is mixed with the water stream being treated in a device called a contactor. The objective of this operation is to maximize the dissolution of ozone into the water at the lowest power expenditure. There is a variety of ozone contactor designs. Principal ones employed in wastewater treatment facilities include:
• Multistage porous diffuser contactors, which involve a single application of an ozone-rich gas stream and application of fresh ozone gas to second and subsequent stages with off-gases recycled to the first stage.
• Eductor-induced, ozone vacuum injector contactors, which include total
494 WATER AND WASTEWATER TREATMENT TECHNOLOGIES MEASUREMENT AND CONTROL
Favorable operational economics and good management practices require high levels of control of the ozonation system. Depending on the specific process of ozone applications, plant size, and design philosophy, the control system may be simple or complex. The trend in Europe is toward highly sophisticated and
Several parameters should be measured to provide a fully operable ozonation system. There should be a means of providing full temperature and pressure profiles of the ozone generator feedgas from the initial pressurization (by fan, blower, or compressor) to the ozone generator inlet. Moisture content is also important. There should be a means of measuring the moisture content of the feedgas to the ozone generator. This procedure should be conducted with a continuously monitoring dew-point meter or hygrometer. Other parameters that
• Temperature, pressure, flow rate, and ozone concentration of the ozonecontaining gas being discharged from all the ozone generators. This is the only effective method by which ozone dosage and the ozone production capacity of the ozone generator can be determined.
• Power supplied to the ozone generators. The parameters measured include amperage, voltage, power, and frequency, if this is a controllable variable.
• Flow rate and temperature of the cooling water to all water-cooled ozone generators. Reliable cooling is important to maintain constant ozone production and to protect the dielectrics in the generation equipment.
• There should be a means to monitor the several cycles of the desiccant drier,
Analytical measurements of ozone concentrations must be made in the ozonized gas from the ozone generator, the contactor off-gases, and the residual ozone level in the ozonized water. Methods of ozone measurement commonly used are the: simple "sniff" test, Draeger-type detector tube, wet chemistry potassium iodide method, amperometric-type instruments, gas-phase chemiluminescence, and ultraviolet radiation adsorption. The use of control systems based on these measurements varies considerably. The key to successful operation is an accurate and reliable residual ozone analyzer. Continuous residual ozone monitoring equipment may be successfully applied to water that has already received a high level of treatment. However, a more cautious approach must be taken with the application of continuous residual ozone monitoring equipment for water that has only received chemical clarification because the ozone demand has not yet been satisfied and the residual is not as stable. Ozone production must be closely controlled because excess ozone cannot be stored. Changes in process demand must be responded to rapidly. Ozone production is costly; underozonation may produce undesired effects and overozonation may require additional costs where off-gas destruction is used.
WATER STERILIZATION TECHNOLOGIES 495 OPERATION AND MAINTENANCE
Ozonation equipment typically has low maintenance requirements. The, airpreparation system requires frequent attention for air filter cleaning/changing and for assuring that the desiccant is drying the air properly. However, both are usually simple operations. Two factors which impact ozone generator operation and maintenance are the effectiveness of the air-preparation system and the amount of time that the generator is required to operate at maximum capacity. Maintenance of the ozone generators is commonly scheduled once a year. However, many plants perform this maintenance every six months. Typically, one man-week is necessary to service an individual ozone generation unit of the horizontaltube type. Dielectric replacement due to failure as well as breakage during maintenance may be as low as 1 percent to 2 percent. An average tube life of ten years can be expected if a feedgas dew point of - 60' is maintained and if the ozone generator is not required to operate for prolonged periods at its rated capacity. Plate-type ozone generators use window glass as dielectrics. However, the same attention to air preparation is taken as with the more expensive glass or ceramic tubes in order to avoid costly downtime. Operations and maintenance of the ozone contactor also requires attention. Turbines require electricity to power the drive motors, while porous diffusers require regular inspection and maintenance to insure a uniform distribution of ozone-rich gas in the contact chamber. It should be noted that serious safety problems exist with servicing some of these units. For example, even after purging the contact chambers with air, maintenance personnel entering the chambers should be equipped with a self-contained breathing apparatus, since the density of ozone is heavier than air and therefore is difficult to remove completely by air purging.
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