Ozone is an unstable gas, having a boiling point of -112° C at atmospheric conditions. Its molecular weight is 48. Ozone is partially soluble in water (approximately 20 times more soluble than oxygen), and has a characteristic penetrating odor which is readily detectable at concentrations as low as 0.01 - 0.05 ppm. Ozone is the most powerful oxidant currently available for use for wastewater treatment. Commercial generation equipment generates ozone at concentrations of 1 percent to 3 percent in air (that is, 2 percent to 6 percent in oxygen). Ozone is unstable in water, however, it is more stable in air, especially in cool, dry air. As a strong oxidant, ozone reacts with a wide variety of organics. Ozone oxidizes phenol to oxalic and acetic acids. It oxidizes trihalomethane (THM) compounds to a limited extent within proper pH ranges and reduces their concentration by air stripping. Trihalomethanes are also oxidized by ozone in the presence of ultraviolet light. Oxidation by ozone does not result in the formation of THMs as does chlorination. A combination of ozone and ultraviolet light destroys DDT, malathion, and other pesticides. However, high dosages and extended contact times that are not normally encountered in drinking water treatment are needed. Ozonized organic substances are usually more biodegradable and absorbable than the starting, unoxidized substances. When ozonation is employed as the final treatment step for potable water systems in water containing significant concentrations of dissolved organics, bacterial regrowth in the distribution system can occur. Consequently, ozonation is not typically used as the final treatment step but rather followed by granular activated carbon filtration and sometimes by the addition of a residual disinfectant. Humic materials are the precursors of THMs. Humic substances can be oxidized by ozonation. Under proper conditions significant reduction in THM formation can be realized when ozone is applied prior to a chlorination step. Because of ozone's instability, it is able to produce a series of almost instantaneous reactions when in contact with oxidizable compounds. One example follows.
In this reaction, iodine is liberated from a solution of potassium iodide. This reaction can be used to assess the amount of ozone in either air or water. For determination in air or oxygen, a measured volume of gas is drawn through a wash bottle containing potassium iodide solution. Upon lowering the pH with acid, titration is effected with sodium thiosulfate, using a starch solution as an indicator. There is a similar procedure for determining ozone in water. A typical ozone treatment plant consists of three basic subsystems: feedgas preparation; ozone generation; and ozone/water contacting. Commercially, ozone is generated by producing a high-voltage corona, discharge in a purified oxygen-containing feedgas. The ozone is then contacted with the water or wastewater; the treated effluent is discharged and the feedgas is recycled or discharged. Ozone's high reactivity and instability, as well as serious obstacles in producing concentrations in excess of 6 percent, preclude central production and distribution with its associated economies of scale. The requirement for on-site generation and application of ozone must yield a cost-efficient, lowmaintenance operation in order to be useful. The feedgas employed in ozonation systems is either air, oxygen, or oxygen-enhanced air. The particular selection of feedgas for each application is based on economics and depends on several factors: total quantity of ozone required; desired concentration of ozone in the feedgas; and fate (recycle or discharge) of the feedgas. For a given ozone generator with a specified power input and gas flow, two to three times as much ozone may be generated from oxygen as from air. The maximum concentration economically produced from air is about 2 percent, while that generated from pure oxygen is approximately 6 percent.
The use of higher concentrations of ozone provides two advantages: capital and operating costs per pound of ozone produced are substantially reduced, and a greater concentration gradient for mass transfer of ozone is provided in the contacting step, yielding increased ozone-utilization efficiency. These advantages, however, must be weighed against the increased cost of oxygen production. Air is generally employed in those applications requiring less than 50 pounds/day of low-concentration ozone. If air is the feedgas, it must be dried and cooled to reduce accumulation of corrosive nitric acid and nitrogen oxides that occur as by-products when the dew point is above 40° C.
Ozone may be produced by electrical discharge in an oxygen-containing feedgas or by photochemical action using ultraviolet light. For large-scale applications, only the electric-discharge method is practical since the use of ultraviolet energy produces only low-volume, low-concentration ozone.
In the electric-discharge (or corona) method, an alternating current is imposed across a discharge gap with voltages between 5 and 25 kV and a portion of the oxygen is converted to ozone. A pair of large-area electrodes is separated by a dielectric (1-3 mm in thickness) and an air gap (approximately 3 mm.) as shown in Figure 6. Although standard frequencies of 50 or 60 cycles are adequate, frequencies as high as 1,000 cycles are also employed.
Only about 10 percent of the input energy is effectively used to produce the ozone. Inefficiencies arise primarily from heat production and, to a lesser extent, from light and sound. Since ozone decomposition is highly temperature dependent, efficient heat-removal techniques are essential to the proper operation of the generator.
The mechanism for ozone generation is the excitation and acceleration of stray electrons within the high-voltage field. The alternating current causes the electrons to be attracted first to one electrode and then the other. As the electrons attain sufficient velocity, they become capable of splitting some 02 molecules into free radical oxygen atoms. These atoms may then combine with02molecules to form 03. Under optimum operating conditions (efficient heat removal and proper feedgas flow), the production of ozone in corona-discharge generators is represented by the following relationships, showing the factors to be considered in the design of these where Y/A = ozone yield per unit area of electrode surface
The following requirements will facilitate optimization of the ozone yield:
The pressure/gap combination should be constructed so the voltage can be kept relatively low while maintaining reasonable operating pressures. Low voltage protects the dielectric and electrode surfaces. Operating pressures of 10 -15 pounds per square inch gauge (psig) are applicable to many waste treatment uses. ^ For high-yield efficiency, a thin dielectric with a high-dielectric constant should be used. Glass is the most practical material. High-dielectric strength is required to minimize puncture, while minimal thickness maximizes yield and facilitates heat
& For reduced maintenance problems and prolonged equipment life, highfrequency alternating current should be used. High frequency is less damaging to dielectric
& Heat removal should be as efficient as possible.
Basic configurations of ozone generators are shown in Figures 6 and 7. The thre designs are the Otto plate, the tube, and the Lowther plate. The least efficient of these generators is the Otto plate, developed at the turn of the century. The tube and Lowther plate units include modern innovations in material and design. The Lowther plate generator is the most efficient configuration due in large measure to advantages in heat removal. In addition to ozone yield, the concentration of ozone is an important consideration. Ozone concentration from a generator is usually regulated by adjusting the flow rate of the feedgas and/or voltage across the electrodes.
LOWTHER PLATE GENERATOR UNIT
Figure 7. Types of ozone generators.
LOWTHER PLATE GENERATOR UNIT
Contactor design is important in order to maximize the ozone-transfer efficiency and to minimize the net cost for treatment. The three major obstacles to efficient ozone utilization are ozone's relatively low solubility in water, the low concentrations and amounts of ozone produced from ozone generators, and the instability of ozone. Several contacting devices are currently in use including positive-pressure injectors, diffusers, and venturi units. Specific contact systems must be designed for each different application of ozone to wastewater. Further development in this area of gas-liquid contacting needs to be done despite its importance in waste treatment applications. In order to define the appropriate contactor, the following should be specified:
^ The objective: disinfection biochemical oxygen demand (BOD) or chemical oxygen demand (COD) reduction to a particular level, trace refractory organics oxidation, and so on.
^ Relative rates of competitive reactions: chemical oxidation, lysing bacteria, decomposition of ozone in aqueous solutions, and so on. ^ Mass-transfer rate of ozone into solution.
^ Wastewater quality characteristics: total suspended solids, organic loading, and so on.
^ Operating pressure of the system. ^ Ozone concentration utilized.
Other considerations for the contacting system itself include contactor type (for example, packed bed, sparged column); number and configuration of contactor stages; points of gas-liquid contact, whether the mix is cocurrent or countercurrent; and the construction materials used. It is clear that designing an ozonation system for even a relatively simple application requires a thorough understanding of many factors in order to employ sound engineering methods and optimization techniques.
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