Figure 7. Types of ozone generators.
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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