Figure 4. Components of a gaseous sodium chlorite-chlorine dioxide generation system.
A major disadvantage of this system is the limitation of the single-pass gas-chlorination phase. Unless increased pressure is used, this equipment is unable to achieve higher concentrations of chlorine as an aid to a more complete and controllable reaction with the chlorite ion. The French have developed a variation of this process using a multiple-pass enrichment loop on the chlorinator to achieve a much higher concentration of chlorine and thereby quickly attain the optimum pH for maximum conversion to chlorine dioxide. By using a multiple-pass recirculation system, the chlorine solution concentrates to a level of 5-6 g/1. At this concentration, the pH of the solution reduces to 3.0 and thereby provides the low pH level necessary for efficient chlorine dioxide production. A single pass results in a chlorine concentration in water of about 1 g/1, which produces a pH of 4 to 5. If sodium chlorite solution is added at this pH, only about 60 percent yield of chlorine dioxide is achieved. The remainder is unreacted chlorine (in solution) and chlorite ion. When upwards of 100 percent yield of chlorine dioxide is achieved, there is virtually no free chlorite or free chlorine carrying over into the product water. The French system can be designed for variable-feed rates with automatic control by an analytical monitor. This has the advantages of eliminating the chlorine dioxide storage reservoir. Production can be varied by 20 equal increments. A 10 kg/h (530 lb/day) reactor can be varied in 0.5 kg/h (26.5 lb/ day) steps over the range of 0-10 kg/h, and this can be accomplished by automatic control with the monitor located in the main plant control panel.
Another approach to chlorine dioxide production is the acid-sodium chlorite system. The combination of acid and sodium chlorite produces an aqueous solution of chlorine dioxide without production of significant amounts of free chlorine. The acid-based process avoids thé problem of differentiating between chlorine and chlorine dioxide for establishing an oxidant residual. This system uses liquid chemicals as the feedstock. Each tank has a level sensor to avoid overfilling. The tanks are installed below ground in concrete bunkers which are capable of withstanding an explosion. There are no floor drains in these bunkers. Any spillage must be pumped with corrosion-resistant pumps. Primary and backup sensors with alarms warn of any spillage. Because of the potential explosiveness, chemicals are diluted prior to the production of chlorine dioxide. The dilution is carried out on a batch basis controlled by level monitors. Proportionate quantities of softened dilution water along with the chemical reagents are pumped to mixing vessels by means of calibrated double-metering pumps. After the reactor is properly filled, an agitator within the container mixes the solution. Dilutions of 9 percent HC1 and 7.5 percent sodium chlorite are produced in the chemical preparation process. The chlorine dioxide is subsequently manufactured on a batch basis. The final strength of the solution is about 20 percent, 90 percent to 95 percent of this is chlorine dioxide and 4 percent to 7 percent is chlorine.
Chlorine is the most widely used disinfectant in water treatment. It appears, however, that it may not be the best disinfectant to use for drinking water where poor-quality raw water or completely recycled water is used. Other reasons for considering alternative disinfection techniques include the possibility that disinfection by chloramines will allow viruses to remain viable or that the inactivated virus particles have viable nucleic acids that may be released within humans, the reduction of germicidal efficiency with elevated pH, and the formation of persistent chlorinated organic compounds. Chlorine dioxide has proven to be a strong oxidizing agent. When free of chlorine, it does not form trihalomethane compounds in drinking water. It is less likely than chlorine to form chlorinated compounds with most organics commonly encountered in raw water supplies. Chlorine dioxide is effective in oxidizing organic complexes of iron and manganese, imparts no taste and odor to treated water, and provides a highly stable, long-lasting oxidant residual.
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