Bromate Formation Control With CO₂ Addition

Ozone is not only a powerful disinfectant that can inactivate two insects (Cryptosporidium and Giardia) which are hard to inactivate by traditional methods of disinfection, but also a strong oxidants that can break down harmful substances in water, so the ozone disinfection process has been widely used in drinking water treatment. But when there is certain concentration of bromide in water, the ozone oxidation process may produce a potential carcinogen-bromate. In this paper, the subjects is the low alkalinity drinking water in South China. A new bromate formation control method-adding CO₂ was discovered, and through chemical kinetics simulation and bubble column experiments the mechanism and effect of bormate control was studied。pH depression can inhibit the direct way and indirect way of bromate formation. Ammonia affect the direct way and direct-indirect way of bromate formation. Adding CO₂ into water has two effects, one is to reduce the pH value which inhibit direct and indirect way of bromate formation, the other effect is adding CO₃^2- and HCO₃^- into water, they can capture OH·and inhibit the indirect way, but the direct-indirect way may be promoted to a certain extent, so that adding CO₂ may resulted in slightly higher bromate formation than pH depression overall, but the increase rate is very small. The combined use of ammonia and CO₂ can inhibit all three pathway of bromate formation. In the case without ammonia addition, with each unit of pH decreased, CO₂ addtion will reduce 38.01⁶5.49% bromate formation. At the end point pH=7.4, CO₂ addtion reduce slightly (4.2%) less bromate formation than adding acid. When pH=6.9, adding acid can reduce the bromate formation even more significantly (8.8%) than CO₂ addtion. When the ammonia-nitrogen was 0.20 mg/L in water, It will reduce 73.9% bromate formation. When the pH value was reduced to 7.4, CO₂ addtion can reduced 11.3% of the bromate formation more than adding acid, as the pH decrease to 6.9. CO₂ addtion can reduce more than 23.5%. It suggests that the combined use of CO₂ and ammonia can inhibit all the three ways of bromate formation. It is a more reasonable approach. CO₂ addtion method has little effect on the bromate formation. There is nearly nothing different between CO₂ premixed contract and through direct aeration from aeration diffuser. So there is no need to add any additional equipment, CO₂ can be directly aerated to bubble column through the diffuser. Adding CO₂ to the disinfection process did not have any side effects to disinfection efficiency, On the contrary it can greatly improve the inactivation efficiency of Bacillus subtilis. It costs only 21 yuan to add CO₂ to every ten thousand tons water , and there is no need to add mixing facilities.