THE USE OF CHLORINE AND AMMONIA TO REDUCE BROMATE FORMATION DURING OZONATION

The Southern Nevada Water System (SNWS) has undertaken a pilot testing program to verify the bromate formation when ozone is added to achieve 2-log Cryptosporidium inactivation and to investigate possible bromate mitigation techniques and its effect on biologically activated filtration. The pilot evaluated three bromate mitigation techniques, the addition of a small amount of prechlorine, the addition of a small amount of preammonia, and the combination of prechlorine and preammonia. Results show bromate formation would likely exceed the proposed MCL of 10 ppb when ozone was added to produce a 2-log Cryptosporidium inactivation and no mitigation technique was used. Bromate formation could be reduced without a mitigation technique by lowering the inactivation to approximately 0.75-log to meet the project goal of 5 ppb and to approximately 1.0-log to meet the MCL. The addition of a small amount of prechlorine did not significantly reduce bromate formation when compared to no mitigation technique. However, preammonia addition and the combination of prechlorine and preammonia addition did significantly reduce bromate formation to 5 ppb at 2-log Cryptosporidium inactivation. Although a dose of 0.5 mg/L of ammonia alone was required to meet the bromate goal, adding 0.5 mg/L of prechlorine followed by ammonia allowed the ammonia dose to be reduced to 0.1 mg/L and still meet the 5 ppb goal. During preliminary testing, it was determined that ozonated Lake Mead water contained an interference with bromate quantitation when using EPA method 300.1. Using EPA method 317 or pretreating the sample with a H+ cartridge eliminated the interference.