| The mechanism by which ammonia inhibits bromate formation when ozonating drinking water was investigated. A kinetic analysis of the chemistry of bromate formation supported earlier hypotheses that ammonia acts by blocking two bromate formation pathways that require free bromine as an intermediate. A third pathway, the so-called Indirect/Direct pathway, does not require free bromine and is not affected by ammonia. Some bromate will therefore continue to form regardless of ammonia concentration.;Ammonia blocks the free bromine pathways by reacting with free bromine to form bromamines, which do not participate in bromate formation. Bromamines are unstable, and their decay opens up the free bromine pathways once more. The stability of bromamines is therefore a key factor in ammonia's ability to inhibit bromate formation. A study of the possible routes of bromamine decay indicated that autonomous decay in water was likely insignificant, while reaction with ozone and natural organic matter (NOM) would lead to substantial bromamine decomposition in an ozone contactor. Ammonia may therefore be more effective at inhibiting bromate formation if the water were first clarified to remove NOM prior to ozonation. The impact of hydroxyl radicals on bromarnine stability could not be assessed due to a lack of a reported reaction rate.;Factors affecting the rate of bromate formation and ammonia depletion when ozonating water containing ammonia were investigated both experimentally and using a deterministic model. Less bromate is predicted to be formed for lower ozone and bromide concentrations, lower pH, and lower temperature. (Bi)carbonate alkalinity may also work synergistically with ammonia to inhibit bromate formation more effectively than either compound alone, due to hydroxyl radical scavenging. In contrast, conditions which promote hydroxyl radical formation (such as hydrogen peroxide formation from reaction between ozone and NOM) may render ammonia less effective.;Of important practical significance is evidence that only a very small amount of ammonia (≈0.05 mg/L as N) is required to block the free bromine pathways of bromate formation. Adding more ammonia will not further limit bromate formation under typical conditions, however greater ammonia doses would be needed to account for ammonia depletion during ozonation. In practice, selection of an appropriate ammonia dose may be best determined by monitoring concentrations in the ozone contactor effluent. An ammonia dose that can provide a contactor effluent residual in the range of 0.05 mg/L is likely sufficient; applying more may not result in further bromate reduction. |
