Evaluation of Emerging Bromate Control Strategies

Abstract

Ozonation of waters containing moderate to high levels of bromide at ambient pH can form bromate at levels that may exceed the United States maximum contaminant level (MCL) of 10 μg/L. A proven technology for minimizing the formation of bromate is ozonation of the water at a reduced (typically acidic) pH. However, in waters with moderate or high alkalinity, the cost of acid and base addition to lower and subsequently raise the pH of the water can be higher than the cost of generating the ozone. To lower costs (minimize the amount of acid and base addition), Metropolitan Water District of Southern California has been conducting a long-term evaluation of alternative bromate-control strategies. Test results indicate that, depending on a utility's by-product goals, that the addition of chlorine/ammonia, ammonia/chlorine or chlorine dioxide upstream of the ozone can be used to significantly minimize bromate formation. This paper provides new insights into emerging bromate-control strategies, as well as potential fatal flaws-the increased formation of other by-products of concern (chlorate, trihalomethanes, N-nitrosodimethylamine) in treated water.

Critical Review on Bromate Formation during Ozonation and Control Options for Its Minimization

“…It is a weak • OH scavenger, which can partly suppress • OH reactions and stabilize dissolved ozone ( k = 5.2–5.7 × 10 8 M –1 s –1 ): , N H 2 C l + O • H → N H C l • + H 2 O In one study, there was no difference in bromate formation for prechlorination contact times 1 or 5 min prior to ammonium addition. This is an indication that monochloramine alone may also play an important role in bromate suppression …”

“…However, because both HOBr and OBr – are oxidized by • OH with similar second-order rate constants (eqs and ), another major benefit of a decrease in pH is increased ozone stability with a lower transient • OH concentration and a lower R ct . A decrease in pH from 8 to 6 typically results in a 50–60% decrease in the level of bromate formation in drinking water, , though a bromate minimization of >90% has also been demonstrated . This approach allows for bromate minimization while meeting disinfection objectives.…”

“…The chlorine–ammonium process was developed to provide enhanced bromate control beyond what is possible with ammonium addition alone. ,, In this process, chlorine is added upstream of ozonation to oxidize bromide to HOBr ( k = 1550 M –1 s –1 ): normalH normalO normalC normall + normalB normalr − → normalH normalO normalB normalr + normalC normall − Five to seven minutes of chlorine contact before ammonium addition is typical, though contact times as short as 1 min have been investigated. ,, Ammonium is then added to form NH 2 Br prior to ozonation (reaction ). Via the formation of NH 2 Br prior to ozonation, the chlorine–ammonium process masks bromine and, in contrast to ammonium addition, can suppress bromate formation initiated by both O 3 and • OH.…”

“…Via the formation of NH 2 Br prior to ozonation, the chlorine–ammonium process masks bromine and, in contrast to ammonium addition, can suppress bromate formation initiated by both O 3 and • OH. The level of bromate formation has been demonstrated to be reduced by 44–94% ,− depending on the treatment conditions (Table and Table S3).…”

“…Instead, bromochloramine can be formed during ozonation ( k = 2.86 × 10 5 M –1 s –1 ): normalN normalH 2 normalC normall + normalH normalO normalB normalr → normalN normalH normalB normalr normalC normall + normalH 2 normalO However, similar to the addition of ammonium, this does not quench the bromine radical pathway. In a comparative study, the ammonium–chlorine process reduced the level of bromate formation from 17 to 3–4 μg/L compared to a value of 2 μg/L for the chlorine–ammonium process . Nevertheless, the ammonium–chlorine process was selected for bromate control because it resulted in a lower level of formation of trihalomethanes (THMs).…”