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

Abstract

Ozone is a commonly applied disinfectant and oxidant in drinking water and has more recently been implemented for enhanced municipal wastewater treatment for potable reuse and ecosystem protection. One drawback is the potential formation of bromate, a possible human carcinogen with a strict drinking water standard of 10 μg/L. The formation of bromate from bromide during ozonation is complex and involves reactions with both ozone and secondary oxidants formed from ozone decomposition, i.e., hydroxyl radical. The underlying mechanism has been elucidated over the past several decades, and the extent of many parallel reactions occurring with either ozone or hydroxyl radicals depends strongly on the concentration, type of dissolved organic matter (DOM), and carbonate. On the basis of mechanistic considerations, several approaches minimizing bromate formation during ozonation can be applied. Removal of bromate after ozonation is less feasible. We recommend that bromate control strategies be prioritized in the following order: (1) control bromide discharge at the source and ensure optimal ozone mass-transfer design to minimize bromate formation, (2) minimize bromate formation during ozonation by chemical control strategies, such as ammonium with or without chlorine addition or hydrogen peroxide addition, which interfere with specific bromate formation steps and/or mask bromide, (3) implement a pretreatment strategy to reduce bromide and/or DOM prior to ozonation, and (4) assess the suitability of ozonation altogether or utilize a downstream treatment process that may already be in place, such as reverse osmosis, for post-ozone bromate abatement. A one-size-fits-all approach to bromate control does not exist, and treatment objectives, such as disinfection and micropollutant abatement, must also be considered.

Keywords: bromate; dissolved organic matter; human carcinogen; ozonation.

Figure 1

Fraction of the reaction of ozone with the bromine radical as a function of the specific ozone dose (the initial ozone dose is taken for the calculations). The blue and red curves represent the lower and higher limits, respectively, of the second-order rate constant for the reaction of Br^• with DOM. The colored area represents the range of the fraction f(Br^• + O₃) (eq 20) for the lower or higher second-order rate constants for the Br^•–DOM reaction (second-order rate constants were obtained from ref (66)) (DOC = 5 mg/L).

Figure 2

Simplified mechanism for bromate formation during ozonation of bromide-containing waters. Adapted from and expanded on the basis of a previous study. Reactions of HOBr/OBr^– with hydrogen peroxide and ammonia are also included.

Figure 3

Fractions of reactions of Br^–, HOBr/OBr^–, and BrO₂^– occurring with ozone or ^•OH as a function of R_ct in the range of 10^–6–10^–9 (^•OH/O₃ concentration ratio). Note that the X-axis and the second Y-axis are reversed.

Figure 4

Overview of bromate mitigation strategies during pretreatment, ozonation, and post-treatment.

Figure 5

Comparisons of dissolution CT and compliance CT for different ozone mass-transfer systems: (a) conventional fine bubble diffuser and (b) sidestream addition.

Figure 6

Tiered approach for the assessment of bromate control strategies based on the results of this work. In the blue area, the focus should primarily be on removing bromide from entering the ozonation process and on minimizing bromate formation through optimizing ozone dissolution, adding specific chemicals that sequester bromide, and/or disrupting bromate formation reactions. In the green area, if these solutions are not viable, then the question of whether ozone should be used should be assessed. The brown area shows options and limitations of downstream treatment for bromate abatement.