Concerns for the environment, operator safety, and liability issues coupled with the 

trend toward more alkaline treatment approaches have changed the acceptability of chlorine. When industrial water systems operate at pH values in the 6.5-7.5 range, chlorine remains the primary toxicant of choice from a technical perspective for microbial control. However, because of the increase in environmental regulations limiting the discharge of chlorine and the popularity of alkaline treatment programs, the industry has turned to alternatives. Bromine chemistry represents cost-effective, oxidizing microbial control especially in alkaline waters and where nitrogen compounds may be present as a contaminant in the industrial water stream.

Similar to chlorine, when bromine is added to water, it ionizes into hypobromous acid and the hypobromite ion. The reaction is shown below.

Like its chlorine analog, hypobromous acid is an extremely strong oxidizing agent. It easily diffuses through the cell wall of the microorganism and reacts with the cytoplasm to form chemically stable bromine-nitrogen bonds with the proteins of the cell. Bromine oxidizes the active sites on certain sulfhydryl groups which constitute intermediate steps in the production of ATP, essential for respiration. The extent of ionization determines the extent of microbiocidal activity. 

This is due to the fact that bacteria carry an overall negative charge. As such, the negative charge associated with the hypobromite ion limits its ability to penetrate the cell wall and oxidize the protein group. The neutral hypobromous acid can enter the bacterial cell unencumbered and provide effective disinfection. One of the major differences between bromine and chlorine is the dissociation constant (pK) for hypobromous acid which is approximately one pH unit higher than that of hypochlorous acid (pK HOBr = 8.7; pK HOC1 = 7.5). 

Therefore, in the more alkaline industrial water systems, bromine is maintained in the active species longer than its chlorine counterpart (Figure 5-23). A comparison of the curves shows that the active hypobromous acid is dominant up to a pH of 8.7, while at a pH of 7.5 there is only fifty percent active chlorine. Clearly, the ability of bromine to function effectively at elevated pH represents a distinct advantage over chlorine.

Various levels of ammonia and/or nitrogen-based organics are commonly encountered in industrial water systems. Bromine and chlorine are similar in their reactions with nitrogen compounds. In the presence of ammonia or amines, both bromine and chlorine will form a series of "animated" complexes forming mono-, di- and trihaloamines as shown below:

The monohaloamines and dihaloamines are the most prevalent species under typical industrial water conditions. At a pH of 8.5, monochloramine formation is virtually complete.

The contrast between chloramines and bromamines is significant in regard to biocidal activity. The chloramine species yields an eighty-fold reduction in the effectiveness of the original hypochlorous acid form. Bromamines, on the other hand, have been shown to have disinfection properties comparable to that of the uncomplexed, hypobromous acid form.

The key to increased biocidal activity is the fact that the reaction between HOBr and NH₃ is reversible; whereas, the reaction between HOC1 and NH3 is not.

Bromine can be fed to a industrial water system via a number of methods. Liquid sodium bromide is supplied as a neutral salt solution and when mixed with an activator, most often liquid hypochlorite or chlorine gas, delivers hypobromous acid to the treated system. The feed equipment is fairly simple and in the case of liquid hypochlorite, relatively safe. Solid bromine donors like the chloro-bromo hydantoins are also popular methods to apply a bromine microbiological control program (Figure 5-25). These materials are supplied as tablets, granules or powders and offer the added advantage of being safe to handle and feed. Unlike chlorine, neither of these two methods of bromination significantly affect the system pH.

The reaction depicting the generation of bromine by sodium hypochlorite or chlorine gas is shown below.

NaBr + HOC1 -> HOBr + NaCl

Generally, bromine programs are controlled in the same manner as chlorine with system water treated to meet the bromine demand and run a slight residual level halogen. Normal applications carry a 0.2-0.4 ppm residual of free available bromine (as C1₂).