Enhanced bromate control during ozonation: the chlorine-ammonia process

Area of Science:

• Environmental Chemistry
• Water Treatment Technologies

Background:

• Bromate, a potential carcinogen, forms during ozonation of bromide-rich waters.
• Current methods like ammonia addition and pH depression may not meet future regulations for bromate levels.

Purpose of the Study:

• To evaluate the efficacy of the chlorine-ammonia process in minimizing bromate formation during water ozonation.
• To elucidate the mechanisms behind bromate reduction using the chlorine-ammonia process.

Main Methods:

• Investigated the chlorine-ammonia (Cl2-NH3) process, involving prechlorination followed by ammonia addition before ozonation.
• Compared bromate formation with conventional ozonation and ammonia-only processes.
• Analyzed the roles of hypochlorous acid (HOCl), ammonia (NH3), and dissolved natural organic matter (DNOM) in the reaction pathways.

Main Results:

• The Cl2-NH3 process reduced bromate formation by fourfold compared to the ammonia-only process.
• Specific mechanisms include HOCl oxidation of bromide, NH3 masking, and DNOM acting as a bromine sink.
• At pH 8, bromate decreased from 35 µg/L to 5 µg/L; further pH depression to 6 reduced it by another factor of four.
• Byproducts like trihalomethanes (THM) and cyanogen chloride (CNCl) remained below regulatory limits.

Conclusions:

• The chlorine-ammonia process is a highly effective strategy for mitigating bromate formation in drinking water treatment.
• This process offers a promising solution for facilities needing to comply with stringent bromate regulations.