Biological Bromate Reduction in the O3 -STEP Filter: Mechanism and Relationship with Denitrification

Abstract

Bromate is a possible human carcinogen that does not naturally exist in surface or groundwater bodies. Its formation mostly results from ozonating bromide-containing water during wastewater treatment or drinking water production. Bromate is difficult to remove from water due to its high solubility and low reactivity in the aqueous environment; however, some bacteria showed the ability to reduce bromate to bromide, but the mechanism is unknown. Previous studies postulated a cometabolic pathway of biological bromate reduction, a side reaction of denitrification using the same enzymes. This study investigates the biological bromate reduction mechanism with a focus on its relationship with the denitrification process.

Wastewater and biologically active granular activated carbon (BAC) from a methanolsupplemented pilot filter called O3 -STEP in the wastewater treatment plant (WWTP) Horstermeer, the Netherlands, were used for research. This study first measured the crucial water quality parameters at eight different heights in the filter to investigate the redox condition’s influence on biological bromate removal. After that, batch experiments were conducted to validate the findings. The filter showed the ability to remove bromate as it lowered the bromate concentration from 2.7 to 0.9 µg/L. Bromate reduction happened at all depths including the supernatant, although the redox conditions significantly changed. Decreasing nitrate and dissolved oxygen (DO) concentrations did not change the bromate reduction rate in the filter. The batch experiments confirmed that nitrate did not affect bromate reduction. However, a DO concentration of 8 mg/L led to a 50% reduced bromate reduction rate compared to anoxic conditions. Experiments with varying chemical oxygen demand (COD, in the form of methanol) concentrations showed an extensive accelerating effect on bromate reduction. This explained why the bromate reduction rate was not lower at high DO levels in the filter, as the high COD concentration promoted bromate reduction. Nitrate reduction was found to have a high positive correlation with bromate reduction in both filter and batch experiments, indicating similarities in their mechanisms. Nitrate reduction happened under highly oxic conditions. The intensive mixing of the granules in the filter may have provided alternating aeration and anoxic conditions for the enrichment of aerobic denitrifiers.

This study is the first study to observe simultaneous nitrate and bromate reduction under oxic conditions. Taken together, biological bromate reduction is likely to be a synergetic cometabolic process of aerobic denitrification. The robustness of the biological bromate reduction under high DO and nitrate conditions enables the O3 -STEP ® filter to steadily produce bromate-free effluents under more extreme influent conditions.