Ammonia removal in bubble column by ozonation in the presence of bromide

Decomposition of ammonia to N2 in wastewater was accomplished by ozonation in the presence of bromide with high efficiency. To acquire the optimum condition for ammonia removal with minimum nitrate formation, the effects of Br/N ratio, pH, and initial ammonia concentration were investigated using a continuous bubble column with an effective volume of 32 l. Synthetic wastewater was fed into the reactor from the top, and O3 gas was introduced from the bottom through a sintered glass diffuser. The effluent pH was automatically maintained at given values with NaOH, and effluent NH4+-N was automatically maintained at 5 mg l−1 by adjusting the O3 production rate. In the process, N2 was the main product of ammonia decomposition with nitrate as the main nitrogen by-product. The formation of nitrate was mainly affected by the Br/N ratio, N load, and pH. The ratio of NO3−-N to influent NH4+-N was nearly linearly related to the Br/N ratio in a log–log plot, and was lower than 5% at a Br/N higher than 0.40 under following conditions: influent NH4+-N, 600 mg l−1; N load, 3.6–4.0 kg m−3 d−1; effluent pH, 6.9. Less nitrate was obtained at lower N loads. The minimum NO3−-N was obtained around pH 6.0. The ratio of NO3−-N to influent NH4+-N decreased with increasing influent NH4+-N and was ca. 1.5% under the following conditions: influent NH4+-N, 1200 mg l−1; effluent pH, 5.8; N load, 2.8 kg m−3 d−1; Br/N, 0.60. An O3 absorption efficiency of more than 95% was obtained, and ca. 5.4 g O3 was needed for removing 1 g NH4+-N under proper conditions. Nitrite formed in the upper part of the reactor when the pH in the upper part was higher than 7.0, and was oxidized to nitrate in the lower part. It was necessary to reduce the vertical pH distribution in the reactor to prevent nitrite formation in the upper part. At an HRT of 2 h or lower and a Br/N>0.40, BrO3−-Br, a toxic byproduct of ozonation, was produced at a level of less than 0.1 mg Br l−1, and it increased with longer HRT or lower Br/N ratio. The minimum concentration of BrO−-Br, an intermediate of ozonation, was obtained at an effluent pH near 6.0. BrO−-Br could be quantitatively decomposed by Na2SO3 under ORP control. The results demonstrated that the process used in this study could be utilized for efficient ammonia removal for such inorganic wastewaters as condensate demineralization wastewater from thermal power plants, etc.