The role of boundary conditions in the bromide-enhanced ozonation process for ammonia nitrogen removal and nitrate minimization

In this study we investigated the effect of some operating conditions, such as the ratio between ammonia nitrogen (AN), ozone and bromide, the initial value of water pH, the amount of bicarbonate used as a buffering substance and the possible interference of organic matter, in a bromide-enhanced ozonation process for the removal of AN. One of the peculiarities of this process is that the oxidation of AN proceeds through the generation of reaction intermediates (bromoamines) and ends with molecular nitrogen (N2) with none or limited generation of nitrate. The results of the ozonation tests carried out at a lab scale demonstrated that nitrate generation could be lowered to less than 10% of the theoretical value when three conditions coexist. Firstly, all the AN was present in the form of ammonium ion (NH4 +); this happened when the pH of the system was lower than 8. Secondly, the ratio between bromide and ozone species was optimized, so as to avoid the presence of free ozone and the consequent direct reaction with AN. Finally, the acidification induced by the oxidation of AN was controlled with a buffering substance (bicarbonate). The generation of inorganic bromoamines from the reaction between AN and bromide inhibits the oxidation of bromide to bromate. In fact, in the tests carried out in this study the formation of bromate was limited to values of less than 0.2% of the molar bromate yield. Organic substances in the form of glucose or potassium hydrogen phthalate (KHP) did not affect the removal rate of AN. Conversely, the presence of substances such as phenol and hydroquinone, that had a higher reactivity towards ozone and HOBr, determined a reduced AN removal rate and a bromate generation of one order of magnitude higher than that observed with KHP and glucose. Finally, all the organic substances employed in the study seemed to have a positive effect in reducing the nitrate generation (-30% of the value obtained with sole AN).