Study of the production of bromate during ozonation of Paris-area source waters: ozonation mode and variation of the physical and chemical parameters
The publication of Kurokawa et al. in 1990 confirming the toxicity of bromate of rats and mice, initiated the research effort that was internationally conducted during the last seven years to better understand the reaction mechanisms of bromate formation during the ozonation of natural waters. Based on the research findings regarding the effect of a number of parameters (bromide, ozone dose, pH, temperature, alkalinity, DOC content, ammonia, ...), predictive models (empirical and reaction kinetic based models), including molecular and/or radical pathways, have been developed with more or less success. Complementary results are still needed to better understand this complex mechanism. The main objective of our work was to evaluate how the seasonal variation of the physical chemical characteristics of Paris-area source waters (i.e. bromide content, temperature, natural organic matter) can affect the production of bromate during ozonation. In order to confirm that lab-scale experiments could be proposed to develop such research program, parallel tests were first conducted at the bench- and pilot-scale based on comparable C∙τ conditions. The lab-scale reactor was a 380 ml glass column (internal diameter: 0.02 m; height: 1.2 m) equipped with a water jacket to allow temperature to be varied and maintained. These reactor was used as a continuous flow reactor with recirculation. The pilot-scale ozonation contactor installed at the Méry sur Oise water treatment plant was comprised of four 30-liter columns in series (diameter: 0.1m ; height: 4m). The first column is used as the application column while the three others are used as residence column. The results have shown that lab-scale ozonation experiments conducted on Méry sur Oise sand filtered water led to similar results compared to pilot ozonation conducted on the same water and at the same temperature (sampled the same day) using the Méry sur Oise pilot-scale reactor. For applied C∙τ that ranged from 4 to 20 mg O3/L.min, similar linear relationship between bromate formation and applied C∙τ was obtained with the two reactors. A survey conducted on the Oise River has shown that the bromide concentration ranged from 40 μg/L (winter period) to 80 μg/L (summer period). If it is already well known that higher the bromide content, higher the bromate formation, our work has also pointed out that even a small increase of the bromide concentration from 80 to 95 μg/L (15 μg/L of bromide spiked as KBr) can significantly impact the bromate formation (same experimental conditions) that, as an example, increased from 16 to 27 μg/L for C.t of 10 at 21 °C. The temperature of the Oise river can vary from 5 °C up to 25 °C. Using carefully controlled temperature conditions, one can observed that the slope of the bromate production versus applied C∙τ increased with increasing temperature (same water). For example, the production of bromate during the ozonation (applied C∙τ=10) of the Méry sur Oise sand filtered water was 7, 10 and 16 μg/L for 5, 13 and 21 °C, respectively. Complementary experiments, have shown that the impact of the variation of the initial bromide concentration was proportionally more important for low-temperature water (5 to 13 °C) than for moderate-temperature water (20 °C). The origin and nature of the water is considered to play a significant role on the formation of bromate during ozonation, however few studies have evaluated the importance of these parameters using carefully controlled experimental conditions. In order to better define how important is the change in bromate production with the modification of the quality of the Paris suburbs water sources, especially the organic content (nature and concentration of the NOM), two sets of experiments were conducted. In the first part of the work, the Méry sur Oise sand filtered water was sampled at three different periods of the year 1996 (June, July and December), and the ozonation experiments were conducted at the same temperature (21 °C) after bromide concentration was adjust to 80 μg/L. The three water samples had the same pH and did not contain ammonia. Significant differences were observed in the bromate production, showing a larger production with the winter water as compared to the summer water. The fact that the winter water was enriched in DOC (3.7 mg/L of DOC) as compared to the two others (2.6 - 2.7 mg/L of DOC) may explain this difference since a larger ozone dose was probably necessary (ozone transfert not controlled because of the small size of the lab-scale reactor) to reach the same applied C∙τ due to a higher ozone consumption from the natural organic matter. The slightly lower alkalinity of the winter sample (200 mg/L as CaCO3 as compared to 250 mg/L CaCO3 for the summer samples) could have led to a less pronounced scavenger effect, condition that favors the radical pathway which is generally predominant. However, it is also known that carbonate species can also promote the formation of bromate due to the production of carbonate radicals. Comparing the results obtained with the water samples collected during the summer period, more bromate was produced in July than in June. The higher hydrophobic character (more aromatic in character) of the NOM of the water sampled in July (SUVA=2.15) as compared to the June sample (SUVA=1.88), characteristic that favor the ozone consumption and consequently the OH radical production, may justify this finding. In the second part of the work, the bromate formation obtained during the ozonation of the three major water sources of the Paris suburbs (sampled after clarification), Oise River, Marne River and Seine River, was compared (same temperature) after the bromide content was adjust to 80 μg/L. Similar results were obtained with the clarified Marne river and Seine River, the two waters showing the same physical chemical characteristics (2.2 and 2.5 mg/L of DOC; pH 7.9 and 7.8; Alkalinity: 225 and 210 mg/L as CaCO3). A lower production of bromate as a function of the applied C∙τ was observed with the clarified Oise river, result that is in contradiction with our previous hypotheses since this water source showed the highest DOC content, the highest SUVA and the lowest alkalinity among the three waters studied. More work needs to be done to better understand the impact of the origin and nature of the NOM on the bromate formation mechanisms. As a general conclusion, this work also confirmed that the physical chemical characteristics of source water (DOC, temperature, alkalinity, bromide content,...) are more important factors as compared to the hydraulic characteristics of the reactor. Keywords .