Optimization of ozone and coagulation processes for bromate control in Istanbul drinking waters

Unplanned transportation systems near water resources have not only caused a dramatic population increase in the water catchment areas of Istanbul but also created major water pollution problems as a result of illegal settlement and industrial and agricultural activities. In this study, water qualities in seven main water reservoirs of Istanbul were monitored for 1 year. The water was found to be highly polluted in all reservoirs. Also bromide levels of these reservoirs were monitored for the first time for the evaluation of applicability of ozone oxidation for the treatment of raw waters. Bromide level was found very high in the Buyuk Cekmece (BC) reservoir. Thus, ozone was applied to determine bromate formation potential of BC raw water and to optimize ozone and coagulation processes for bromate control in bromide-containing BC surface water. An optimum ozone/TOC ratio and aluminum sulfate dose were determined as 0.8 mg/mg and 40 mg/l for the control of bromate formation. TOC (50%) was removed after coagulation of previously ozonated BC water.

Fig. 6. Effect of pre-ozonation on the TOC removal. process. As presented in Fig. 6, 22% TOC removal was achieved at the optimum alum dose of 60 mg/L Al2(SO4)3. The pH decreased with increasing alum dose, and it was found to be around 6.5 using an optimum alum dose of 60 mg/L. A pre-ozonation experiment was performed with a 22 mg/L.min ozone dose in 1 min; the zzone/TOC threshold was previously found to be about 0.8 mg/mg. Both pH and TOC change during ozonation was insignificant (<2%). The bromate concentration was found to be 8 µg/L after ozonation. Alum coagulation was carried out Bromide levels of seven drinking water reservoirs in Istanbul were followed for 1 year. In terms of bromate formation during ozonation, bromide levels in Elmali, Alibeykoy and Sazlidere were found to be insignificant, while bromide levels in Omerli and KC may be significant to form bromate during ozonation. The highest bromide level was observed in the BC reservoir due to seawater intrusion, and it was at maximum amounts during the summer because of the decreasing water level in the reservoir. Aluminum sulfate was used for the coagulation of BC raw water without using preozonation. An optimum aluminum sulfate dose was determined as 60 mg/L and a maximum of 22% TOC was removed after coagulation. Pre-ozonation was applied before the coagulation process and the optimum ozone/TOC ratio for the control of bromate formation during ozonation of BC water was found to be 0.8 mg/mg. After pre-ozonation, alum coagulation was applied, and it was found that pre-ozonation enhanced the efficiency of alum coagulation. The optimum aluminum sulfate concentration achieved was 40 mg/L and about 50% TOC was removed at the optimum aluminum sulfate dose. Aluminum sulfate coagulation was also carried out for the removal of 54 µg/L bromate concentration that was formed in BC water after applying an over-ozone dose (ozone/TOC ratio higher than 0.8 mg/mg). Bromate removal was insignificant at the optimum alum concentration. Bromate removal of 26% was gained using 100 mg/L alum concentration. Results indicate that aluminum sulfate coagulation after pre-ozonation is not applicable for bromate removal in BC water.