ABSTRACT
Several pilot plant experiments were done to assess the ozone doses that may be applied for the combination of ozonation and Granular Activated Carbon (GAC) filtration. The objective of these experiments was to relate (i) formation of bromate, (ii) formation of Assimilable Organic Carbon (AOC), and (iii) the Ct (mg O3/l · min) value achieved to the ozone dose applied.
All the experiments conducted show that ozone doses lower then 0.8 mg/l result in less than 10 μg/l, which is the standard proposed for the European Union. Doses up to 3 mg/l can be applied when taking into account that about 96% of bromate formed will be removed by reverse osmosis, which is planned as the final step for the extension of the treatment capacity at the Amsterdam Water Supply’s Leiduin plant.
Ozonation was found to increase the concentration of AOC. The higher the ozone dose, the lower the AOC/O3 ratio obtained: an ozone dose of 0.75 mg/l increases the concentration of AOC for more than 50% of the maximum 170 μg Ac-C/l that were formed for doses up to 3 mg/l. No pronounced seasonal effect was observed regarding the AOC formed for a given ozone dose.
An ozone dose of 1.5 mg/l may be expected to result in the Ct value sufficient for, at least, 2-log inactivation of viruses and 1-log inactivation of Giardia cysts, while a dose of 3 mg/l may be expected to result in at least 14-log and 7-log inactivation of viruses and Giardia cysts, respectively. Disinfection in the full-scale plant can be higher, if the short-circuiting in ozone contact tanks can be limited. No pronounced seasonal effect was observed regarding the disinfection credit achieved for a given ozone dose.
