Full Description

This paper describesa CO2 addition system that was installed at the Alameda County Water District's treatment plant in California. CO2 was added to lower the pH of the raw water before it enters the preozone contactor. The goal of CO2 addition was to reduce the formation of bromate with ozonation. Preliminary tests were conducted to evaluate the effectiveness of CO2 addition for bromate control, and its impact on the overall treatment cost. The highlights of the information presented in this paper include: a user-friendly CO2-chemistry chart that was used to determine the relationship between CO2 dose and pH of natural waters, and full-scale results obtained confirmed the accuracy of the chart in predicting the impact of CO2 dose on water pH; comparison of the measured CO2 doses to calculated doses suggested that the CO2 transfer efficiency was close to 100%; under the raw water conditions tested, the addition of 25 mg/L CO2 was required to lower the pH of the water from 8.0 to 7.1; the tests conducted before and after the installation of the full-scale CO2 feed system showed that bromate reduction can be achieved with CO2 addition; and, adding CO2 reduced the ozone dose required to meet a specific disinfection CT value. However, the cost increase due to the increase in CO2 dose is greater than the cost savings due to the decrease in ozone dose. Therefore, the plant's CO2 addition strategy should focus only on minimizing bromate formation. Finally, it was noted that, while pH depression reduces bromate formation, it also reduces the ozone decay rate, which in turn increases the ozone CT through the contactor. Since bromate formation is a function of both pH and ozone exposure (expressed as CT), it is conceivable that moderate depression in pH without reducing the ozone dose may actually result in an increase in bromate formation. Includes 3 references, tables, figures.

Product Details

Edition: Vol. - No. Published: 06/16/2002 Number of Pages: 16File Size: 1 file , 520 KB