The effects of ozonation pathways on the formation of ketoacids and assimilable organic carbon (AOC) in drinking water

In the United States most drinking water plants use chlorine for disinfection (98%). However, concern relating to the formation of disinfection by-products has resulted in 332 (OzoneNews, 2000) facilities having replaced chlorination with ozonation. Although ozonation is effective at removing pathogens and organic pollutants, ozonation can result in an increase in the amount of biodegradable organic matter in the treated water, which can enhance the regrowth of bacteria in the distribution system. Moreover, the reaction of ozonation by-products with chlorine during post-disinfection leads to the formation of potentially toxic disinfection by-products (DBPs). The reaction of ozone with NOM (Natural Organic Matter) can occur by two different pathways: reaction of molecular ozone and the reaction of OH radicals, which are produced from the decomposition of molecular ozone. The molecular ozone reaction with NOM would preferentially form AOC and decrease precursors of DBPs rather than that of OH radical reaction. As such, the pathways for the reaction of ozone with NOM were controlled by the factors that affect the pathways of ozone decomposition to control the formation of AOC and DBPs. The ratios of [OH·]/[O₃] under the various ozonation conditions were determined to investigate the pathways forming AOC and DBPs. The effect of operational parameters such as temperature, pH, HRT (Hydraulic Retention Time) and initial ozone concentration on the formation of AOC (Assimilable Organic Carbon), ketoacids, which is a portion of AOC as well as ozonation by-product, during ozonation and DBPs during post-chlorination were investigated. In addition, inorganic additives such as hydrogen peroxide, bicarbonate ion and phosphate ion, which affect the pathways of ozone decomposition, were used to control the formation of AOC, ketoacids and DBPs during ozonation. The conditions where the molecular ozone reaction predominated resulted in an increase in the formation of AOC and resulted in the formation of DBPs precursors.