CONTROL OF CHLORINOUS ODOR IN DRINKING WATER BY OXIDATION PROCESSES AND CHARACTERIZATION OF CHLORINOUS ODOR PRECURSORS BY FRACTIONATION TECHNIQUE

The increase in customer’s complaints about chlorinous odor in drinking water has led water utilities to seek methods to improve the water quality. A promising treatment process known for musty odor removal is ozonation. During ozonation, ozone (O3) and its decomposition product (•OH) are utilized to oxidize micropollutants prior to chlorination. Ozonation, however, produces bromate ion, a probable carcinogen in humans. The simplest way to elevate •OH and minimize bromate ion at the same time is an advanced oxidation process (AOP). This study attempted simultaneous control of chlorinous odor and bromate ion in drinking water treated by ozonation and AOP (O3/H2O2 process) with operation in batch and continuous modes. The contribution of trichloramine (NCl3, a suspected odor compound causing chlorinous odor in drinking water) to chlorinous odor in drinking water was evaluated using the triangle sensory test and headspace GC-MS analysis. Furthermore, to effectively formulate treatment process for chlorinous odor control, aquatic organic matter in Yodo River was characterized using a fractionation technique in order to seek the major contributors to chlorinous odor. To evaluate the effectiveness of ozonation and AOP in controlling chlorinous odor, a bench-scale ozonation reactor was operated in batch mode under various reaction conditions. The strength of chlorinous odor, indicated by “Threshold Odor Number (TON)”, decreased by approximately 50% for the water treated by ozonation and AOP compared to chlorination alone. Changes in the concentrations of ammonium ion and bromide ion, as well as pH, had no effects on odor strength reduction. Increasing the O3 dose to extremely high levels led to slight improvement of chlorinous odor removal, while typical O3 doses (1-2 mg/L) seemed to have no effect on chlorinous odor reduction. Trichloramine was minor odor compound in chlorinous odor. The effect of ozonation and AOP on chlorinous odor removal in a more practical setup was evaluated with an ozone column contactor. Results of chlorinous odor removal obtained using this continuous reactor quantitatively corresponded with those obtained using the bench-scale reactor, indicating that differences in operation mode have no effect on chlorinous odor reduction. Although trichloramine was a minor odorous compound in water treated by conventional treatment processes, trichloramine can cause a limitation of odor control by ozonation and AOP (note that ammonium ion, which is a trichloramine precursor, cannot be oxidized by O3 and •OH). The lowest odor strength of source water treated by ozonation and AOP was approximately 30. The molar ratio of H2O2 to O3 at 0.5 (AOP0.5) was found to be effective for the formation of •OH. Furthermore, it was observed that the oxidation of organic compounds in natural river water leads to the formation of ammonium ion. Based on the musty odor removal by ozoantion and AOP, AOP0.5 with an O3 dose of 1 mg/L should be utilized for simultaneous control of musty odor and bromate ion below their Japanese standard values when the concentrations of musty-odor compounds (e.g., geosmin and 2-methylisoborneol (2-MIB)) in source water increase to 77 ng/L and 40 ng/L for geosmin and 2-MIB, respectively. When the concentrations of geosmin and 2-MIB are higher than 125 ng/L and 83 ng/L, respectively, AOP0.5 with higher O3 dose is necessary. Source water treated by rapid sand filtration (RSF) mainly consisted of hydrophobic acid (HoA), hydrophilic acid (HiA), and transphilic (Trs), while in the water treated by ozonation, hydrophilic fractions (HiA, hydrophilic neutral (HiN), and Trs) were dominant. Base fraction (Bas), including hydrophilic base (HiB) and hydrophobic base (HoB) fractions, was the major fraction causing chlorinous odor in water treated by RSF. Chlorinous odor in ozonated water was mainly produced during the chlorination of HiA and HiN fractions. With the fact that ozonation followed by granular activated carbon (GAC) or biological activated carbon (BAC) is normally used, and trichloramine formation from ammonium ion in ozonated water may not be negligible. Thus, the treatment of ion exchange after ozonation/AOP and GAC is recommended in order to eliminate the remaining odor precursors such as ammonium ion. To my parents, brother and sister, and all of my friends