Figure 8 presents the results of TOC removal efficiency in the ozonation and O3/UV processes. The removal efficiency of TOC in the batch ozonation is below 6 %. This evidence suggests that the electrophilic character of ozone could only oxidize and destroy a small amount of the aromatic structure and unsaturated bond of organic matter without mineralizing the organic carbon to form carbon dioxide as well as the destruction by hydroxyl radical. Therefore, the reduction of these organic precursors in the ozonation process is very limited. The removal efficiency of TOC for three model compounds was found to be over 40 % in the O3/UV process, which suggests that the higher hydroxyl radical exposures (O3/UV) could effectively reduce the TOC concentration. The effect of alkalinity on removal of TOC was presented in Figure 8 which indicates that the natural inhibitor (alkalinity) could be negligible because of the insignificant removal efficiency of TOC in ozonation.
Organic compounds with aromatic structures or conjugated double bonds would absorb light in the ultraviolet wavelength range, commonly 254 nm (UV254). SUVA is defined as a ratio between the ultraviolet absorbance (UV254) and the concentration of TOC in water, i.e., UV254 (m-1)/TOC (mg/L). The change of the value of SUVA is shown in Figure 9. The most aromatic structure and conjugated double bonds are destroyed by ozone and hydroxyl radical resulting in the high UV254 decrease, which results in the low value of SUVA. According to an Edzwald and Van (1990) study, when the value of SUVA is smaller than 2, the composition in the sample is mostly non-humics, low hydrophilic materials, and low molecular weight. In other words, the sample contains relatively small amount of aromatic moieties. Therefore, the lower SUVA after the ozonation and O3/UV processes indicates that ozone and hydroxyl radical can effectively destroy the aromatic structure and also reduce chlorinated
by-products formation potential (Rook, 1976). As shown in Figure 9, the difference of SUVA for the three model compounds is insignificant, because of their similar benzene structure, to which the attack of ozone following Crigee mechanism and the nonselective reactivity of hydroxyl radical result in having similar TOC and UV254 removal.
Formation of Ozonation by-Products
According to a Glaze study (1986), the ozonation by-products include aliphatic aldehyde, hydrogen peroxide, organic peroxide, and saturated carboxylic acid. Among them, aldehyde is the most concerned because of its harmful to human health.
Aldehyde consists of formaldehyde, acetaldehyde, glyoxal, and methyl glyoxal that are commonly found and investigated in ozonation process. Figure 10 shows the formation of the ozonation by-product (aldehyde) for resorcinol at different levels of pH and alkalinity treated by the ozonation and O3/UV processes. In this study, the principal aldehyde formation is formaldehyde, especially at high pH. For instance, at pH 9 the ratio of formaldehyde in aldehyde formation is up to 70 %, while at pH 7 is 50 %, and pH 5 is 39 % in resorcinol. This formation suggests that hydroxyl radical (formed at pH 9) could destroy organic compound and generate shorter chain by-products such as formaldehyde than ozone molecule (formed at pH 5). In general, the order of the aldehyde formation concentration is O3 (pH 9) > O3 (pH 7) > O3 (pH 5). Similar observations for phloroglucinol and p-hydroxybenzoic acid were also found in this study.
As shown in Figure 10, the addition of alkalinity would decrease the aldehyde concentration in the indirect ozone process. The phenomenon conforms to the
concentration to inhibit oxidation reaction and result in less aldehyde formation. In the O3/UV process, the higher hydroxyl radical exposure reduces TOC by 40 % and further oxidization results in lowering aldehyde concentration to 2 µg/L. In summary, the order of aldehyde formation with respect to the ozonation process is O3 (pH 9;
Alk=0) > O3 (pH 9; Alk =60) > O3 (pH 7; Alk=0) > O3 (pH 7; Alk=60) > O3 (pH 5) >
O3/UV. It was thus concluded that the ozone and hydroxyl radical could break the aromatic structure and destroy organic precursors in the ozonation process.
Formation of Chlorination by-Products
Among the chlorine demands for these three model compounds, resorcinol is the lowest. It could be explained that the two activating –OH groups in resorcinol are situated at vicinal position to stabilize the transition state of the reaction through the donation of electron density. Therefore, the electrophilic addition and substitution reactions by chlorine easily occurs, which leads to low chlorine demand (Boyce and Hornig, 1983). However, the symmetric structure for phloroglucinol flanked with three –OH groups may form a resonance-stabilized intermediate, which could confine the hydrolysis and decarboxylation with C–C bond cleavage on the aromatic structure and result in more chlorine demand (Chang et al., 2006). For p-hydroxybenzoic acid, the moderately deactivating group (–COOH) would lower the electron density on aromatic structure, but not for the symmetric structure such as phloroglucinol.
Therefore, the order of chlorine demand is strictly depended upon the physical and chemical property of the model compounds and followed by P > PHBA > R. In this investigation, the destruction of organic precursors by hydroxyl radical results in the higher chlorine consumption than ozone molecular during the chlorination process,
and the inhibition of alkalinity would increase the chlorine consumption. The detailed experimental data are listed in Table 4.
Chlorination of natural organic matter results in the formation of various chlorination disinfection by-products (DBP). Among all DBP, the THM and HAA are considered as the principal disinfection by-products which cause public health concerns for safe drinking water. The comparison of specific DBP formation potentials (DBPFP) and DBP yield coefficient (D) between the ozonation and O3/UV processes are also shown in Table 4. As mentioned earlier, the ozone and hydroxyl radical could change the properties in the three model compounds by destroying the aromatic structure, which leads to more reduction of chlorine demand and DBPFP. In the O3/UV process, the 40% TOC reduction performed by the hydroxyl radical would also enhance the reduction of DBPFP. Therefore, the reduction of DBPFP by the O3/UV process is much higher than that by the ozonation process. The relationship between DBP formation and chlorine demand could be evaluated by the DBP yield coefficient (D). Table 4 shows the values of D in different processes. The order of D is similar to the order of DBPFP as O3/UV system < < ozonation.