Disinfection of water containing natural organic matter by using ozone-initiated radical reactions

Abstract

Ozone is widely used to disinfect drinking water and wastewater due to its strong biocidal oxidizing properties. Recently, it was reported that hydroxyl radicals ((.)OH), resulting from ozone decomposition, play a significant role in microbial inactivation when Bacillus subtilis endospores were used as the test microorganisms in pH controlled distilled water. However, it is not yet known how natural organic matter (NOM), which is ubiquitous in sources of drinking water, affects this process of disinfection by ozone-initiated radical reactions. Two types of water matrix were considered for this study. One is water containing humic acid, which is commercially available. The other is water from the Han River. This study reported that hydroxyl radicals, initiated by the ozone chain reaction, were significantly effective at B. subtilis endospore inactivation in water containing NOM, as well as in pH-controlled distilled water. The type of NOM and the pH have a considerable effect on the percentage of disinfection by hydroxyl radicals, which ranged from 20 to 50%. In addition, the theoretical T value of hydroxyl radicals for 2-log B. subtilis removal was estimated to be about 2.4 x 10(4) times smaller than that of ozone, assuming that there is no synergistic activity between ozone and hydroxyl radicals.

FIG. 1.

Inactivation of B. subtilis spores in pH-controlled distilled water. (a) Absence of t-butanol. (b) Presence of t-butanol.

FIG. 2.

Inactivation of B. subtilis spores in water containing humic acid. [DOC]₀ = 1.2 mg/liter at pH 7.1 and 1.5 mg/liter at pH 8.2.

FIG. 3.

Inactivation curves of B. subtilis spores in Han River water. [DOC]₀ = 2.4 mg/liter.

FIG. 4.

Formation of hydroxyl radicals in water containing NOM (pH 8.2, [pCBA]₀ = 1.92 μM). (a) Water containing humic acid ([DOC]₀ = 1.5 mg/liter). (b) River water ([DOC]₀ = 2.4 mg/liter).

FIG. 5.

Comparison of T values in several types of water for achieving 2-log B. subtilis spore inactivation (A, absence of t-butanol; B, presence of t-butanol).

FIG. 6.

Comparison of the slope (a) and lag phase (b) of inactivation curves determined by the delayed Chick-Watson model. Relative slope is defined as (slope in the absence of t-butanol under each experimental condition)/(slope in the presence of t-butanol at pH 8.2). Relative intercept of the x axis is defined as (intercept in the absence of t-butanol under each experimental condition)/(intercept in the presence of t-butanol at pH 8.2). A, absence of t-butanol; B, presence of t-butanol.

FIG. 7.

Theoretical inactivation curves of B. subtilis spores by hydroxyl radicals.