Chlorine dioxide and ozone facilitated disinfection of selected bacteria in aqueous systems.
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Date
2018
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Abstract
Chlorination is the most commonly used disinfection technology for the control of pathogenic
microorganisms in drinking water or wastewater treatment. However, the reactions of chlorine
with natural organic matter in water have been found to produce harmful by-products including
trihalomethanes, haloacetic acids, and haloacetonitriles. Regulations on these deleterious
disinfection by-products keep increasing and have consequently focussed considerable
attention on the use of alternative chemical disinfectants. The design and operation of an
efficient disinfection system at a water treatment facility aim at providing adequate control of
microbial threats and simultaneously satisfying regulatory requirements on by-products.
Achieving these require an in-depth understanding of the inactivation kinetics of the
disinfectant on potential target organisms.
In this study, the microbial inactivation kinetics of chlorine dioxide (ClO2) as an alternative
chemical disinfectant for water treatment was monitored on two Gram-negative bacterial
species: Escherichia coli (ATCC 35218) and Pseudomonas aeruginosa (ATCC 27853) and a
gram-positive: Staphylococcus aureus (ATCC29313) under varied conditions of disinfectant
concentration, pH, temperature and bacterial density in an oxidant demand free water. Further
studies were conducted to investigate the effect of ClO2 on bacterial outer cell membrane
permeability, the cytoplasmic membrane integrity, inhibition of intracellular enzyme activity
and changes in cell morphology by TEM to elucidate the bactericidal mechanism of action of
ClO2. In addition, autochnous bacteria from urban wastewater were exposed to chlorine dioxide
and the susceptibilities monitored and compared by a culture-dependent heterotrophic plate
count technique and culture-independent 16S rRNA gene-directed polymerase chain reaction
(PCR) based denaturing gradient gel electrophoresis (DGGE). Furthermore, the influence of
four organic solvents commonly discharged from industrial lines into wastewater systems,
namely, ethanol, methanol, ethyl acetate and dimethyl sulfoxide (DMSO) on ozone absorption,
stability and consequent inactivation of Escherichia coli (ATCC 25218) and Staphylococcus
aureus (29213) in water were also examined.
Chlorine dioxide showed strong and rapid disinfection capabilities at relatively lower dosages
with significant influences by pH and temperature. However, the efficiency generally appeared
unaffected by changes in bacterial density. The PCR- DGGE technique showed that 1.0 mg/L
was sufficient to inactivate three predominant bacterial species from an urban wastewateridentified as Arcobacter suis F41, Pseudomonas sp strain QBA5 and Pseudomonas sp B-AS-
44, whereas a significant population of other species such as Pseudomonas sp CCI2E was
observed to presumably remain viable to 5.0 mg/L chlorine dioxide whilst the heterotrophic
plate count method indicated complete elimination of bacteria at 3.0 mg/L. ClO2 was not found
to inactivate bacteria by inflicting gross morphological damages to the cell wall, but instead,
increases the permeability of the outer cell membrane, disrupts the integrity of the inner
cytoplasmic membrane which leads to the efflux of intracellular contents of the cell and hence,
resulting in the overall cell death. The presence of ethyl acetate and DMSO were observed to
significantly enhance ozone absorption and stability in water with a consequent increase in
bacteria inactivation efficiency whilst methanol-containing water rather accelerated the
decomposition of ozone. The findings herein provide further knowledge to enhance the
disinfection operations at a water treatment plant when ClO2 or O3 is applied.
Description
Doctoral degree. University of KwaZulu-Natal, Durban.