Chlorine dioxide and ozone facilitated disinfection of selected bacteria in aqueous systems.

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.