Microbial Inactivation: Gaseous or Aqueous Ozonation?

Abstract

For decades, ozone has been known to have antimicrobial properties when dissolved or generated in water and when utilized in its gaseous form on different substrates. This property (the ability to be used in air and water) makes it versatile and applicable to different industries. Although the medium of ozonation depends on the specific process requirements, some industries have the inherent flexibility of medium selection. Thus, it is important to evaluate the antimicrobial efficacy in both media at similar concentrations, an endeavor hardly reported in the literature. This study provides insights into ozone's efficacy in air and water using two Gram-negative bacteria (Escherichia coli NTCC1290 and Pseudomonas aeruginosa NCTC10332), two Gram-positive bacteria (Staphylococcus aureus ATCC25923 and Streptococcus mutans), and two fungi (Candida albicans and Aspergillus fumigatus). For gaseous ozonation, we utilized a custom-made ozone chamber (equipped with ultraviolet lamps), whereas an electrolysis oxygen radical generator was applied for ozone generation in water. During gaseous ozonation, the contaminated substrates (fabric swatches inoculated with bacterial and fungal suspensions) were suspended in the chamber, whereas the swatches were immersed in ozonated water for aqueous ozone treatment. The stability of ozone nanobubbles and their resulting impact on the aqueous disinfection efficiency were studied via dynamic light scattering measurements. It was observed that ozone is more effective in air than in water on all tested organisms except Staphylococcus aureus. The presented findings allow for the adjustment of the treatment conditions (exposure time and concentration) for optimal decontamination, particularly when a certain medium is preferred for ozonation.

Figure 1

Dipslides used for the enumeration of different organisms applied in this study (a) C. albicans, CA (b) A. fumigatus, AF (c) E. coli, EC (d) S. aureus, SA (e) P. aeruginosa, PA (f) S. mutans, SA. BC represents bacterial contamination, whereas CAF represents the contaminated area fraction.

Figure 2

Experimental setup for (a) gaseous ozone disinfection (Copyright permission from Elsevier. Adapted from Figure 1 in Epelle et al.) and (b) aqueous ozone disinfection. An accurate comparison was enabled by utilizing the same ozone dosage (ozone concentration × time) and temperatures, for both gaseous and aqueous treatments. The volume of gaseous ozonation chamber is 0.2 m³.

Figure 3

Analysis of aqueous and gaseous ozone stability. Generation (a) and decomposition cycles (b) of ozone in water, with first-order kinetic plots (c). Size distribution of ozone bubbles in solution, at different times after generating 4 ppm ozonated water (d). Three separate runs of the bubble size distribution, which were obtained, ∼10 min after ozonation (e). Variation of ozone bubble polydispersity index (PDI), with time after ozonation (f). Zeta potential variation with time after ozonation (g). Typical gaseous ozone treatment cycle (h), showing the decomposition profile (i) and the first-order ozone decomposition plot (j). Error bars represent the standard deviation of at least three separate measurements.

Figure 4

Effect of the contact duration on microbial reduction at 2 ppm ozone concentration in air and water, for the different microorganisms applied. Microbial log reduction plots are shown beside the percentage reduction plot for each organism to provide better insights into the air-water differences and the variation with respect to time. For gaseous ozonation, RH = 50 ± 2%, whereas T = 18 °C for both gaseous and aqueous ozonation.

Figure 5

Effect of the contact duration on microbial reduction at 4 ppm ozone concentration in air and water, for the different microorganisms applied. Microbial log reduction plots are shown beside the percentage reduction plot for each organism to provide better insights into the air-water differences and the variation with respect to time. For gaseous ozonation, RH = 50 ± 2%, whereas T = 18 °C for both gaseous and aqueous ozonation.

Figure 6

Retainment of the biocidal properties of ozonated water against E.coli. The initial concentration of aqueous ozone utilized is 4 ppm.

Figure 7

SEM of the fabric swatch used in this study (the substrate for disinfection (a); ozone treated fabric inoculated with S. aureus (b); S. mutans (c); P. aeruginosa (d) E. coli (e); and C. albicans (f–h)). Red arrows indicate regions of cell damage by the action of gaseous ozone (10 ppm for 10 mins). In f, regions 1 and 2 are magnified to give g and h.