Archive

Posts Tagged ‘air sterilizing’

Why do we need air disinfection? (3)

September 2, 2010 Leave a comment
 
Procedure for air disinfection using ozone
 
In order to test for the effectiveness of ozone in reducing airborne bacteria, a conference room with area about 12m2 was selected for testing. As high level of ozone is required to kill viruses, bacteria and spores, the disinfection process was carried out when humans, animals and plants were evacuated.
 
Depends on the size of the room, an ozone generator (PIE Ozonation) with 2g/hr output was chosen. The capacity of the chosen ozone generator has the ability to maintain high concentration of ozone (0.5 – 5 ppm) inside. Circulation fan was placed in the room to ensure good distribution of ozone. After closing all the windows and doors, the ozone generator was turned on by remote device located outside to begin ozonation process. Concentration of ozone was monitored using a digital ozone sensor (Ecosensor). Different levels of ozone (0.5, 2.5 and 5 ppm) were tested to determine the optimal value for killing as much microorganisms as possible. After turning off the ozone generator, ozone level began to drop as it was undergoing self-decomposition to oxygen.
 
For safety reason, no people should enter the room until the level of residual ozone is below 0.02 ppm. In general, ozone concentration drops to below 0.02 ppm in a hour after ozonation, therefore people should wait for at least one hour (after turning off the generator) before entering the “ozonated” room.
 
The total airborne bacteria in the conference room was measured before and after each ozonation. Measurement was carried out using an Andersen N-6 single-stage sampler with Tryptone Soya Agar (Oxoid) in petri dish. 283L of air was taken for each sampling. The petri dish was incubated at 35oC for 48 hrs before counting. The disinfection efficiency of ozonation at different concentration was tabulated in Table 1.
 
Table 1. Reduction of Airborne Bacteria after Ozonation

Ozone conc.

0.5 ppm

2.5 ppm

5 ppm

Before Ozonation

592 CFU/m3 

612 CFU/m3 

552 CFU/m3 

After Ozonation

169 CFU/m3 

42 CFU/m3 

57 CFU/m3 

Reduction %

71.5%

93.1%

89.6%

The results show that ozone is effective in reducing airborne bacteria. At higher ozone level, the sanitizing effect increased. Over 90% of airborne bacteria could be reduced at 2.5 ppm concentration. Further increase of ozone concentration to 5 ppm does not beneficial in bacteria reduction percentage.
 
Unlike laboratory experiments conducted by Kowalski et al (1) that could remove 99.99% airborne bacteria after ozonation, the best reduction percentage in our case was around 93% only. High removal percentage could not be achieved because the conference room was not 100% sealed. Doors should be opened briefly during each air sampling (for placing a new agar dish on the sampler) and air exchange from outside was unavoidable.
 
For safety reason, excessive high concentration ozone should be avoided and the lowest ozone concentration that could kill most of the microorganisms should be selected as optimum. Depends on the contamination level, 0.5 – 2.5 ppm ozone level is adequate for air disinfection.
 

Conclusion

Experimental data shows that ozone is effective in reducing airborne bacteria of unoccupied room. Over 90% of airborne bacteria could be reduced after ozonation. As viruses are generally more susceptible to ozone than bacteria, it could assume that all viruses are killed if large percentage of airborne bacteria are removed. Ozone is a gas that has good penetration capacity and powerful oxidizing power, thus its disinfection efficiency is superior to UV radiation and HEPA filter. As ozone disinfection is conducted in unoccupied room only and all the residual ozone will be decomposed after the treatment, ozone toxicity to human is therefore not a concern. Given the advantages of strong oxidizing power, good penetration capacity and no harmful residues left after the treatment, ozone is recommended to be used in disinfection of SARS-contaminated environments.
 

Advertisements