Abstract
The use of cold plasma jets for inactivation of a variety of microorganisms has recently been evaluated via culture-based methods. Accordingly, elucidation of the role of cold plasma in decontamination would be inaccurate because most microbial populations within a system remain unexplored owing to the high amount of yet uncultured bacteria. The impact of cold atmospheric plasma on the bacterial community structure of wastewater from two different industries was investigated by metagenomic-based polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) utilizing 16S rRNA genes. Three doses of atmospheric pressure dielectric barrier discharge plasma were applied to wastewater samples on different time scales. DGGE revealed that the bacterial community gradually changed and overall abundance decreased to extinction upon plasma treatment. The bacterial community in food processing wastewater contained 11 key operational taxonomic units that remained almost completely unchanged when exposed to plasma irradiation at 75.5 mA for 30 or 60 s. However, when exposure time was extended to 90 s, only Escherichia coli, Coliforms, Aeromonas sp., Vibrio sp., and Pseudomonas putida survived. Only E. coli, Aeromonas sp., Vibrio sp., and P. putida survived treatment at 81.94 mA for 90 s. Conversely, all bacterial groups were completely eliminated by treatment at 85.34 mA for either 60 or 90 s. Dominant bacterial groups in leather processing wastewater also changed greatly upon exposure to plasma at 75.5 mA for 30 or 60 s, with Enterobacter aerogenes, Klebsiella sp., Pseudomonas stutzeri, and Acidithiobacillus ferrooxidans being sensitive to and eliminated from the community. At 90 s of exposure, all groups were affected except for Pseudomonas sp. and Citrobacter freundii. The same trend was observed for treatment at 81.94 mA. The variability in bacterial community response to different plasma treatment protocols revealed that plasma had a selective impact on bacterial community structure at lower doses and potential bactericidal effects at higher doses.
Highlights
Industrial, agricultural, and domestic wastewater must be treated to eliminate pathogenic microorganisms and prevent their transmission through the environment
The bacterial community of food processing wastewaters showed a gradual decrease in species richness to 90.9 and 54.5% when treated with plasma at 75.5 mA for 60 and 90 s, respectively
No significant changes in food wastewater community structure were observed in response to short-term plasma irradiation at lower dosages; elevated plasma dosage for even a relatively short time destroyed most of the bacterial populations, and extension of exposure time resulted in elimination of all bacterial groups
Summary
Industrial, agricultural, and domestic wastewater must be treated to eliminate pathogenic microorganisms and prevent their transmission through the environment. Conventional wastewater treatment processes do not guarantee disinfection and elimination of these organisms (Howard et al, 2004). Non-thermal atmospheric pressure plasmas (APPs) have been recognized as a new paradigm in biomedical applications and materials processing. APPs are vacuum-less generated plasmas with gas temperatures much lower than the electron temperature, even approaching room temperature. Owing to their low temperature and absence of a vacuum, APPs have widespread physical, chemical, and biomedical applications. Low temperature APPs (Becker et al, 2005) have the potential for application in decomposition or detoxification of gaseous materials, surface treatment, sterilization, protein destruction, decontamination, food processing, teeth bleaching, dental cavity treatment, blood coagulants, treatment of living tissue, wound care, deposition, etching, and synthesis of carbon nano-tubes, sources of UV and excimer radiation, and as reflectors or absorbers of electromagnetic radiation (Deng et al, 2007a,b; Harbec et al, 2007; Fridman et al, 2008; Niemira and Sites, 2008; Daeschlein et al, 2009; Kong et al, 2009; Lee et al, 2009; Lloyd et al, 2009; Choi et al, 2010; Yasuda et al, 2010)
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