Fast inactivation of microbes and degradation of organic compounds dissolved in water by thermal plasma

The multifunctionality and the advantages of thermal plasma for the fast inactivation of viable cells and degradation of organic compounds dissolved in waste water are presented. A complete bacterial inactivation process was observed and studied using a thermal plasma treatment source with very short application times, in particular for Staphylococcus aureus bundle spore survival. The survival curves and analyses of the experimental data of the initial and final densities of S. aureus bacteria show a dramatic inhibitory effect of the plasma discharge on the residual bacteria survival ratio. As the exposure time increased, the inactivation process rate increased for direct exposure more than it did for indirect exposure. The evaluation of direct and indirect exposure was based on the analysis of the ultraviolet spectrum from the absorbance spectra of the organic compound dye called benzene sulfonate (C₁₆H₁₁N₂NaO₄S) and of viable cells called S. aureus. Organic compounds were degraded and viable cells were killed in a short time by thermal plasma. Moreover, analyses of total carbon, total organic carbon, and total inorganic carbon showed a fast decrease in organically bound carbon, however, this was not as fast as the absorbance spectra revealed by the exposure time increasing more for direct exposure than indirect exposure. After 100 s of exposure to the organic compound dye the removal had a maximun of 40% for samples with indirect exposure to the plasma and a maximum of 90% for samples with the direct exposure. For both samples, where some organic contaminants still remained in treated water, four electrolytes (KCl, NaCl, Na₂SO₄, and CH₃COONa) were added to be effective for complete sterilization, reaching a purity of 100%. A proposal is made for an optimized thermal plasma water purification system (TPWPS) to improve fast inactivation of microbes and the degradation of organic compounds dissolved in water (especially for direct exposure rather than indirect exposure) using a hybrid plasma torch with an electrical power of 125 kW (500 V–250 A) producing a high-temperature (10 000 K–19 000 K) plasma jet with a maximum gas consumption of 28 mg s⁻¹.