An atmospheric microwave plasma argon was used for the inactivation of bacteria E. coli.The employed device, called Axial Injection Torch (or TIA for Torche à Injection Axiale), consisted of a microwave power source, a waveguide and a gas supply system.Using this argon plasma source, we studied the effects of the exposure time, the exposure distance, the input power, and the gas flow rate on the reduction rate of Escherichia coli cells.The first part of the study was carried out with a static sample exposed to the plasma and then in the second part the sample was set in motion relative to the plasma jet.A log reduction number of E. coli of 4 (10 -4 CFU/mL) was obtained with UV and active species, for UV only a log of 1 (10 -1 CFU/mL) was obtained.

ABSTRACT The aim of this study was to investigate the effects on the cell membranes of Escherichia coli of 2.45-GHz microwave (MW) treatment under various conditions with an average temperature of the cell suspension maintained at 37°C in order to examine the possible thermal versus nonthermal effects of short-duration MW exposure. To this purpose, microwave irradiation of bacteria was performed under carefully defined and controlled parameters, resulting in a discontinuous MW exposure in order to maintain the average temperature of the bacterial cell suspensions at 37°C. Escherichia coli cells were exposed to 200- to 2,000-W discontinuous microwave (DW) treatments for different periods of time. For each experiment, conventional heating (CH) in a water bath at 37°C was performed as a control. The effects of DW exposure on cell membranes was investigated using flow cytometry (FCM), after propidium iodide (PI) staining of cells, in addition to the assessment of intracellular protein release in bacterial suspensions. No effect was detected when bacteria were exposed to conventional heating or 200 W, whereas cell membrane integrity was slightly altered when cell suspensions were subjected to powers ranging from 400 to 2,000 W. Thermal characterization suggested that the temperature reached by the microwave-exposed samples for the contact time studied was not high enough to explain the measured modifications of cell membrane integrity. Because the results indicated that the cell response is power dependent, the hypothesis of a specific electromagnetic threshold effect, probably related to the temperature increase, can be advanced.

Flow cytometry is a technology that simultaneously measures and analyses multiple physical characteristics of single particles, usually cells, as they flow in a fluid stream through a beam of light. The properties measured include a particle’s relative size (represented by forward angle light scatter), relative granularity or internal complexity (represented by right-angle scatter), and relative fluorescence intensity. These characteristics are determined using an optical-to-electronic coupling system which records how the cell or particle scatters incident laser light and emits fluorescence. A wide range of dyes, which may bind or intercalate with different cellular components, can be used as labels for applications in a number of fields, including molecular biology, immunology, plant biology, marine biology and environmental microbiology. Interest in rapid methods and automation for prokaryotic cell studies in environmental microbiology has been growing over the past few years.

Although many information is currently available about sludge minimization processes in wastewater treatment plant (WWTP), few data are available about their fundamental mechanisms especially microbial changes. In order to clarify the relationship between sludge reduction efficiency and both chemical and biological modifications, the effects of thermal treatment on activated sludge were investigated by combining the monitoring of cell lysis using flow cytometry (FCM), organic matter solubilization, floc structure and biodegradability. For the maximal temperature (95 degrees C) applied, COD, proteins, HLS and sugars solubilization degrees reached 12.4 (+/- 1.3)%, 18.6 (+/- 1.8)%, 9.6 (+/- 1)% and 7.4 (+/- 1.9)%, respectively, showing clearly the transfer of organic matter from the particulate to the soluble fraction of the sludge. The results from FCM analysis showed that thermal treatment induces a progressive cell lysis when increasing temperature from 50 to 95 C. However, the impact on floc structure seemed to be limited as floc destructuration was limited to the temperature elevation at 50 degrees C. Above 50 degrees C floc size distribution remained almost constant. The results from biodegradability tests did not show any improvement of the intrinsic biodegradability after the thermal treatment. Only the increase of the specific digestion rate was observed. This study which provides a complete investigation of chemical, physical and biological effects of thermal treatment allows a better knowledge of fundamental mechanisms involved during heat treatment to improve sludge reduction processes. (C) 2011 Elsevier B.V. All rights reserved.