Effect of voltage and oxygen on inactivation of E. coli and S. typhi using pulsed dielectric barrier discharge

Highlights

• The development of a DBD reactor to evaluate the bacterial inactivation in water.

• Efficiency of treatment with a 6 log10 reduction for E. coli and S. typhi.

• 12 kV with oxygen was the minimum voltage to inactivate E. coli and S. typhi.

Abstract

This work presents the study of the voltage and oxygen effect on bacterial inactivation in water using a pulsed dielectric barrier discharge (DBD) under atmospheric pressure, where Escherichia coli (E. coli) and Salmonella typhi (S. typhi) bacteria were used as model microorganisms. A cylindrical DBD reactor was developed and tested in applications to assay the efficiency of bacterial inactivation in water on a volume of 500 mL flowing continuously throughout the system assisted with a peristaltic pump at 4.4 ± 0.1 mL/s. The efficiency of the treatment reached a 6-log₁₀ reduction for both E. coli and S. typhi bacteria at 10⁶ CFU/mL of concentration at the end of the first cycle of treatment at a minimum voltage of 12 kV with oxygen bubbling gas, concluding that there was a minimum voltage to produce inactivation of E. coli and S. typhi samples. Bacterial inactivation without the oxygen condition contrasted with the high rate of inactivation with oxygen at relatively low voltage discharges.

Introduction

The growing interest in the process of plasma discharge in liquids and its application is discussed in several studies published in the past [1], [2], [3]. The state of knowledge about the interactions of electric discharges in liquids is less advanced than in gases. Its effect on biological material varies and is sometimes contradictory. In recent years, some aspects of plasmas in and in contact with liquids and the gas-liquid interaction have been investigated. Still, understanding gas-liquid interface phenomena where neutral species, ions, electrons, and positive and negative photons are involved remains a challenge.

A particular interest in electric discharges effects on living cells is closely related to those mentioned earlier. Electric discharges in liquids have been an important field of study due to their broad range of applications such as chemical synthesis, material processing and environmental remediation to cite some ones, showing at the time to be a promising technology for the treatment of drinking water because of their physicochemical properties. These properties have been exploited as a method to inactivate microorganisms [4], [5], [6], [7], [8], [9], [10].

In this regard, two fundamental insights mechanisms of plasma-cell interactions are commonly accepted: (1) Biological effects are significantly caused by plasma-induced changes of the liquid environment of cells, and (2) Reactive oxygen and nitrogen species (RONS) generated in or transferred into the liquid phase play a dominating role in biological plasma effects. However, other mechanisms such as electric fields or charged particles interacting directly with the cell are studied [11], [12].

It is known that electric discharges in water initiate the formation of active chemical species, UV radiation, electric fields and shock waves [13], achieving a synergistic effect on bacteria inactivation. Some researchers had related some of these agents with bacterial inactivation [14], [15], but, despite these studies, it is not yet understood how the inactivation and, in general, the elimination of microorganisms can take place.

However, it is well known that different free radicals and ions formed during an electrical discharge play an essential role in the biological processes of cells reacting extra- and intracellularly. These radicals and ions are also involved in different cellular information processes, resulting in oxidation [16] and genetic damage that at last kills the cell [17]. Experimentally, it has been shown that electrical discharges containing oxygen have a highly germicidal effect owing to the generation of oxygen-based chemical species, such as atomic oxygen (O) and ozone (O₃) [15]. Likewise, in some discharge experiments, the formation of highly oxidant chemical species such as the OH radical and hydrogen peroxide (H₂O₂) has been observed [18].

In the case of ozone, this is a well-known disinfecting agent used in many applications, including water decontamination. Studies of ozone as a bactericidal agent are widely reported. Still, some of these studies highlight the limited utility of ozone associated with its generation outside the system, limiting its effect. This disadvantage is mainly because ozone is produced in a plasma reactor and then transferred to the application site [19], [20]. On the contrary, in our system, ozone is generated directly in the treatment reactor, avoiding the transfer and allowing the treatment of a continuous water flow.

From the physical point of view, electric discharges effect is compared to the treatment by pulsed electric fields (PEF) [21], [22], where both require the application of a high voltage between two electrodes. In this case, two mechanisms of microorganisms inactivation have been proposed, these are electric shock and electropermeabilization or electroporation [23]. Schoenbach et al. [24] reported that pulsed electric fields of 10 kV/cm and tens of microseconds induced permeability in the membrane of bacteria, a process known as lysine or lysis. For bacterial inactivation, pulsed electric fields were established with durations in the range of nanoseconds to microseconds and electric fields ranging from 10 kV/cm to more than 100 kV/cm.

Although the study of the inactivation of bacteria in water has been reported in the last two decades, in this work it was determined the best conditions in which Escherichia coli (E. coli) and Salmonella typhi (S. typhi) bacteria were inactivated using pulsed dielectric barrier discharges (DBD) in water flowing continuously. The applied voltage was decreased from 25 kV to low voltage discharge of 10 kV to optimize the DBD reactor performance, reducing the applied energy but maintaining the bacterial inactivation efficiency.