## 1 Introduction  
In recent years, investigations of plasma-liquid interactions have become an important topic in plasma science and technology, not least being stimulated by the accelerating field of plasma medicine. Consequently, electrical discharges in combination with liquids have been extensively studied. Resulting from plasma-liquid interactions, very complex physical as well as chemical processes occur starting from gas phase chemistry via multiphase species transport, mass and heat transfer up to interfacial reactions and very complex liquid phase chemistry. In the case of low-temperature plasma sources working at atmospheric air conditions (cold atmospheric plasmas--CAP), this chemistry is mainly determined by reactive oxygen and nitrogen species transferred from the gas/plasma phase into the liquid phase or generated in the liquid phase by secondary reactions [1; 2]. Several electrode configurations and modes of electrical operation for liquid treatment were investigated. Generally, these discharges can be ignited directly in the liquid [3], between one electrode above the liquid and the surface of the liquid itself serving as the counter electrode [4; 5; 6], and in close vicinity to the water surface, e.g., by a dielectric barrier discharge [7] or with a gliding arc discharge [8]. Configurations using plasma jets [9] or high-voltage-fed hollow needles [10] and electro-spraying were also investigated [11]. The discharges can be energized with DC, AC, or pulsed high voltage [4; 12; 13]. Besides the various ways of operating these discharges, their physical properties like breakdown voltage, discharge current and morphology, gas temperature, and light emission are discussed. As liquids, different kinds of water were used (distilled water, saline solution, buffered solutions, or tap water). For comprehensive summaries of the broad variety of discharges in combination with liquids, please refer to [1; 14; 15]. Regarding the treatment of water, changes of its properties have been examined by many researchers [6; 7; 8; 9; 16; 17; 18; 19]. In all cases of discharges operated in contact with or in close vicinity to water, acidification (except for tap water) and nitration was observed. Additionally, it was described that chemical contaminations, e.g., with pharmaceuticals, in the water were decomposed over the course of the plasma treatment [20; 21; 22]. Further experiments revealed that the plasma-treated water became antimicrobial [5; 7; 18]. In these experiments, bacteria such as _Staphylococcus aureus_ and _Escherichia coli_ were significantly inactivated. Several tests were performed with bacterial suspensions, i.e., the microorganisms to be inactivated were present during plasma treatment of the liquid (direct plasma treatment). However, it was also demonstrated that plasma-treated liquids show antibacterial activity if the microorganisms were added immediately after plasma treatment of the liquid (indirect plasma treatment) [18; 23]. [PERSON] et al. [24] have shown that distilled water which was plasma treated for 20 min retained its antibacterial activity for 7 days. [PERSON] et al. [18] discussed that the plasma-generated species hydrogen peroxide and nitrite decrease but are still present 1.5 h after the treatment was finished. According to the present state of knowledge, reactive oxygen and nitrogen species (ROS and RNS) play an important role in plasma-mediated inactivation of microorganisms [25]. These findings have given rise to the conclusion that plasma treatment of aqueous liquids might be useful to produce biologically-active formulations for several applications in the medical field like disinfectants or antiseptics [26; 27]. Many of the aforementioned experiments were performed with rather small volumes of water, i.e., in the range of milliliters. For the characterization of the plasma itself and the antimicrobial properties, as well as the chemistry of the liquids, this volume is sufficient. To enable relevant conditions for applications using plasma-treated liquid, larger volumes are required.