## 2 Types of Plasma Discharge for Water Treatment  
In recent times, attention has been paid to atmospheric pressure non-thermal plasma technology due to its low energy requirements. Discharge methods under normal pressure include pulsed corona discharge, dielectric barrier discharge (DBD), plasma jets, gliding arc discharge, and glow discharge, etc. [31], which can be processed according to different processing requirements. Figure 2 illustrates the different types of plasma generator, the plasma generation process and the wastewater treatment process.  
DBD is the workhorse of plasma technology. This approach is based on reducing the power usage of plasma devices. An electrical breakdown occurs when a sufficiently high voltage is delivered between two electrodes under air pressure [32]. It comprises two electrodes, at least one of which is coated with a dielectric barrier material such as glass, ceramic, quartz, and so on (Figure 2A). When the potential across the gap hits the breakdown voltage, the dielectric can cause a high number of micro-discharges and reduce the likelihood of electrode etching and spark creation [29; 33]. Therefore, DBD plasma can maintain uniformity and stability when decontaminating samples with a large area [34; 35; 36].  
As shown in Figure 2B, the plasma jet is another type of cold plasma discharge. The plasma jet has a structure made up of two concentric electrodes within a nozzle through which the carrier gas travels, the outer of which is grounded and the inner of which is connected to an external power supply. The inner electrode is frequently shocked with a high voltage at a high frequency, inducing ionization of carrier gases such as helium, oxygen (O\({}_{2}\)), and other gases or gas combinations [37; 38]. These gases are also helpful in pushing the stream carrying active species out of the electrodes [39]. In addition, the simple structure and easy operation of plasma jets seem to have a wider range of applications in wastewater treatment.  
The gliding arc is a common generator of thermal plasma, but it may also create non-thermal plasma under certain conditions [39]. In comparison to other kinds of discharges, gliding arcs combine the advantages of thermal and non-thermal plasmas, which have higher operating pressure, power and plasma density [39; 40]. Figure 2C depicts one kind of gliding arc discharge, which is extensively employed for liquid treatment. The arc is pushed away by the injected stream along the two divergent electrodes, pushing away from the ignition point, and moving along the electrode gap to their tips, where it forms a large plume of plasma. The plasma plume is placed close enough to the aqueous dye solution, so it lacks the surface of the liquid and allows chemical reactions to occur at the plasma-solution interface, resulting in highly reactive species for the purpose of degrading organic contaminants [41]. This technique is widely recognized for its ability to degrade numerous organic components found in water at atmospheric pressure [41; 42].  
Corona discharge frequently occurs through the imposition of high voltage on sharp electrodes (Figure 2D), such as tips, pinpoints, or thin wires [39]. Generally, a high voltage is connected to the pin electrode, and the plate electrode is connected to the ground [43]. The gas-phase pulsed corona discharge has proven the most energy-efficient plasma for water treatment, and achieves the highest degree of decontamination efficiency [29; 44]. It is worth mentioning that all these techniques undergo a mineralization process that leads to the disintegration of different organic contaminants, i.e., the transformation of compounds into inorganic intermediates, H\({}_{2}\)O and CO\({}_{2}\). Many studies have demonstrated the potential of the different plasma to degrade toxic pollutants and stable substances [45].