## 3 Water Purification
The concentrations of these main reactive species, such as those of NO\({}_{2}\) and H\({}_{2}\)O\({}_{2}\), are proportional to the length of the plasma jet because a larger interface between the water and plasma jet enables more reactions for oxidizing agent production. In 2018, [PERSON] et al. [80] designed a water purifier based on plasma technology. They showed that the optimized gas flow rate was 4 L/min based on the concentrations of NO\({}_{2}\), NO, and H\({}_{2}\)O\({}_{2}\) generated. Pathogens were eliminated from the water by exposing a prototype to the plasma stream and its UV radiation. To convert the polluted water into a gas-liquid mixture, the authors used a water pump that accelerated the polluted water to high speeds. Finally, to obtain the liquid form of clean water, the mixture was slowed down. The authors showed that comparedwith other wastewater treatment methods, this new plasma-water treatment system was more efficient and cheaper. Block diagram of plasma-based water treatment method and photograph of water treatment plant is shown in Figure 6.  
When the plasma touches the solution, its interaction with the solution can lead to two types of electron-induced reactions. One is between the accelerated electrons and a very thin layer on the liquid surface. This process can lead to dissociative electron attachment. The second is related to the aqueous electrons that do not participate in reactions near the liquid surface and can produce OH\({}^{-}\) and H. It is worth noting that the reaction rate can be found to depend on the discharge parameters, such as the discharge current. Both the electron density (and flux) and energy distribution of electrons in the plasma are related to the discharge current, as indicated by the following relation as shown in Equation (11) [81, 82]:  
\[n_{e}=\frac{j}{\mu E_{e}} \tag{11}\]  
where \(n_{e}\) is the electron density, \(j\) is the discharge current density (current per unit area), \(E\) is the electric field, and \(\mu\) is the electron mobility. When the discharge current increases, the electron density in the plasma increases, and the ratio of the reactive species produced by the plasma in the aqueous solution consequently increases as well. These results imply that increasing the discharge current in the plasma could enable acceleration of the electron transfer rate. Another water treatment technique involves use of O\({}_{3}\) plasma. In 2019, [PERSON] et al. [83] investigated the effects of this type of plasma on peat water. Injecting O gas into an area with a high voltage produces O\({}_{3}\) plasma. This plasma can initiate different mechanisms for purifying water. Passing through the corona plasma unit will produce O\({}_{3}\) plasma. By passing the O\({}_{3}\) through a charged tube with water in the presence of internal high stresses, an oxidizing compound such as H\({}_{2}\)O\({}_{2}\) as a hydroxide radical is produced. This mechanism will produce more effective OH radicals, O\({}_{3}\) and H\({}_{2}\)O\({}_{2}\) that have an oxidation ability that can be used to destroy germs and decompose materials that cannot be degraded by conventional methods. The used O\({}_{3}\) and OH radicals will break down into H\({}_{2}\)O and O\({}_{2}\). These particles will be released as treated water with high dissolved O content and clarity that meets clean water standards. Compared with the other conventional methods, the plasma and liquid interaction produces more oxidizers that affect the oxidation level. The characteristics of this approach are as follows: (1) as ordinary air can produce O\({}_{3}\) plasma, additional consumables are not needed; (2) the corresponding organic compounds decompose relatively effectively; (3) as consumables are not needed, the costs of and infrastructure for consumables can be neglected; and (4) O\({}_{3}\) plasma can be used as final stage in water treatment because it is inherently modular. O\({}_{3}\) plasma also can be employed for the elimination of micro contaminants.