## 2 Unique Characteristics of the Feedstock for SCWO Plants
### Wastewater  
The SCWO reaction of many types of organic wastewater has been investigated, such as pharmaceutical wastewater [34], coal chemical wastewater [35] and dyeing wastewater [36]. No matter what kind of wastewater, the chemical oxygen demand (COD), chemical composition and pH value of the wastewater should be detected and measured at first, which can significantly affect the operation of an SCWO system.  
COD refers to the concentration of organic matter. Organic matter in any concentration can be dissolved in SCW, but usually the COD of the feedstock for the industrial SCWO plant should not be too high (e.g., less than 120,000 mg/L). In order to improve the oxidation efficiency, excess oxygen was injected into the system to react with the organic matters. An extremely high COD of feedstock means a terribly high concentration of oxygen would be injected into the reactor. As a result, once the feedstock and the oxygen mixes together, a large amount of heat will be released due to the drastic oxidation reaction, leading to the over-heating of the reactor. Moreover, a high value of the local oxygen concentration may lead to severe corrosion of reactor materials.  
The chemical composition of feedstock is another parameter that needs to be paid attention to. Although the main target of the SCWO plant is to decompose the organic matter in the wastewater, some wastewater also contains considerable amounts of inorganic salts, such as K\({}^{+}\), Na\({}^{+}\), Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\). The solubility of these inorganic salts decreased to thousands of ppm in SCW, separating out and depositing on the inner wall of equipment or pipelines. With the accumulation of the salt deposits, the pipelines could be totally blocked and the SCWO system would be forced to shut down. The most important equipment in an SCWO system for salt precipitation is the pre-heater or the heat exchanger, because the wall temperature of these vessels is the highest. Salt deposits crystallize once the saturation point is reached, and the crystalline salt will stick onto the inner wall of the vessels and result in the plugging of the equipment in several minutes. Moreover, the polymerization of the organic materials in an oxygen-free environment (heat exchanger) may happen, forming insoluble char or tar and resulting in scaling of heat-transfer surfaces or plugging [37].  
An acidic feedstock would improve the risk of severe corrosion of heat exchangers. Many research studies show that the corrosion of constructional materials in a subcritical environment is higher than that in supercritical water [38; 39; 40]. This is due to the solubility of the corrosive ions, such as H\({}^{+}\) and Cl\({}^{-}\), reaching the highest value in a near-critical temperature environment. For example, [PERSON] et al. [41] found that the corrosion rate of stainless steel 316 can reach 340 mm/year in subcritical water containing oxygen, sulfuric acid and ammonium ions. An acidic feedstock is definitely a disaster for an SCWO plant and it should be strictly avoided. Considering the fact that some oxidation reactions of organic matters also produce acids, the pH value of the effluents should also be taken into account because the effluents also go through a subcritical state.  
There are many nitrogen-containing organic materials in wastewater to be treated by the SCWO process. The nitrogen element in organic matters can be directly transferred into N\({}_{2}\) by an SCWO reaction at a temperature higher than 700 \({}^{\circ}\)C or with the aid of a catalyst [42]. However, most of the commercial SCWO plants were operated with the highest temperature of less than 650 \({}^{\circ}\)C, to decrease the equipment investigation [43]. As a result, the nitrogen element in organic matter is converted to ammonia gas and is dissolved in effluent. The ammonia gas is toxic and has a pungent smell, which is a risk for the health of the operating personnel.