## 3 Safety Management Strategy
### Salt Deposition Management of the Heater  
The start-up process of an industrial SCWO plant is a very challenging step, since the inorganic salts in feedstock are likely to precipitate on the inner walls of the heating modules [28]. During a start-up process, the electric heater is the only heating source of the system, which contains several tubes surrounded by heating modules. In a heater, the feedstock is heated from room temperature to supercritical temperature. The pipe walls of the electric heater are directly heated. Inorganic salts in feedstock are easier to crystalize near the pipe walls, because the temperature of the inner wall is the highest. As a result, the heater is likely to be blocked by salt precipitating during a start-up process. Some efforts should be made to minimize the risk of salt deposition.  
A novel heater made especially for an SCWO system was developed by our research group. The structure of the heater is shown in Figure 2. The core components of the new heater are similar to those of the traditional one, namely, some tubes and surrounding heating modules, but there are several special designs to solve the salt deposition breakdown. Firstly, the new heater is divided into many parallel units, and each unit contains only one tube. The temperature and pressure of the inlet and the outlet of every unit is measured, and the pressure difference between the inlet and the outlet of every unit is automatically calculated and used for evaluation of the degree of blockage. By this method, the heating process of the feedstock from room temperature to supercritical temperature can be divided into separated parts. If the heater is blocked due to salt deposition, the operating personnel can easily find out where the salt precipitation happens by the difference of pressure between the inlet and the outlet of each pipe.  
Figure 1: A simplified SCWO system diagram for showing especially the principle of temperature management of the reactor.  
Secondly, the length of the heating modules that are fixed on the tubes is only a half of the whole length, and the remaining surface of the tube is covered by thermal insulation materials. The heating module surrounding a half part of the surface is placed on a sliding rail, and it can be moved along the lengthwise direction of the tube easily. The thermal insulation material on the tube is dismountable. When the salt deposition occurs in one tube the thermal insulation material is dismounted, and the heating module is moved to the other side of the tube. Therefore, the salt deposition area in the tube can be quickly cooled in air. The precipitated salt dissolves again when the temperature of feedstock in the block area decreases below the saturation point of the salts. It is notable that the movable heating modules and the dismountable thermal insulation material are only installed on the tubes where salt deposition is likely to occur.  
Apart from the two methods introduced above, a new ultrasonic desaling technique is under development by our research group. Ultrasonic cleaning is an efficient method for removing the fouling layer by destroying the surface morphology of inorganic salts via the acoustic cavitation effect [12,47], which is widely used for the descaling of heat transfer equipment [48]. Moreover, ultrasonic treatment is an eco-friendly method for the disposal of sludge [49,50]. As shown in Figure 2, some ultrasonic working heads are welded onto the tube. During a start-up process, the ultrasonic generators are turned on to decrease the risk of salt precipitation. However, the obstacle for using the ultrasonic facilities is that the function distance of a working head is limited. Only when a large number of ultrasonic working heads are installed on one tube can a satisfactory descaling effect be achieved. The descaling technique is still being investigated by our research group, and some results on the effect of ultrasound on delaying the salt deposition and improving the removal efficiency of the sludge will be published in the future.