# A Review of the Water Desalination Technologies
## 2 The State of the Art
### Reverse Osmosis (RO)
The Isobaric Devices are recent solutions used to transfer energy from the brine flow to the saline feedwater flow without intermediate energy conversions. The idea is depicted in Figure 9:0  
Figure 9:1 A schema of a RO unit with a Dual Turbine System. Figure 9:2 A schema of a RO unit with a Hydraulic Energy Management Integration (HEMI) System. Figure 9:3 A schema of a RO unit with a Pressure Exchanger. The saline feedwater is divided in two flows: the first one is pressurized by the main HPP, driven by an electrical motor, and the latter one is pressurized by the ERD. The HPP treats a part of the total saline feedwater flow, so this component is smaller in comparison with the previous cases, considering the same freshwater flow. At the same time, this kind of ERD shows higher energy efficiency, reducing the total energy expenditure in the desalination process. A solution is the Rotary Pressure Exchanger (RPX), depicted in Figure 9:4 Inside the device, a ceramic matrix is taken into rotation by the brine flow, which enters with a tangential speed component [45]. The channels inside the rotative matrix (part C in the picture) are alternatively connected to the High-Pressure Brine and the High-Pressure Feedwater pipes or to the Low-Pressure Feedwater and the Low-Pressure Brine pipes. In the first case, the channel, previously filled by feedwater, is now refilled by brine, pushing the feedwater inside the High-Pressure Feedwater pipe to nearly the same high pressure as the brine. In fact, a very limited pressure drop is measured due to the rotational motion of the channel matrix. Similarly, in the second case, the saline water is pushed by the feedwater, refilling the channel with feedwater at low pressure [37]. The process is practically continuous because of the high rotary speed of the matrix and the number of internal channels. The same idea is applied in another way by the Dual Work Exchanger Energy Recovery (DWEER(tm)), proposed by Calder AG (now Flowserve Corporation) [37]. The DWEER system is presented in Figure 9:5 In detail, the device is composed of two cylinders, with two commanded valves (LinX(r) valve) to control the brine flow, and four automatic valves for the saline feedwater flow [46]. Figure 9:6 The working principles of a Rotary Pressure Exchanger. Figure 9:7 The working principles of a Piston-type Work Exchanger. According to the position of the LinX valve, the brine flow from the desalination unit can be introduced in one piston or in the other one. As the high pressure of the input water, the automatic valve (on the left side of the picture) is opened, transferring the saline feedwater to the booster pump. During this step, the piston is filled with brine water. Changing the position of the LinX valve, now the brine side of the cylinder is connected to the brine discharge pipe. As the water from the feedwater pump has a greater pressure than the brine discharge, the automatic valve is open, refilling the piston with saline water. The system is composed of two cylinders, which allows one to be filled with saline water (low pressure) and the other with brine (high pressure) in alternation [37].