Desalination installation system design
The desalination installation is a complete system, with feed water input and separate discharge pipes for concentrate and permeate. The data of the input and output pipes should always be compared with water analysis, feed water pressure and salt retention.
A membrane filtration system is usually designed to attend a continuous process. The choice for a continuous process can be made, because the process conditions, such as feed water flow and permeate flow, are continuous.
The same goes for Reverse Osmosis systems. These are also designed to attend a continuous process with a continual permeate flow and a system recovery that is steady.
Systems that consist of more than one stage are called multi-stage systems. These systems can reach higher system recoveries, without exceeding the single element recovery limits. To gain a recovery of up to 70%, two stages must be implemented in the feed water treatment system.
For higher recoveries, three stages must be used. These values are based on the assumption that standard pressure vessels with six elements are used. For shorter vessels the number of stages must be doubled.
Plug-flow and concentrate recirculation
The plug-flow concept is the standard Reverse Osmosis system design for water desalination applications.
Schematic representation of an installation with concentrate recirculation options
The number of elements in each pressure vessel
Reverse Osmosis systems are usually designed for a specific permeate flow. To achieve this flow, a number of membrane elements is required. The number of membrane elements that is placed within the installation depends upon the designed flux.
Feed water pressure
A certain feed pressure is required, depending on the system design. The flux, the energy loss in the system and the osmotic pressure determine the feed pressure that the system requires. The required feed pressure will increase when the membrane elements are becoming contaminated through the years. A feed pump that enables a higher flow than the flow that is theoretically required will than be implicated to keep the feed pressure continual. A feed pump that increases the feed pressure by 25% will be satisfactory in practise.
During monitoring of the system, measurements will take place of the flow, pressure and conductivity of the water. To check the hydraulic affectivity of the system the feed pressure per stage and the permeate flow need to be measured. The feed pressure depends upon the temperature of the feed water. When feed water temperatures are low, more pressure is required to achieve the same recovery that would be reached when feed water temperatures are high. When water temperatures fluctuate, one needs to normalize the permeate flow, to enable comparison with the starting situation.
Within desalination installations, there is a course environment when it comes to corrosion of separate parts of the system. Because of this, the material needs to possess a certain resistance to corrosion. This goes for external parts, which are exposed to a salty atmosphere (spillage, leaks), as well as for internal parts. Corrosion of external system parts can usually be prevented by providing them with a surface layer (painting, galvanizing) and by periodic maintenance of the system and closing of leaks.
In a desalination system the concentrate is released under high pressure, that is why it is important to win back energy from the concentrate flow. This can be done by application of a pressure exchanger. The concentrate flow from the membranes is directed through the pressure exchanger, where it directly transfers energy to part of the incoming feed water with maximum affectivity.
* Graphic images come from FILMTEC membranes