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.
The designer of a Reverse Osmosis system will aim at the lowest possible membrane pressure and installation costs and maximum recovery and salt retention.
The recovery of a desalination installation for brackish water is around 85%. It depends on the solubility of suspended solids that are present in the feed water. During seawater desalination a recovery of forty to fifty percent is desirable. The recovery of seawater desalination depends on the osmotic pressure of the feed water and the applied membrane types during the desalination process.
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.
Schematic representation of a continuous process
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.
Variations in water temperature and fouling degree of the feed water are compensated by adjustments of the feed pressure.
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.
Schematic representation of a two-stage 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.
When the system recovery is higher, more membrane elements have to be connected in series. A typical two-stage system uses a stage ratio of 2:1 for the desalination of seawater with high dissolved solids contents.
Plug-flow and concentrate recirculation
The plug-flow concept is the standard Reverse Osmosis system design for water desalination applications.
The feed water is passed through a plug-flow system only once. A fraction of the feed water passes a membrane to produce permeate. The rest of the feed water is not derived of salts and will become more and more concentrated.
When the number of membrane elements in a plug-flow system is too low to achieve a high enough system recovery, concentrate recirculation may be implicated. During recirculation part of the concentrate is directed back to the feed water side of the module. The recycled concentrate mixes with the feed water and will be treated once more.
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.
For the desalination of seawater the limiting factor is the maximum feed pressure; this may not exceeds 69 bars.
Based on the designed flux, the production per unit membrane can be determined:
Production per element = flux * element surface
Number of elements = permeate flow / production per element
Number of pressure vessels = number of elements / number of elements per vessel
By means of the permeate flow and the required recovery, the feed water flow is calculated:
Feed water flow = permeate flow / recovery
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.
When the system is started up, the initial situation is recorded. All relevant parameters should be registered and noted in a log. Based on this data the performance of the installation can be examined and regulated after the system has been put into action.
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.
When the installations function correctly the conductivity of the permeate is low, because of the removal of univalent and bivalent ions. When a leak is situated in the membrane element, the conductivity will increase. That is why conductivity measurements are applied.
These measurements take place in the permeate collection drain. Measurements can be performed for each individual stack or for all present stacks.
A sufficient monitoring of the system will enable the user to know when the system needs cleaning.
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.
Despite the fact that materials are protected against potential corrosion, they also need to be able to be resistant to pressure, vibrations and changing temperatures.
To prevent corrosion and chemical reactions in the part of the system where pressures are low (<10 bar), such as in membrane elements and pressure vessels, non-metals such as PVC and fibreglass are often applied. For high-pressure parts (10-70 bar), such as pumps, drains and lids, one needs to use metals to provide them with the same kind of protection.
PVC and some metals cannot sufficiently resist corrosion. When they start corroding they can contaminate membranes. When corrosion protection is in order, we need to keep this in mind.
The main material that is used for high-pressure parts is stainless steel. The benefit of stainless steel is that it is resistant to corrosion and erosion corrosion. Stainless steel is rarely stricken by galvanic corrosion.
Pipes and parts of the installation should follow up demands of the company where the installation is located. Pipes and components of the installation are usually built using the following materials:
Candle filters and pressure vessel: polypropylene filter in PVC or stainless steel vessel
Pumps: stainless steel
Low-pressure pipes: PVC
High-pressure pipes: stainless steel
Cleansing system: PVC or other chemical-resistant synthetic material
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.
The feed water flow now has the same volume as the concentrate flow. It will be directed to a small booster pump that corrects the hydraulic losses of the flow.
The upgraded feed water flow will join the feed water flow from the high-pressure pump.
In an installation that uses a pressure exchanger, the high-pressure pump will give off 41% of the energy, the booster pump will give off 2% of the energy and the pressure exchanger will give off the remaining 57%. The pressure exchanger does not use any external energy, so the total energy savings will be 57%.
By practising pressure directly to incoming seawater in a membrane system, a reduction of 60% in size of the high-pressure pump can be achieved. This does not only save energy; it also saves purchase costs of high-pressure pumps.