Reverse Osmosis Desalination
Importance of Desalination
Around 70% of the earth’s surface is covered in water, but only 2.5% of that is fresh water and roughly 70% of that fresh water is captured as ice in the polar caps and glaciers. This means that only 0.75% of all water on earth is left for usage by humans. Approximately 2.2 billion people live without a secure supply of water, which makes about 27% of the population. Given that water is the foundation of life on earth this is a tragic statistic. Innovations for fresh water solutions are needed more than ever. Approaches like desalination aim for the 97.5% of unusable seawater as a new source of fresh water.
Figure 1: diagram of water distribution on earth
Reverse Osmosis (RO) Process
The process of desalination describes the separation of mineral components in saline water. One of the main methods to achieve this is the reverse osmosis desalination, which is the most used for smaller and medium sized desalination plants due to the prompt availability and constant flow of pure water once the plant is installed. Thanks to the improvement in energy consumption of this method, in recent years it has becoming more and more popular. Still, improvements in the current technologies are needed to make it a more accessible method to source drinking water. New technologies that reduce energy consumption are helping to achieve a broader availability.
To desalinate the seawater, the salt molecules have to get separated from the water. To do this, one method is to filtrate the seawater by reverse osmosis as shown in figure 2. Previously to reverse osmosis, bigger molecules and particles have to be removed through various stages of filtration and then lastly the reverse osmosis takes place. It uses a semi-permeable membrane to filter out molecules of size 0.1 – 1 nm.
Figure 2: stages of filtration for reverse osmosis
Figure 3: breakdown of particles in figure 2
It is called reverse osmosis, because it has to overcome the osmotic pressure. Osmosis is a natural process that occurs when two solutions in a closed environment containing different concentrations of solute are separated by a semi-permeable membrane, that holds back solute but not solvent. Here an imbalance of chemical potential occurs. The solution with a lower concentration of solute has a higher chemical potential than the higher concentrated solution. Water molecules will pass through the membrane to reach equilibrium of chemical potential. Due to being in a closed environment, the increase in volume of the solution with the higher concentration of solute results in an increase of pressure – osmotic pressure.
Figure 4: osmosis in a -shaped tube
For the separation of seawater (typically 35,000 ppm, 20°C), the reverse osmosis filtration system must overcome an osmotic pressure of 27 bar to prevent inward flow of pure permeate across the membrane. To have a practical flow of permeate (pure water), the working pressure should be double the osmotic pressure. Typical working pressure for a reverse osmosis seawater desalination plant is between 50 and 70 bar. In practice this is a very high pressure to produce, equivalent to a range of 500 to 700 meters of a column of water.
To prevent the saturation of the membrane, a type of filtration called crossflow filtration is needed, as shown in figure 5. With this type, the solute (salt) doesn’t get retained in the membrane, instead it flows out as concentrate (brine).
Figure 5: cross section of crossflow filtration
It takes high amount of energy to pressurize the feed (seawater) to the working pressure level. From figure 5 one can tell that this high pressure is maintained in the concentrate. This means that in the production of permeate all the energy used to pressurize the feed is wasted on the concentrate. This has been a concern since the invention of reverse osmosis desalination, and over the years several systems have been developed to recover pressure energy from the concentrate and use it to pressurize the feed.
The different devices that were developed to recover the energy from the concentrate are grouped as Energy Recovery Devices (ERD). The next paragraph shows our revolutionary high pressure solution comprised of high pressure pump and ERD, and how it is compared to other high pressure solutions with ERD.
Abacus Resales Solutions for Seawater Desalination
The SALINO® Pressure Center revolutionizes seawater reverse osmosis desalination with its patented-award winning solution. Its compact design consisting of a 4-in-1 technology makes it extremely space-saving and therefore perfect for compact containerized systems. It combines high-pressure pump, energy recovery device, booster pump and electric motor in one single unit.
Figure 6: model of RO plant with booster pump in series to pressure exchanger
Figure 7: model of RO plant with booster pump connected to pressure exchanger
Other energy recovery systems need more components than the SALINO® Pressure Center because they use one more motor to power an additional booster pump (figures 6 & 7) that compensates the pressure difference, which is not necessary with the SALINO®, as shown in figure 8. These extra pump and motor are part of the isobaric pressure exchanger or are connected in series to it. There are also the turbocharger pressure exchangers, like the ones shown in figures 6 and 7, that have simple configurations as the SALINO®. But since the SALINO® is a positive displacement equipment, it has much higher efficiency at working pressures than the turbochargers, that are instead rotodynamic equipment.
Figure 8: model of a RO-plant using the SALINO®-All-In-One solution
SALINO® Pressure Center
Low pressure feed water (colored in light blue) flows into the high pressure pump (left part of the SALINO®, video 01:35). This flow is pushed forward to the high pressure side (colored in purple) by axial pistons that rotate on an angled plate. In the high pressure side, the feed water is filtrated by the RO membrane.
For this case, the membrane has a recovery rate of 45% which means that 45% of the feed water is filtrated by the RO membrane and becomes pure water. The remaining 55%, now high pressure brine (purple), are led back into the energy recovery device.
In the energy recovery device, the process happens in reverse (video 01:55). High pressure brine moves the pistons, which rotate on an angled plate. The angle here is smaller than on the pump side, proportionally to the total volume of fluid. As the brine pushes the pistons and rotates the pump shaft, energy is returned to the system, and the brine loses pressure. So, the brine exiting the device (light blue) has low pressure again.
Depending on the configurations of the plant the ERD can give back up to 70% of the power initially put into by the motor (video 02:15), making it very efficient.
Using an ERD is crucial to minimize total energy consumption of the plant, which in the case of the SALINO® can be up to 70% due to its simple and highly efficient design.
The 4-in-1 simple SALINO® solution has the best energy efficiency and lowest life cycle costs in its class. It is easy to install and operate, and highly reliable to run. For the small and medium-sized systems in industry, ships and hotels, the SALINO® Pressure Center units are the ideal solution.
The following table shows the working range of the SALINO® models.
Figure 9: table picturing working range of SALINO® models
