Desalinization of water using reverse osmosis is one of the greatest inventions of man. Desalinization eliminates salt components from a water sample as a way of providing fresh drinkable water.
The world’s landmass is huge, and an even bigger surface area is covered with water. Given this fact, water should never be a problem as long as one has access to this water. However, the main problem is that most of this water is salty.
We do not have much use for salty water as compared to freshwater. One could even die of thirst when lost at sea due to dehydration. Drinking this salty water is even more harmful than not drinking at all. Research indicates that saltwater consumption is dangerous and excessive intake may even lead to deaths due to dehydration. As such, the presence of a technology that could transform some of this water into a fresh and drinkable one would be a lifesaver.
Many technologies have been availed to help solve this problem. One of the earliest and simplest methods is distillation. In a nutshell, the salty water is placed in a suitable container and heated till it evaporates. The steam is then condensed and used as freshwater. However, the problem with this technology is the amount of thermal energy that is used to make it happen.
Hence, comes in the process of salt removal through the use of reverse osmosis.
What is Reverse Osmosis Technology?
Reverse osmosis is simply the passage of a solvent (the liquid component of a mixture) from a region in which it is in high concentration to that which it is in low concentration via a semipermeable membrane.
To understand how reverse osmosis works, it is important to shed light on what osmosis is by itself. Osmosis denotes the movement of a solvent from a region in which it is in low concentrations to that which it is in high concentration via a semi-permeable membrane.
The main difference between the two processes is the direction that the solvent takes in relation to the semi-permeable membrane. For reverse osmosis to take place, there needs to be a source of pressure that forces the water through the membrane. As such, some amount of energy is also required for this process to be a success.
A Short History of Reverse Osmosis Technology
The idea was first brought to light by Scientist Abbe Nollet. The modern reverse osmosis technology was developed through both collective and independent approaches. The developers and inventors managed to build the technology to its modern status over the past century.
The technology has constantly improved and increased in its efficiency. The technology was readily accepted in the industrial scale of desalinization. This was due to the considerable efficiencies in the energy used as compared to distillation.
As of today, the technology is widely used for many different applications instead of just the production of drinkable water.
Some Technical Aspects and Definitions In Reverse Osmosis
The Feed-water is the raw water that enters the osmotic chamber. The Membrane is the layer that selectively permits the flow of water from one end to the other to separate it from the salt components. The reverse Osmosis module is the system containing the membrane. The Permeate refers to the freshwater which is a product of the action of the reverse osmosis process. Osmotic potential is dependent on the concentration of salt in the seawater. The Flow rate is the degree of movement of the water across the membranes and is dependent on the osmotic potential and the pressures induced by the pump.
For reverse osmosis to take place within the confines of a treatment chamber, the pressure induced by the pump to the raw water needs to be greater than the osmotic pressure within the treatment module. However, if the pump pressure does not exceed the osmotic pressure, the permeate will revert and travel back to the raw water side through osmosis.
Design Process of the Reverse Osmosis System
Before a Reverse osmosis plant is set up, the economics involved in its intended operations must be analyzed carefully. The key design considerations of an RO design include:
- The Feed-water (Saline Water)
- Membrane
- Pumping requirement
- Pressure Control
- Permeate (Desalinated water)
The Feed-water (saline water)
The main factor that has the potential to affect the entire system is the source of the Feed-water. The selection of the water source affects the operation of the entire system. Water with high turbidity will give rise to the need for more treatment steps before the desalinization process. Furthermore, the concentration of the salts in the water and the production demands are also considered. If multiple sources are available, the one with the best quality is selected.
Good quality water will improve the efficiencies of the membrane. As such, it is common practice to include pre-treatment techniques in the feed-water. These initial processes eliminate the microbes and suspended matter from the water.
Raw water that is of unsatisfactory quality undergoes treatment similar to the convention water treatment plants in most municipalities. These include coagulation, flocculation, clarification, and disinfection. A further filtration system ensures that microscopic constituents that might remain are removed.
The Membrane
The Reverse Osmosis membranes are often highly efficient. However, they are seldom perfect and tend to allow a small percentage of salt to pass.
The unique properties of a particular membrane together with its surface area are crucial. These will affect the flow rate. One of the most used membrane installation types is the spiral configuration. This often prevents any backflows; hence proper pre-treatment is required. Some portion of the Feed-water is employed to clean the membrane surface.
Pumping Requirements
The pump pressure acts on the raw water to produce the permeate. As a result, the flow rates within the modules are dependent on the pump and the salinity of water itself. The rate is ultimately the difference between pump and osmotic pressure. A similarity in the two will lead to zero flow flow rate. Hence, this shows the need for a pump that is reliable in achieving this goal.
Two main types of pumps are often used. The selection of the pump depends on its use. In largescale seawater desalinization plants, the centrifugal pump is often preferred. However, the small-scale applications feature the use of positive displacement pumping.
Pumping needs to be done for both the permeate and the salt concentrate. Furthermore, the amount of Feed-water is always greater than that of the fresh permeate. Furthermore, the concentration of the saltwater within the desalination chamber is much greater compared to that of the Feed-water. Due to these reasons, the actual energy used in the process far exceeds the theoretical expectations of the same.
Pressure Control
Since the practical energy used in the process is great, an overpressure (pressure exceeding the theoretical expectations) is often applied. However, design improvements have been made to increase the efficiencies of the systems. For instance, connecting multiple osmotic chambers in series. A similar magnitude of pressure is applied to all of them while the saline water flows through all of them. Since the osmotic pressure rises between the different chambers, the rates of flow within them decrease along with the permeate.
Valves are often included to regulate the passage of water within the system.
The Permeate (Desalinated water)
The quality of freshwater obtained depends on the salt concentration levels in the raw water as well as the kind of membrane used. Some small salt concentrations may still be present in the permeate. Due to this, the RO process may be done a second time. Some small quantities of microorganisms may be present due to faults and leakages in the system. Hence, this leads to the need for further post-treatment.
Main Applications of Reverse Osmosis
The main applications of reverse osmosis include:
- Sea-Water Reverse Osmosis (SWRO)
- Food and Beverage Industry – Beer and wine production takes advantage of the technology. RO process also aids in the concentration of juices.
- Small Scale Saline Water Conversion Devices – Many novel techniques are constantly being produced to match the water demands in many parts of the world. These systems include portable devices that can be easily purchased. The devices are used in homes, explorers, and other similar applications. Several mineral water production companies also utilize RO technology.
- Military Use – the military has an advanced RO system to aid with the flexibility of the water treatment services. The permeate is then disinfected and either discharged or stored for later use.
- Chemical production.
- Health industry.
Sea Water Reverse Osmosis
SWRO is used to convert seawater into drinkable water. The process has great efficiencies due to the economies of scale. The membranes operate on a scale of 0.001 micrometres. This is the highest possible form of the purification system. Other systems used to remove particles from water do not match this scale.
While most systems use seawater in their feeds, other sources of salty water are used as well. These include water from wells and other sources. Such water is known as Brackish water. Hence, this demonstrates that the raw water used often varies in degree of concentration.
Sea Water Desalinization Process Steps (SWRO)
- Feed-water Intake – The seawater is directed into the treatment plant via pipes.
- Pre-treatment – The raw water has to be subjected to some preliminary steps to prepare it for efficient passage through the membrane. The solid particles are first screened before directing the water into cartridge filters. Chlorine is infused into the water to remove the microorganisms. It must be neutralized to prevent corrosion of the membranes. The acidity level is then changed and stabilized.
- Pumping Stage – Often pumps delivering high pressures are used in the process. However, the decision to use them is dependent on whether an energy recovery system is utilized. Furthermore, the use of these recovery devices leads to almost half the savings on the total energy. The energy is often used to supplement the pressure of the primary pumps into the system.
- Reverse Filtration – The water then flows to the membrane system. A fraction of water passes through the membranes while the rest is ejected as waste, and might be used for energy recovery. Greater efficiencies of permeate volume are attained with greater pressures in large treatment plants.
- Energy Recovery – There are a variety of devices that can be installed to ensure that the energy used is recovered. Hence, the total energy used for pressure is subsided. These include turbines and recovery pumps among others.
- Post-treatment – These include the treatments subjected to the permeate to make it suitable for the intended need. Chemicals such as lime might be induced to prevent rusting of the pipes and contact areas. The acidity is also adjusted and the essential minerals four human consumption can be infused. Some disinfection can also be made to ensure that all disease-causing organisms are destroyed.
Advantages of Reverse Osmosis Technology
- The RO technology does not require rainwater input, unlike most other treatment procedures. Hence, the raw brackish water is often in large quantities from sources such as seawater and boreholes.
- The technology does not cause pollution as does the other desalinization systems. Through reverse osmosis, neither the marine ecosystem nor the other environments are tampered with. However, the disposal of the brine (concentrated saltwater from the chamber) must be done to ensure no harm is caused to the marine environment.
- The efficiency of the process makes it superior to the other systems. The production on a large scale uses a lot of energy to induce pressure used for the process. However, most of this energy is often recycled so that the losses are minimized.
Disadvantages of Reverse Osmosis Technology
- The technology makes use of lots of Feed-water and only a small fraction of freshwater is obtained. This is especially true for most small-scale treatment technologies. However, on an industrial scale, the efficiency of the system is much higher.
- While the RO process might lead to complete salt removal from the water samples, it also eliminates the essential ions. The ions are naturally present in mineral water. Remineralisation processes are often included in seawater treatment. However, this does not offer the complete edition of the essential minerals.
- Salt removal is often advantageous in areas that are close to seas and oceans. However, those that are in far places and at great heights lack the necessary brackish water resource. Hence, the expense challenges make it uneconomical to transport and apply the technology. In such areas, it would be more economical to use other sources of freshwater supply such as rainwater harvesting.
Does reverse Osmosis Really Work?
Yes, reverse osmosis works and is highly efficient.
It is safe to drink desalinized water as long as it is properly disinfected.
The pores of the membranes used in RO process are so small that relatively large ions and molecules are retained on one side, and only the water molecules are allowed to pass.
Though lots of energy is used in the process, it can still be reused.
The recovered energy is in form of heat energy. The heat is especially generated within the RO chamber as a product of the actions of the components. For instance, the frictional forces of the water as it edges its way through the membrane produce heat. Some also result from the passage of the highly pressurized salinized water across the valves. As a result, a great portion of this heat is held in the highly salinized water remain after the process.
Some areas employ the use of solar power to run the pumps. Solar energy has several technology applications in the fresh water production and may even be used for solar-powered desalination.
Other alternatives to reverse osmosis are available.
The other types of desalinization processes include:
- Forward osmosis
- Distillation
- Freeze-thaw method
- Electro-dialysis
- Solar desalinization
Conclusion
Reverse osmosis has been responsible for several water treatment developments in the world since its inception. With the constant upgrades and research, its efficiency keeps improving.
The technology has aided nations, communities, armed forces with the solution to freshwater. Furthermore, it has availed this water to many industries whose operations would have otherwise been paralyzed had they lacked the water.
More importantly, reverse osmosis has provided sustainable freshwater solutions to millions of people. Hence, this has led to the improvement of lives and development on a global scale.