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Reverse osmosis and thermal desalination are the most common methods of desalination. Understanding the differences is essential for implementing effective desalination projects.

Reverse osmosis and thermal desalination top the list of ways to turn saline water into fresh water

According to UNICEF, more than 2 billion people live in regions of the world where the water supply is insufficient, and half the world’s population could be facing water scarcity by 2025. By 2030, around 700 million people could be displaced because of severe water scarcity. With these supply problems, there is a growing need to tap into alternative sources of water to meet the demand.

While 71% of the Earth’s surface is covered by water, only 3% of this water is fresh, and most of it is locked in ice caps, glaciers, snow, or permafrost. Since the oceans contain about 96.5% of the Earth’s water, and saline groundwater makes up an additional 0.93%, transforming saline water into fresh water is crucial for addressing water scarcity in many parts of the world.

Reverse osmosis and thermal desalination are the two most common methods of desalinating seawater and brackish water in regions where conventional water sources are limited.

Reverse osmosis (RO) uses membranes to separate salt and impurities from saline water under high pressure. Its versatility and recent gains in energy efficiency have made it the most common method.

Thermal desalination relies on heating seawater to produce steam, which is then condensed into fresh water. While effective for high-salinity water, it typically requires more energy than reverse osmosis. Understanding the differences between these two methods is essential for implementing effective desalination projects worldwide. Below is a breakdown of how each method works.

How Membrane Desalination Works

Semipermeable membranes are at the heart of reverse osmosis desalination systems. These membranes have very tiny pores that allow water molecules to pass through while trapping larger molecules, effectively filtering out salts and contaminants.

In reverse osmosis desalination plants, seawater goes through a pretreatment stage, during which large particles and impurities are removed. The seawater is then pressurized with pumps, preventing pure water from passing through the membrane to dilute the saltwater.

The pressurized seawater is then forced against the semipermeable membranes. Its water molecules are pushed through the membrane while salts, minerals, and other impurities are left behind.

The water that passes through the membranes is now much less salty and is suitable for drinking water, agricultural irrigation, or industrial processes.

As water molecules from the saline side cross the membranes, a concentrated solution of salt and impurities known as brine or reject water is left behind. Proper disposal or treatment of the brine is essential to minimize its impact on the environment.

Once the fresh water has been collected, it may undergo additional treatment to adjust pH, to add minerals to improve taste, or to disinfect it to meet higher drinking water standards.

Pros and Cons of Reverse Osmosis Desalination

Membrane desalination offers several advantages as well as some limitations. Some of the advantages include:

  • Scalability: RO desalination systems can be scaled up or down to meet specific water needs, making them suitable for small and large applications. Packaged plants like those in our NIROBOX™ line are especially scalable, compact, and easy to install.
  • Reliability: RO technology is established and reliable. Systems can operate efficiently for extended periods with minimal maintenance.
  • Energy Efficiency. Compared to other desalination technologies, such as thermal desalination, RO typically requires less energy, especially with recent advances in membrane technology and system design.
  • Reduced Environmental Impact. RO desalination produces less brine compared to other desalination methods, reducing its impact on marine ecosystems. Also, it doesn’t require the burning of fossil fuels, resulting in lower greenhouse gas emissions.
  • Flexible Feedwater Sources. RO can treat seawater, brackish water, and wastewater, providing flexibility in water supply management.

However, while RO desalination offers many advantages, there are some drawbacks to be aware of, including:

  • Membrane Fouling. In RO, membrane fouling is caused by the accumulation of particles, minerals, and biological matter on the membrane surface. It’s a potential issue, but Fluence’s extensive experience in designing and operating seawater reverse osmosis and membrane systems ensures optimal performance. Our NIROBOX™ modular desalination units typically include a clean-in-place (CIP) system for effective membrane cleaning. We also provide clients with detailed procedures and operations and maintenance (O&M) contracts to maintain membrane efficiency for many years. Additionally, Fluence offers build-own-operate plant options or engineering and procurement contracts that include O&M, ensuring long-term operational success and minimized downtimes.
  • High Upfront Costs. The initial capital investment required for building RO desalination plants can be substantial, making it economically challenging for some. However, this can be overcome with flexible financing solutions such as leasing and build-own-operate (BOO) contracts.
  • Brine Disposal. Although reverse osmosis produces less brine than other methods, it still generates concentrated waste that requires proper Typically, brine is discharged back into the ocean. We ensure particles are removed and waste is neutralized before discharge. Fluence engineers take special care to minimize environmental impacts by conducting thorough area studies and bathymetric surveys. If there are backwashes that need additional treatment, Fluence can provide an optimal process for removing particles before discharge. Fluence designs systems according to the highest desalination market standards to protect marine ecosystems. By using long discharge pipes and dispersion models, we ensure brine is effectively diluted deep in the sea, preventing harm to marine life.

How Thermal Desalination Works

Thermal desalination relies on the principle of evaporation to separate water from salt. In this method, seawater is heated to produce steam, which is then condensed back into liquid form, leaving behind salts and impurities. There are different thermal processes, including multistage flash distillation, multiple-effect distillation, and vapor compression distillation. While thermal desalination can be effective for treating high-salinity water sources and is less susceptible to membrane fouling, it typically requires higher energy inputs than reverse osmosis.

Pros and Cons of Thermal Desalination

As with membrane desalination, there are pros and cons of using thermal desalination to produce fresh water from seawater. Some of the advantages include:

  • High-Quality Water Production. Thermal desalination produces high-quality fresh water with minimal impurities. This process effectively handles variations in feedwater quality and salinity without the risk of membrane fouling.
  • Effective for High-Salinity Sources. This method is particularly effective for treating high-salinity water sources.
  • Alternative Energy Sources. To reduce costs, thermal desalination can be powered by various sources, including waste heat from industrial processes, solar energy, or geothermal energy.

There are also drawbacks to be considered, such as:

  • High Energy Consumption. Thermal desalination requires significant energy for heating seawater, making it generally less energy-efficient than methods such as reverse osmosis. This can lead to increased operational costs and environmental impacts, particularly if fossil fuels are used.
  • Higher Capital Costs. The process may have higher capital costs and maintenance requirements than other desalination technologies.
  • Brine Disposal Challenges. Like membrane desalination, thermal desalination generates concentrated brine that requires proper disposal. Improper disposal can pose environmental challenges and harm aquatic ecosystems.

Despite these drawbacks, thermal desalination remains a viable option for producing fresh water in regions with abundant heat resources or specific water quality requirements. However, advancements in membrane technology, such as reverse osmosis, have led to increased use of this more energy-efficient and cost-effective desalination method in many applications.

Fluence has years of experience in financing, designing, constructing, and operating desalination plants in regions where fresh water is in short supply. Contact Fluence to learn more about our desalination solutions, including our NIROBOX™ packaged desalination plant, for providing safe drinking water to communities and high-quality process water for industrial operations anywhere in the world.

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