What is Reverse Osmosis?

How the most popular desalination technique turns seawater into drinking water. 

Every living being needs water to survive, making it one of the most precious resources on our planet. Although water covers 70% of the Earth’s surface, only 2-3% is freshwater, used by homes, farms, and industry. The other 98% of the Earth’s water is salt water, which is undrinkable.  

As our world faces increasing water scarcity linked with water quality decline and the overuse of slow-to-renew groundwater sources, finding ways to provide access to clean water is crucial. Desalination technologies transform seawater into fresh water, offering a solution to water scarcity challenges, but they require high energy consumption. 

One of the most efficient desalination technologies is reverse osmosis (RO): a water purification process that uses a semi-permeable membrane to separate the water and salt, creating freshwater and a concentrated brine (salt solution). So, how does it work? 

How does reverse osmosis work?  

If osmosis is the spontaneous movement of water molecules across a semi-permeable membrane to create two solutions of equal concentration, reverse osmosis uses pressure to invert this process, creating two different solutions.  

Imagine two containers—one full of seawater, the other holding fresh water—with a special filter in between that has extremely tiny holes that only allow water molecules to pass through (the semi-permeable membrane). 

When pressure is applied to the container with the saltwater, it forces the water through the membrane, leaving the salt and other impurities behind.  

The result is clean, drinkable water on one side, and the brine—a concentrated salt solution—on the other. 

Although it may sound easy, let’s take a deeper look at the process. First, water is taken from the sea and undergoes an initial treatment process to eliminate impurities such as oil, seaweed, rubbish, etc. Following this, the saltwater is subjected to reverse osmosis, resulting in two streams: fresh drinking water and brine, the latter containing all the impurities unable to pass through the membrane.  

The brine is then diluted before being returned to the sea to avoid high concentrations of salt, which could harm the ecosystem. The freshwater passes through a remineralization and chlorination process, after which it is stored in tanks and sent to the distribution network for consumption.  

Making more (water) with less (energy) 

Reverse osmosis is the most widespread desalination technology, used in many water-scarce regions across the globe to turn seawater into freshwater for drinking, farming, and other needs. With global water demand increasing — linked with the growing global population and economy — in a progressively variable climate and worsening water quality, efficient and secure water treatment technologies are more crucial than ever. 

However, despite RO offering the opportunity to make freshwater from abundant saltwater resources, a lot of energy is required to create the high pressure needed to push water through the membrane, making the process more energy-intensive than conventional water sources (e.g. surface water or groundwater). 

One new promising advancement in RO is the development of Low-pressure Multistage System Reverse Osmosis (LMS RO), which reduces energy use by employing multiple stages at lower pressure, thus lowering the overall energy needs of the RO process. Compared with a conventional system, LMS RO reduces energy consumption by approximately 20% — a marked improvement, but further improvements are needed to improve overall sustainability. 

One new promising advancement is the development of Low-pressure Multistage System Reverse Osmosis (LMS RO), which reduces the overall energy needs of the RO process.

Researchers are working hard to find ways to make desalination processes more energy-efficient, including incorporating renewable energy sources and proposing novel technological integrations that increase efficiency. The LIFE INDESAL project, for example, is testing an integrated circular seawater desalination solution that incorporates LMS RO alongside circular technologies that recover energy and minerals for use within the desalination plant itself.  

Innovations like these are essential to aligning the dual goals of building water security, and climate resilience: By improving the energy efficiency – and thus both economic and environmental cost – of alternative water resources, we can reduce the pressure on conventional resources (i.e. groundwater and surface water) without undermining climate mitigation goals.   

A water-secure future 

Rising water demands must be met, and reducing vulnerability to water stress requires the diversification of water sources. Desalination, alongside regenerated water, has a key role to play in building water security. In the context of our changing climate, promising advancements like LMS RO give hope that the future of desalination can strike a balance between necessity and sustainability, quenching humanity’s growing thirst in harmony with the planet.