Reverse osmosis

We can clean water using a thin skin.

This skin has tiny holes. They are too small for salt. They are too small for germs.

We use pressure to push the water. This push forces water through the skin.

The salt stays on one side. The clean water goes to the other side.

This helps us make water from the sea. It also helps make juice thicker.

Clean water is very helpful.

Reverse osmosis is a way to clean water. It uses a thin layer called a membrane.

This membrane acts like a very fine screen. It has tiny holes in it. These holes are too small for salt or germs. They only let tiny water molecules pass through.

In nature, water likes to move toward salt. This is called osmotic pressure. To clean water, we must do the opposite. We use a lot of pressure to push the water. This push forces the water through the membrane. The salt and other things stay behind. The clean water flows to the other side.

Many places use this to make drinking water. Large plants turn seawater into fresh water.

It is also used in food making. It can make fruit juice thicker. It helps make maple syrup by removing water from sap. This saves energy because you boil it less. It even helps make low-alcohol beer.

Reverse osmosis is a special way to clean water. It is a process used to make water safe to drink. This method can remove salt and other tiny bits from water. It can even remove biological things like bacteria. Many people use it to turn seawater into fresh water. This is very important for making water for people to drink.

This process works by using a semi-permeable membrane. A membrane is a thin layer that acts like a very fine screen. It has tiny holes that are much smaller than a grain of sand. These holes let small water molecules pass through easily. However, larger molecules like salt or germs cannot fit. To make this work, we must apply pressure to the water. This pressure pushes the water through the membrane while the salt stays behind.

People have studied how water moves for a long time. Jean-Antoine Nollet first saw osmosis in 1748. For 200 years, it was only something seen in labs. In 1950, researchers at UCLA began studying how to use it for salt removal. Later, Sidney Loeb and Srinivasa Sourirajan found ways to make better membranes. John Cadotte also found a way to make very good membranes. Most commercial membranes are made using his method today.

There are many big machines that use this technology. In 2013, the largest plant was in Sorek, Israel. By 2019, there were about 16,000 desalination plants around the world. About half of these plants are in the Middle East and North Africa. In 1977, Cape Coral, Florida became the first US city to use it on a large scale. By 1985, that city had the largest low-pressure plant in the world. It could produce 56.8 million liters of water every single day.

We see reverse osmosis in many parts of our daily lives. You might have a small system on your kitchen counter to clean tap water.

It is also used in the food industry to help make things. It can make fruit juices thicker without using heat. Maple syrup makers use it to remove water from sap. This helps them save energy because they do not have to boil it as much. It is even used to make low-alcohol beer.

Reverse osmosis, or RO, is a sophisticated water purification process. It uses a semi-permeable membrane to separate water molecules from other substances. This technology is vital for creating potable water, which is water safe for drinking. RO can remove dissolved chemicals, suspended particles, and biological substances like bacteria. It is used extensively in large industrial processes and municipal water systems. By separating the pure solvent from the unwanted solutes, RO provides a reliable way to clean water.

To understand how it works, we must first look at natural osmosis. In forward osmosis, a solvent moves through a membrane from a low solute concentration to a high solute concentration. This movement is driven by osmotic pressure, which is a reduction in the Gibbs free energy of the system. Reverse osmosis works by applying external pressure to reverse this natural flow. This pressure must overcome the natural osmotic pressure of the solution. When pressure is applied, the purified solvent passes through the membrane while the solute is retained on the pressurized side.

The mechanism of RO is different from standard filtration. Most membrane filtration relies on straining, or size exclusion. In those processes, pores are typically 0.01 micrometers or larger to catch particles. RO, however, is a form of hyperfiltration. It involves solvent diffusion across a membrane that is often nonporous or uses nanofiltration with pores as small as 0.001 micrometers. The primary removal mechanism depends on differences in solubility and diffusivity. Because the pores are so small, RO can remove particles larger than 0.2 nanometers.

Membranes vary in their design and how they are manufactured. Early researchers like Sidney Loeb and Srinivasa Sourirajan developed asymmetric membranes. These consist of a very thin "skin" layer supported by a much thicker, porous substrate. Later, John Cadotte discovered a method called interfacial polymerization. He used m-phenylene diamine and trimesoyl chloride to create membranes with high flux and low salt passage. Most commercial RO membranes today are made using this specific method.

The history of this science spans several centuries. Jean-Antoine Nollet first observed osmosis through semi-permeable membranes in 1748. For the next 200 years, it remained mostly a laboratory phenomenon. In 1950, researchers at the University of California at Los Angeles (UCLA) began investigating osmotic desalination. By the mid-1950s, researchers at UCLA and the University of Florida were testing seawater desalination. While they saw results, the flux, or the rate of flow, was too low for commercial use.

Today, reverse osmosis is a massive global industry. By 2019, approximately 16,000 desalination plants were operating worldwide. These plants produced huge amounts of fresh water, with about half of the capacity located in the Middle East and North Africa. In the United States, Cape Coral, Florida, was a pioneer. In 1977, it became the first US municipality to use RO at scale. By 1985, Cape Coral operated the world's largest low-pressure RO plant, producing 56.8 million liters per day.

RO technology is used in many specialized ways across different sectors. In the food industry, it concentrates liquids like fruit juices without using heat. This is helpful for heat-sensitive substances like proteins and enzymes. Maple syrup producers use RO to remove 75% to 90% of the water from sap. This reduces the energy needed for boiling. In the dairy industry, it concentrates whey to produce protein powders. Even the wine industry uses RO machines to manage production processes.

Finally, RO serves critical roles in specialized environments. The military uses large-scale Reverse Osmosis Water Purification Units (ROWPU) to provide clean water. These units can remove 99.98% of a raw water's total dissolved solids. In homes, countertop RO systems use multiple stages, including carbon filters, to protect the membrane. These filters remove chlorine, which can damage thin-film composite membranes. From cleaning wastewater in Singapore to maintaining reef aquariums, RO remains a cornerstone of modern water management.