What Is Reverse Osmosis and How Does It Work?

Osmosis is a process that happens naturally. When two solutions of different concentration sit either side of a semi-permeable membrane, water moves from the dilute side to the concentrated side. Reverse osmosis turns that flow around: pressure above the osmotic pressure is applied to the concentrated solution, and water passes to the clean side of the membrane, leaving dissolved substances behind.

In industrial terms this means filtration at 0.0001 micron. At that scale salts, heavy metals, bacteria, viruses and dissolved organics are separated from the water. For comparison, a bacterium is around 0.2 micron and a virus around 0.02 micron.

The pressure required depends on the salinity of the feed water. For mains or well water 10–15 bar is enough; seawater needs 55–70 bar. That pressure requirement is why energy is the largest cost item in seawater desalination.

Energy recovery devices (ERD) address exactly this. Water leaving through the concentrate line still carries high pressure; an ERD transfers that pressure back to the feed line and reduces the load on the pump. A system without energy recovery consumes 6–8 kWh per m³, while systems with an ERD come down to 3–4 kWh. For a plant producing 100,000 m³ a year that is a difference of hundreds of thousands of kilowatt-hours.

What really determines the life of an RO system is pre-treatment. If water reaching the membrane carries suspended solids, free chlorine or scale-forming ions, the membrane fouls quickly. That is why every properly built RO system has a multimedia filter, an activated carbon filter and antiscalant dosing ahead of it.

The most common mistake in system selection is setting capacity without a water analysis. Two plants of the same capacity will need entirely different membrane configurations and show different energy consumption if their feed water differs. Engineering should always begin with the analysis.