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How Reverse Osmosis Filters Work

By Finlay Gilkinson – 28/06/2025

Reverse osmosis (RO) is a powerful water purification technology used in homes, industries, and even large-scale water treatment facilities. It’s known for producing clean, safe drinking water by removing a wide range of contaminants. But how exactly does it work? Let’s break it down in a clear and concise way.

How Reverse Osmosis Filters Work

What Is Reverse Osmosis?

Reverse osmosis is a filtration process that removes impurities from water by forcing it through a semi-permeable membrane. Unlike standard filters that trap large particles, RO uses a membrane with tiny pores—small enough to block contaminants like salts, minerals, bacteria, and even some chemicals, while allowing water molecules to pass through.

The “reverse” in reverse osmosis refers to the process working against the natural flow of osmosis. Osmosis occurs when water moves from a less concentrated solution (like pure water) to a more concentrated one (like salty water) through a semi-permeable membrane to balance concentrations. In RO, external pressure is applied to reverse this flow, pushing water from a more concentrated solution to a less concentrated one, leaving contaminants behind.

How Does a Reverse Osmosis System Work?

A typical RO system involves several stages to ensure thorough purification. Here’s a step-by-step look at the process:

  1. Pre-Filtration: Water first passes through pre-filters, which remove larger particles like sediment, dirt, and chlorine. These filters protect the delicate RO membrane from damage and extend its lifespan. Common pre-filters include sediment filters and carbon filters.
  2. Reverse Osmosis Membrane: The heart of the system is the RO membrane. Water is forced under pressure through this semi-permeable membrane, which has pores as small as 0.0001 microns. This size is tiny enough to block dissolved solids (like salts, heavy metals, and minerals), bacteria, and some organic compounds, while allowing water molecules to pass.
  3. Pressure Application: A pump or household water pressure pushes water against the membrane. This pressure is critical because it overcomes the natural osmotic pressure, forcing water to move from the contaminated side to the clean side.
  4. Contaminant Removal: As water passes through the membrane, contaminants are left behind in a concentrated form, called brine or reject water. This waste is typically flushed away, ensuring the membrane doesn’t clog.
  5. Post-Filtration: After the RO membrane, water may pass through additional filters, like a polishing carbon filter, to remove any remaining tastes or odours, ensuring crisp, clean water.
  6. Storage and Delivery: Purified water is stored in a tank until needed. When you turn on the RO faucet, water is dispensed, sometimes passing through a final filter for extra purity.

    7. What Does Reverse Osmosis Remove?

    RO systems are highly effective at removing a wide range of contaminants, including:

    • Dissolved salts and minerals (e.g., sodium, calcium, magnesium)
    • Heavy metals (e.g., lead, arsenic, mercury)
    • Nitrates and sulphates
    • Some organic compounds (e.g., pesticides, PFAS)
    • Fluoride and chlorine

      • However, RO isn’t perfect. It may not remove all volatile organic compounds (VOCs) or certain gases, which is why additional filters are often used in the system.

      Benefits and Considerations

      Benefits:

      • High Purity: RO produces some of the cleanest water possible, ideal for drinking and cooking.
      • Versatility: It’s effective against a broad spectrum of contaminants.
      • Improved Taste: By removing impurities, RO enhances the flavor of water.

        • Considerations:

        • Water Waste: RO systems produce reject water, which can be a concern in water-scarce areas. Some systems recycle this water to minimize waste.
        • Maintenance: Membranes and filters need regular replacement to maintain performance.
        • Mineral Removal: RO removes beneficial minerals like calcium and magnesium, which some systems reintroduce via remineralization filters.

          • Applications of Reverse Osmosis

          • Home Systems: Under-sink RO units provide clean drinking water.
          • Industrial Use: RO purifies water for manufacturing, food processing, and pharmaceuticals.
          • Desalination: Large-scale RO plants convert seawater into freshwater for communities.

            • How Reverse Osmosis Filters Work

            Reverse osmosis is a highly effective water purification method that relies on a semi-permeable membrane and applied pressure to remove a wide range of contaminants. By combining pre-filtration, the RO process, and post-filtration, these systems deliver clean, safe, and great-tasting water. While there are considerations like water waste and maintenance, the benefits make RO a go-to solution for households and industries alike. If you’re looking for reliable water purification, understanding how RO works can help you make an informed choice for your needs.

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