How Reverse Osmosis Actually Works: A Clear Guide
Reverse osmosis removes many dissolved salts and contaminants by pushing water through a semi-permeable membrane under pressure. Clean water passes through as permeate, while concentrated reject water carries much of the unwanted material away. This guide explains how reverse osmosis actually works without unnecessary jargon, including membrane structure, TDS readings, water waste, maintenance, and the system’s limits.
If you are comparing treatment options, start with your source-water test. A home water quality testing guide can help you identify which contaminants matter before you choose an RO system.
How Reverse Osmosis Actually Works: Osmosis First
Osmosis is a natural process. Water moves through a semi-permeable membrane from a solution with fewer dissolved solids toward one with more dissolved solids. This movement continues until the pressure created by the concentration difference reaches balance.
Plant roots use a related process to draw water from soil. The membrane lets water molecules move more easily than many dissolved substances, so the two sides gradually become more alike in concentration.
Reverse osmosis changes that direction. A pump applies pressure to the more concentrated side, such as tap or well water. When the applied pressure is greater than the water’s natural osmotic pressure, water is pushed through the membrane toward the less concentrated side. Dissolved contaminants remain behind and become more concentrated in the reject stream.
The U.S. Environmental Protection Agency identifies reverse osmosis as a treatment technology that can reduce many dissolved inorganic contaminants, including salts and some metals. The EPA also stresses that performance depends on the specific system, source water, and maintenance. In other words, an RO label alone is not proof that a unit removes every contaminant.
How Reverse Osmosis Actually Works Inside the Membrane
A household RO membrane is usually a tightly wound spiral element rather than a flat sheet. Thin layers are wrapped around a central tube, creating a large treatment surface inside a compact housing. Feed water flows across the membrane surface, while treated water moves inward through the layers.
The membrane commonly has three main layers:
- Polyester support layer: provides strength and helps the membrane hold its shape.
- Polysulphone microporous layer: supports the active layer while allowing water to move through the structure.
- Polyamide thin-film layer: forms the selective barrier where most separation occurs.
The active polyamide layer has pores of approximately 0.0001 microns, or about 0.1 nanometres. For scale, a human hair is roughly 70,000 nanometres wide, a bacterium about 1,000 nanometres, and a virus about 100 nanometres. A water molecule is approximately 0.27 nanometres across.
| Particle or substance | Approximate size |
|---|---|
| Human hair | 70,000 nm |
| Grain of sand | 90,000 nm |
| Bacterium | 1,000 nm |
| Virus | 100 nm |
| Water molecule | 0.27 nm |
| RO membrane pore | About 0.1 nm |
Water molecules are small enough to move through the membrane. Many dissolved salts, heavy metals, larger organic molecules, bacteria, and particles are rejected. Some substances are held back because they are too large; others are rejected because the membrane’s chemistry interacts with the electrical charge of dissolved ions.
Why RO TDS Changes and What a Meter Measures
A TDS meter does not identify individual contaminants. It measures electrical conductivity and converts that reading into an estimate of total dissolved solids, usually shown in parts per million, or ppm. Salts and minerals conduct electricity, while very pure water conducts much less.
A lower TDS reading is useful evidence that dissolved material has been reduced, but it is not a complete safety test. A TDS meter cannot tell you whether water contains arsenic, lead, bacteria, or a particular pesticide. For those questions, use laboratory testing or a treatment system certified for the contaminant of concern.
- Distilled or highly purified water may read close to 0 ppm.
- Typical Dhaka tap water may read 300–800 ppm.
- Coastal district groundwater can reach 1,000–3,000 ppm.
- After RO treatment, water commonly reads about 10–50 ppm.
Actual readings vary with local geology, season, temperature, plumbing, and filter condition. Compare feed water and purified water rather than relying on one number alone.
Use this formula to estimate membrane rejection:
Rejection rate (%) = (1 - Permeate TDS / Feed TDS) × 100
For example, if feed water measures 600 ppm and permeate measures 30 ppm, the rejection rate is 95%. A healthy new household membrane often achieves about 92–97% rejection, although the manufacturer’s specification is the best benchmark.
The Four Water Streams in an RO System
A household RO purifier normally manages four different water flows. Knowing them makes it easier to diagnose low production, high waste, or a storage tank that fills slowly.
- Feed water: source water from a tap, well, or building supply entering the purifier.
- Permeate: treated water that passes through the membrane and goes to the faucet or tank.
- Concentrate or reject water: water carrying retained dissolved solids toward the drain.
- Tank water: permeate stored in a pressurised holding tank for later dispensing.
The ratio of permeate to feed water is called the recovery rate. Standard household RO systems often recover 25–33%, meaning they produce about 25–33 litres of purified water for every 100 litres entering the system. Advanced designs can reach around 50% recovery, but pressure, temperature, membrane condition, and water chemistry all affect the result.
Reject water is not automatically unusable. It may be suitable for floor cleaning, toilet flushing, or gardening where local water quality and plant sensitivity allow. Do not use it for drinking or cooking unless it receives appropriate treatment.
For a broader comparison of filtration methods, see this guide to RO versus UV and activated carbon water filters. It can help explain why one system may suit dissolved salts while another is better for microbes or taste.
Why Pre-Filters Matter Before RO Treatment
The membrane is effective but sensitive. Pre-filters protect it from substances that would block, oxidise, or permanently damage the active layer.
- Sediment filters trap sand, rust, silt, and other particles before they reach the membrane.
- Activated carbon filters reduce chlorine and some organic chemicals. Chlorine can oxidise and degrade the polyamide layer, especially in chlorinated municipal water.
- Anti-scalant or softening treatment helps control calcium and magnesium deposits in hard water.
Without working pre-filters, a membrane that might last 18–24 months can fail in roughly 3–6 months. A sudden drop in flow, rising permeate TDS, unusual taste, or continuous drain flow can indicate a blocked pre-filter, low pressure, scaling, or a damaged membrane.
Filter schedules vary by water quality and usage. A practical RO filter replacement and maintenance checklist should include pre-filter changes, leak checks, tank pressure, sanitation, and membrane performance.
What Reverse Osmosis Does Not Remove Reliably
RO is powerful, but no treatment method removes everything equally well. Dissolved gases such as hydrogen sulphide, carbon dioxide, and radon can pass through because they are small and do not behave like dissolved salts. Aeration or specialised carbon treatment may be needed.
Some very small organic compounds, including certain pesticides and volatile organic compounds, may also pass through to some degree. Activated carbon after the membrane can provide additional protection, depending on the compound and cartridge design.
RO also removes beneficial minerals such as calcium and magnesium along with unwanted substances such as arsenic and lead. A mineraliser cartridge can add some minerals back, but it does not replace testing the source water or confirming that the system is certified for the contaminants you need to reduce.
How to Check Whether an RO Membrane Is Failing
Testing rejection monthly is a practical maintenance check. It takes about two minutes when you already have a TDS meter:
- Measure feed water before it enters the membrane.
- Measure permeate after it leaves the membrane, before mineralisation if possible.
- Apply the rejection-rate formula and compare the result with the manufacturer’s specification.
| Rejection rate | Likely condition |
|---|---|
| 92–97% | Excellent; membrane is performing well. |
| 85–92% | Good, but worth monitoring and planning for replacement. |
| 75–85% | Degraded; investigate the system and replace within 1–3 months if confirmed. |
| Below 75% | Likely failed; check pressure, filters, and meter accuracy before replacing. |
These ranges are useful field guidelines, not universal pass-or-fail rules. Feed-water temperature and pressure can change production and readings. If TDS rises suddenly, check the pre-filters, membrane housing seals, storage tank pressure, flow restrictor, and meter before blaming the membrane.
Naturally, water moves through a semi-permeable membrane from the side with fewer dissolved solids toward the side with more. The movement continues until the concentration difference reaches balance.
An RO pump pressurizes the more concentrated side, such as tap or well water. When applied pressure exceeds osmotic pressure, it reverses the natural direction of water movement.
Water molecules move through the selective membrane more easily than many dissolved substances. The membrane is usually a tightly wound spiral element, not a simple open screen.
Clean water leaves as permeate, while concentrated salts and other retained material flow away in the reject stream. That drain flow helps prevent buildup on the membrane surface.
Reverse Osmosis FAQ
Does RO remove bacteria and viruses?
The membrane can reject many microorganisms because they are much larger than its effective separation scale. However, a damaged membrane, poor seals, contaminated storage tank, or dirty faucet can reintroduce risk. Disinfection and routine servicing still matter.
Is low TDS proof that water is safe?
No. TDS estimates dissolved conductive material, not every contaminant. Laboratory testing and certified treatment claims are better ways to assess a specific water-quality concern.
Why does an RO system send water to the drain?
The reject stream carries concentrated salts and other retained material away from the membrane. Without this flow, contaminants would build up at the membrane surface, reducing performance and increasing scaling.
How often should an RO membrane be replaced?
Many household membranes last about 18–24 months under suitable conditions, but hard water, chlorine exposure, heavy use, low pressure, and poor pre-filter maintenance can shorten that period. Use rejection performance and the manufacturer’s instructions rather than a calendar alone.
Follow these steps to understand and evaluate an RO system:
- Test the source water first. Identify the dissolved solids, metals, hardness, chlorine, and other contaminants that need treatment.
- Understand the pressure requirement. RO needs applied pressure greater than the water’s osmotic pressure to push water through the membrane.
- Use pretreatment to protect the membrane. Sediment and carbon filters help reduce particles, chlorine, and other substances that can damage or foul the membrane.
- Separate permeate from reject water. The membrane allows relatively clean water through while sending concentrated contaminants to the reject stream.
- Check performance with more than TDS alone. Compare feed and product-water readings to estimate rejection, but use laboratory testing and certified claims for specific contaminants.
- Maintain the complete system. Replace filters, sanitize storage components, verify pressure, inspect for leaks, and replace the membrane when performance declines.
Does reverse osmosis waste water?
Yes. Standard household systems commonly recover 25–33% of feed water, so a substantial portion becomes reject water. Systems with better recovery can reduce waste, but they usually require suitable pressure, controls, and careful management of scaling.
In short, reverse osmosis works by using pressure to overcome osmotic pressure and move water through a selective membrane. It can greatly reduce dissolved solids, but reliable results depend on correct pretreatment, adequate pressure, sanitary storage, testing, and maintenance.