What Is Osmosis — and What Makes Reverse Osmosis Different
To understand reverse osmosis, you first need to understand regular osmosis. Osmosis is a natural process where water moves through a semi-permeable membrane from a region of lower dissolved solids concentration to a region of higher concentration. This is how plant roots absorb water from soil — water naturally moves toward the more concentrated solution.
Reverse osmosis does the opposite. By applying pressure greater than the natural osmotic pressure, water is forced through a semi-permeable membrane from the more concentrated solution (your contaminated tap water) to the less concentrated side (pure water). The pressure literally reverses the natural direction of water flow — hence "reverse" osmosis.
The RO Membrane: What It Actually Looks Like
An RO membrane is not a flat sheet — it is a tightly wound spiral of thin-film composite (TFC) material. The membrane is made of three layers:
- A polyester support layer — provides structural strength
- A polysulphone microporous layer — the intermediate support
- A polyamide thin-film layer — the actual barrier where filtration happens
The polyamide layer has pores of approximately 0.0001 microns (0.1 nanometres). To put this in scale:
| Particle | Size |
|---|---|
| Human hair | 70,000 nm |
| Grain of sand | 90,000 nm |
| Bacterium | 1,000 nm |
| Virus | 100 nm |
| RO membrane pore | 0.1 nm |
| Water molecule | 0.27 nm |
Water molecules (H₂O) are small enough to pass through the membrane. Dissolved salts, heavy metals, most organic molecules and all larger particles are physically too large to pass through. They are rejected by the membrane and flushed away in the reject water stream.
Why the TDS Number Changes: What a TDS Meter Actually Measures
A TDS (Total Dissolved Solids) meter does not measure specific contaminants. It measures the electrical conductivity of water and converts this to an estimate of dissolved solid concentration in parts per million (ppm). Dissolved minerals and salts conduct electricity; pure water does not.
This is why:
- Pure water (like distilled water) reads close to 0 ppm
- Typical Dhaka tap water reads 300–800 ppm
- Coastal district groundwater can read 1,000–3,000 ppm
- After RO treatment, water typically reads 10–50 ppm
The rejection rate of your RO membrane is calculated as:
Rejection rate (%) = (1 - Permeate TDS / Feed TDS) × 100
So if your source water is 600 ppm and your purified water is 30 ppm, the rejection rate is (1 - 30/600) × 100 = 95%. A healthy new membrane should achieve 92–97% rejection.
The Four Water Streams in an RO System
A functioning RO purifier manages four distinct water flows:
- Feed water — your source tap or well water entering the system
- Permeate — the purified water that passes through the membrane (what you drink)
- Concentrate / Reject water — the water carrying all the rejected dissolved solids, flushed down the drain
- Tank water — the permeate stored in the pressurised holding tank ready for dispensing
The ratio of permeate to reject water is called the recovery rate. Standard household RO systems have recovery rates of 25–33% — meaning 25–33 litres of purified water for every 100 litres of feed water. Advanced systems achieve 50% recovery.
Why RO Systems Need Pre-Filters
An RO membrane is highly effective but also fragile. Three things destroy it prematurely:
- Chlorine — oxidises and degrades the polyamide thin-film layer within weeks of unprotected exposure. This is why a carbon pre-filter (which removes chlorine) is mandatory before the membrane in any chlorinated municipal supply.
- Sediment and particles — larger particles clog and physically damage the tightly wound membrane. A sediment pre-filter is the first stage for this reason.
- Scaling — calcium and magnesium can precipitate on the membrane surface in hard water, reducing flow and rejection. An anti-scalant or water softener upstream prevents this.
Without functional pre-filters, an RO membrane that should last 18–24 months can fail within 3–6 months.
What RO Does NOT Remove — and Why This Matters
The RO membrane is extremely effective but has limitations worth knowing:
- Dissolved gases — gases like hydrogen sulphide, carbon dioxide and radon are small enough to pass through the membrane. Post-treatment aeration or carbon filtration is needed for these.
- Certain pesticides and VOCs — very small organic molecules can sometimes pass through. Activated carbon post-filtration addresses this.
- Minerals (beneficial and harmful equally) — RO removes calcium and magnesium along with arsenic and lead. This is why mineraliser cartridges exist in premium systems.
How to Know If Your RO Membrane Is Failing
Test your membrane rejection rate monthly with a TDS meter:
- Measure your source (feed) water TDS
- Measure your purified (permeate) water TDS
- Calculate rejection rate:
(1 - permeate TDS / feed TDS) × 100
| Rejection Rate | Membrane Condition |
|---|---|
| 92–97% | Excellent — membrane is healthy |
| 85–92% | Good — monitor closely, plan for replacement |
| 75–85% | Degraded — replace within 1–3 months |
| Below 75% | Failed — replace immediately |
A rejection rate below 85% means your purified water is not as clean as your purifier is implying. This test takes two minutes and costs nothing if you already own a TDS meter.