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Complete technical guide
How reverse osmosis actually works.
The science, engineering and mechanics behind one of the most effective purification technologies there is, from the physics of osmotic pressure to what happens inside the membrane.
- Membrane pore size
- ~0.1 nm
- Dissolved salt rejection
- 95–99%
- Operating pressure
- 3–80 bar
- Stages in a full system
- 5
Osmosis, then reverse it.
Natural osmosis moves water from the dilute side of a membrane to the concentrated side, with no energy at all. Reverse osmosis pushes back with more pressure than osmosis can exert, and the water turns around. Try it.
Seawater's osmotic pressure is about 27 bar. The flow shown is proportional to the net pressure, applied minus osmotic. Illustration, not to scale.
Water moves through a semi-permeable membrane from low solute concentration to high, without external energy. Nature is equalising: a cucumber in brine loses water until the concentrations balance, which is why it shrivels. The pressure behind that flow is the osmotic pressure; for seawater, about 27 bar.
Water flows: dilute → concentrated, no energy.
RO applies mechanical pressure greater than the osmotic pressure, forcing water the other way: from the salty side to the pure side. Only water molecules pass. Ions, bacteria, viruses and organics are rejected and leave as brine. Seawater desalination needs 55 to 80 bar; tap water, 3 to 10.
Pressure applied: concentrated → pure, energy required.
π = i·M·R·T
i is the van 't Hoff factor, M the molarity, R the gas constant and T the temperature in kelvin. For NaCl in seawater (about 35 g/L) this comes to roughly 27 bar. The pump must exceed it before any permeate is produced at all.
The five filtration stages.
A complete RO system is a pipeline where each stage protects the next. Skip the pre-filtration and the membrane is destroyed within weeks.
- Sediment
- Carbon
- RO membrane
- Post-carbon
- Tank & tap
- 1
Sediment pre-filter (5 to 50 micron)
The first line of defence. A spun polypropylene or wound-string cartridge captures sand, silt, rust flakes and turbidity, which would otherwise clog and abrade the carbon and RO stages. Typically 50 µm for coarse pre-filtration, then 5 µm fine.
- Removes
- Dirt, sand, rust
- Pore size
- 5–50 µm
- Replace
- Every 6–12 months
- 2
Activated carbon pre-filter
Granular activated carbon or a carbon block adsorbs chlorine, chloramines, VOCs, herbicides, pesticides and many pharmaceuticals. Critical, because chlorine attacks the membrane's thin-film composite layer. One gram of activated carbon can have more than 1,000 m² of surface.
- Removes
- Chlorine, VOCs, taste, odour
- Purpose
- Protects the RO membrane
- Replace
- Every 6–12 months
- 3
RO membrane, the core stage
A thin-film composite membrane wound in a spiral around a permeate tube. Under high pressure it rejects 95 to 99% of dissolved salts, heavy metals, bacteria, viruses, nitrates and most organics. What passes is permeate; what does not is concentrate, or brine. Home systems typically split 25 to 50% permeate. A good TFC membrane rejects at least 98% of NaCl.
- Removes
- Salt, metals, bacteria, viruses
- Pore size
- ~0.1 nm
- Replace
- Every 2–5 years
- Energy
- The high-energy stage
- 4
Post-carbon polishing filter
Stored water can pick up a slight taste from the tank or tubing. A coconut-shell carbon filter removes it before dispensing. A finishing stage, purely sensory.
- Removes
- Residual taste and odour
- Replace
- Every 12 months
- 5
Storage tank and dedicated tap
Membranes produce slowly, a home membrane 50 to 100 gallons per day (about 2 to 4 litres per hour), so a pressurised tank of 3.2 to 4 gallons holds water ready. An air charge of 6 to 8 psi pushes it to the tap, and a pressure switch stops the membrane when the tank is full.
- Stores
- 3–10 litres (home systems)
- Output
- Drink-ready water
Inside the RO membrane.
Modern membranes are thin-film composite, spiral-wound elements: engineering at the scale of nanometres.
Polyamide active layer
About 200 nm. The rejection layer, where ions are blocked: highly cross-linked aromatic polyamide formed by interfacial polymerisation.
Polysulfone support
About 40 µm. A porous sub-layer with ~20 nm pores that gives mechanical support but rejects nothing itself.
Polyester fabric backing
About 120 µm. Non-woven fabric for stability, so the sheet can be wound under tension without tearing.
Spiral-wound element
Sheets and spacers wound around a central permeate tube. A standard 4″ × 40″ element packs about 7 m² of active membrane.
Two mechanisms at once: size exclusion, with pores smaller than salt ions, and electrostatic repulsion, since the slightly negative surface repels anions such as Cl⁻ and SO₄²⁻. Neutral water molecules, 0.28 nm across, diffuse through the polymer. Together they reject 95 to 99% of dissolved ions.
Warmer water is less viscous and passes more easily. A 10 °C drop in feed temperature cuts output by about 30 to 35%, which is why specifications are always quoted at 25 °C.
The leading cause of degradation, in four kinds: scaling (mineral precipitation), biofouling (bacterial biofilm), colloidal (silica, clay) and organic (humic acids, proteins). Pre-treatment and clean-in-place chemistry extend membrane life.
What the RO membrane removes.
Rejection varies with contaminant type, molecular size, ionic charge and membrane specification.
| Category | Examples | Rejection | Primary stage |
|---|---|---|---|
| Dissolved salts | Sodium, potassium, chloride, calcium, magnesium, sulfate | 95–99% | RO membrane |
| Heavy metals | Lead, arsenic, mercury, chromium | 96–99% | RO membrane |
| Bacteria | E. coli, Salmonella, Legionella, Pseudomonas | >99.9% | RO membrane |
| Viruses | Norovirus, rotavirus, hepatitis A, Cryptosporidium | >99% | RO membrane |
| Nitrates | Agricultural run-off; a health risk for infants at high levels | 85–95% | RO membrane |
| Pesticides and PFAS | Glyphosate, atrazine, PFOA, PFOS (“forever chemicals”) | 88–97% | RO + carbon |
| Pharmaceuticals | Antibiotics, hormones, ibuprofen, caffeine | 90–98% | RO + carbon |
| Chlorine | Disinfection by-products, trihalomethanes | >99% | Carbon pre-filter |
| Fluoride | Added to municipal supplies; natural in groundwater | 88–96% | RO membrane |
Dissolved gases such as CO₂, hydrogen sulphide and radon are too small and pass through, as do some low-molecular-weight solvents. Chlorine damages the membrane and must be removed upstream: the membrane itself does not reject it.
Only the water molecule fits.
The pore is so small that water cannot flow through on its own. Pressure has to overcome both viscous resistance and osmotic back-pressure. Sizes below on a logarithmic scale.
- Sand grain~500,000 nm
- Human hair70,000 nm
- Bacterium (E. coli)~2,000 nm
- Microfiltration100–1,000 nm
- Ultrafiltration1–100 nm
- Virus (norovirus)~38 nm
- Nanofiltration0.5–5 nm
- Salt ion (Na⁺)0.5 nm
- Water molecule0.28 nm
- RO membrane pore~0.1 nm
Flux through the membrane is proportional to the applied pressure minus the osmotic back-pressure. More pressure gives more flow, but excess pressure compacts the membrane over time: stay inside the manufacturer's rated range.
From a kitchen to a city.
The same membrane principle, from a countertop appliance to a desalination plant.
- Point of use
Under-sink
The most common home system, on the cold line under the kitchen sink. Typically four to six stages with a dedicated tap and a 3 to 4 gallon tank.
- 50–100 GPD
- 3–8 bar feed
- On demand
Tankless / instant flow
A built-in booster pump and small buffer tank deliver water on demand, with faster flow and no wait for a tank to refill.
- 400–600 GPD
- No large tank
- Industry
Commercial & industrial
Food and beverage, pharmaceutical manufacturing, boiler feed. Multiple elements in series and parallel, often with softeners, UV and deionisation polishing.
- 1,000–100,000+ GPD
- Clean-in-place required
- Desalination
Seawater RO (SWRO)
Large plants turning seawater into drinking water. Sophisticated pre-treatment and high pressure; pressure-exchanger energy recovery brings power to about 2 kWh/m³.
- 55–80 bar
- ~2 kWh/m³ with ERD
What RO does not do perfectly.
Extraordinarily effective, with real trade-offs that matter depending on the application.
- 01
Water waste (reject ratio)
A standard home unit sends 3 to 4 litres to drain for every litre purified: recovery of only 20 to 30%. Industrial systems reach 40 to 50%; high-efficiency home systems with permeate pumps, 50 to 75%.
- 02
Minerals removed too
Calcium, magnesium, potassium and trace minerals are rejected along with the harmful ones. The water is essentially demineralised and slightly acidic, pH 5.5 to 6.5 as CO₂ passes through. A calcite post-filter can re-mineralise it.
- 03
Slow flow (tank systems)
A standard 75 GPD membrane makes about 280 mL per minute, far too slow to dispense directly, so a tank is needed. Cold water cuts production by roughly 30 to 35% per 10 °C.
- 04
Maintenance
Pre-filters every 6 to 12 months, post-carbon yearly, membrane every 2 to 5 years. A neglected carbon pre-filter lets chlorine destroy the membrane; a saturated post-filter can leach bacteria; tanks need occasional sanitising.
- 05
Energy
Home systems without a pump use no electricity at all. Modern SWRO with pressure exchangers is down to about 2 kWh/m³, a 60 to 75% reduction on older systems.
Where RO is used.
Wherever ultra-pure or salt-free water is required.
Drinking water
Under-sink systems, municipal polishing, bottled water
Desalination
Seawater to potable water for cities, islands and ships
Pharmaceutical
Purified Water and Water for Injection to USP/EP standards
Food & beverage
Brewing, soft drinks, dairy, juice concentration
Semiconductors
Ultra-pure water for wafer fabrication, above 18 MΩ·cm
Boiler feed water
Hardness and dissolved solids that cause scale and corrosion
Car washing
Spot-free rinse without mineral marks
Hydroponics
A blank slate for precise nutrient control
Haemodialysis
Ultrapure water where patient safety is critical
The core insight
“Reverse osmosis does not filter water. It forces water molecules through a barrier that contaminants are physically too large to follow. Everything else, from scaling and fouling control to energy recovery, is engineering built around protecting that single step.”
Reverse osmosis questions.
The questions we are asked most, answered briefly.
How much pressure does reverse osmosis need?
The applied pressure must exceed the osmotic pressure of the feed. Seawater has an osmotic pressure of about 27 bar, and seawater desalination runs at 55 to 80 bar. Tap water treatment needs only 3 to 10 bar.
What does an RO membrane remove?
Typically 95 to 99% of dissolved salts, 96 to 99% of heavy metals, more than 99.9% of bacteria and more than 99% of viruses, plus most nitrates, pesticides, PFAS and pharmaceuticals when combined with carbon stages.
What does RO not remove well?
Dissolved gases such as CO2, hydrogen sulphide and radon pass through, as do some low-molecular-weight solvents. Chlorine is not rejected and damages the membrane, so it must be removed upstream by carbon.
How long does an RO membrane last?
Typically two to five years, provided the sediment and carbon pre-filters are changed every 6 to 12 months so particles and chlorine never reach it.
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