Osmosis, then the reverse
Osmosis is a natural process. When two solutions of different strength are separated by a semi-permeable membrane, water moves from the weaker side to the stronger side to even them out. Plant roots draw water from the soil this way.
Reverse osmosis turns this around. A pump applies pressure to the salty side, higher than the natural osmotic pressure. Water is forced back through the membrane towards the clean side, while most of the dissolved salts cannot pass. The clean water is called permeate, and the concentrated stream left behind is called reject.
Why pre-treatment comes first
An RO membrane is thin and delicate. Mud, silt, iron, scale and chlorine can all foul or damage it, and a damaged membrane is costly to replace. Pre-treatment is there to protect it, and a membrane is only as good as the filters in front of it.
The right mix depends on the source. Clean municipal water may need only dechlorination and a cartridge filter, while borewell water with iron or high hardness, or recycled water from an STP, needs much more. A full water analysis before design shows which problems the pre-treatment has to handle, and saves money on membranes later.
- Sand or multimedia filters remove suspended particles and turbidity.
- Activated carbon filters remove chlorine, odour and some organics.
- Cartridge filters catch fine particles just before the high-pressure pump.
- Ultrafiltration (UF) is used where the feed water is poor, such as treated sewage or surface water.
- Antiscalant dosing helps stop hardness salts forming scale on the membrane.
What RO removes, and reject water
RO removes most dissolved salts, including hardness, chlorides, sulphates and nitrates, along with many other contaminants. Nominal rejection of common salts is typically 85 to 98 percent, depending on the membrane and the water. It also removes bacteria and particles, although treated water is usually disinfected as well for safety in storage.
Not all the feed water becomes permeate. The share that does is called recovery. The rest leaves as reject, carrying the salts the membrane held back. Pushing recovery too high concentrates salts until they scale the membrane, so the design balances water saving against membrane life. Reject can often be reused for floor washing or gardening, depending on its quality.
What affects output
An RO plant does not give the same output every day, and a drop is not always a fault. Three factors make the biggest difference. Operators should log them every day along with permeate flow and TDS, so that a real problem can be told apart from a normal seasonal change.
These factors also interact. A plant designed for summer conditions will deliver less in a cold winter unless it was sized with a margin. Rather than raising pump pressure beyond the design limit to chase output, which can damage membranes, it is safer to find out what has changed and correct the cause.
- Pressure: too little feed pressure means low output and poorer quality.
- Temperature: cold water passes through the membrane more slowly, so output drops in winter.
- Feed TDS: the saltier the incoming water, the more pressure is needed and the lower the output.
Looking after the membranes
Good care is mostly routine. Replace cartridge filters on schedule, backwash sand and carbon filters, and keep antiscalant dosing running. Watch the pressure difference across the membranes and the permeate quality. A steady rise in pressure or TDS is an early sign of fouling.
When that happens, membranes are cleaned in place with suitable chemicals. Regular cleaning keeps them working well for years, while neglect shortens their life sharply. An engineer can review the logs and water analysis if output or quality keeps falling.