RO Water Purifiers: How Reverse Osmosis Works, and When You Need It

A conventional domestic RO rejects roughly 3 litres for every litre it purifies. For a family purifying 15 litres a day that is about 45 litres daily and over 16,000 litres a year. In a city where you buy tanker water, that is a line on your monthly bill rather than an abstraction.

'Zero waste RO' is an overstatement: reverse osmosis physically requires a concentrate stream, because the whole mechanism depends on carrying rejected salts away from the membrane. What genuinely exists is high-recovery RO, which recirculates reject water to reach roughly 50% recovery instead of 25%. KENT's Save Water technology and AO Smith's Advanced Recovery Technology are the two most substantive implementations.

The other half of the answer costs nothing: run the reject line into a bucket and use it for mopping, flushing and washing vehicles. Avoid using it on plants if your input water is hard, because the concentrated salts accumulate in soil.

• Typical domestic membrane life is 12–24 months, but 10–12 months above roughly 1,200 ppm input.
• Chlorine is the main killer. An exhausted carbon pre-filter lets chlorine reach the membrane and chemically destroys the thin-film polyamide layer — turning a ₹400 saving into a ₹3,000 replacement.
• Sediment abrades the membrane surface, which is what the sediment pre-filter prevents.
• Iron fouls membranes rapidly. If your water leaves orange staining, you need iron removal upstream, not a bigger RO.
• Hardness scales the membrane. Above roughly 2,000 ppm, antiscalant dosing ahead of the membrane becomes appropriate.
• Running dry or at inadequate inlet pressure stresses both pump and membrane.