The RO membrane: when it is really dead, and when the unit is

A membrane is really dead when, with fresh pre-filters and a pump that makes proper pressure, the purified water's TDS has climbed to a large fraction of the inlet and a flush does not bring it back. That is the test, and it rules out the two things that get blamed on the membrane most often: choked sediment and carbon filters, and a tired pump. Both produce the same symptoms — slow fill, low flow, sometimes higher TDS — and both are cheaper to fix. Change the membrane only after you have eliminated them.

What the membrane does and how it ages

The membrane is a tightly rolled sheet that lets water through and holds back dissolved salts. It needs two things: clean water reaching it (the pre-filters' job) and pressure pushing water through it (the pump's job). Take either away and it under-performs while perfectly healthy.

It ages in three ways:

• Scaling. Calcium and magnesium from hard water crystallise on the surface. Flow drops; TDS rises slowly. Much of Nellore's borewell water does this, and the coastal belt's brackish bores do it faster.
• Fouling. Silt and organic film coat it when the sediment filter was left too long. Flow drops sharply.
• Wear. Over years the sheet simply lets more salt through. TDS creeps up month after month with no flow change.

Scaling and fouling respond to a flush, at least partly. Wear does not.

The signs, and what else causes each

Sign	Membrane?	Could also be
Output TDS creeping up over months	Yes, classic	TDS controller set high; pump weak
Tank takes much longer to fill	Possibly	Choked pre-filters; pump; low inlet pressure
Reject water runs constantly, tank never full	Sometimes	Float valve; solenoid; restrictor
Flow fine but water tastes salty	Yes	Controller blending too much raw water
Flow drops suddenly, not gradually	Rarely	Sediment filter, kinked tube

The sudden-versus-gradual distinction matters. Membranes fail gradually. A purifier that was fine last week and dribbles this week has a blocked pre-filter or a pump problem, not a dead membrane.

How to tell, step by step

1. Change or confirm the pre-filters. If the sediment and carbon candles are more than six months old on borewell water, change them first. Many "dead membrane" calls end here.
2. Check the pump is making pressure. A technician reads this directly; at home, a pump that hums quietly and a reject line that runs with a steady stream is a rough sign it is working. A pump that chatters, or a reject that trickles, is not.
3. Flush the membrane. The low water flow visit on our rate card — membrane flush and pump check — is ₹350.
4. Measure output TDS after the flush. If it has come back down to a small fraction of the inlet and the fill time has improved, the membrane lives. If it is still high, or drops and then climbs back within weeks, it is worn and replacement is the honest answer.

A new membrane is charged at MRP and shown to you before fitting. Insist on a TDS reading of the output after the new one goes in; that number is your baseline for the next two years.

Why membranes die early in Nellore

• Pre-filters changed on a calendar written for cleaner water, while the borewell sent silt and high TDS through them. A starved membrane works at low flow and scales quickly.
• The overhead tank running dry every summer evening, the pump sucking air, and the membrane seeing surges of pressure and nothing.
• Tanker water of unknown hardness in summer that nobody measured.
• A TDS controller set to blend in more raw water so the output "tastes better", which means the membrane's work is being undone downstream and nobody notices the membrane itself declining.

A membrane fed clean, pressured water on moderate TDS often lasts three years or more. One fed silty, hard water through clogged candles can be done in a year.

When it is the whole unit, not the membrane

Replace the purifier rather than the part when:

• The body or housings have gone brittle — they crack when a technician opens them, and every service becomes a leak.
• It needs a membrane, pump and adapter together. Three middle-tier parts at MRP on an old unit is usually more than it is worth.
• Parts for the model are no longer available, so every fix is an improvisation.
• The unit is the wrong type for your water — an RO on low-TDS municipal supply, or a UV-only unit on a 1,000 TDS borewell — and you have been fighting the symptoms of that for years.
• The tank is stained inside and cannot be cleaned to a state you trust.

Keep it when the body is sound, it is the right type for your water, and it needs one part. A membrane, even at MRP, is a fraction of a new purifier, and a technician who has just serviced the rest of the unit knows whether the rest is worth keeping. Ask them plainly: "Would you put this membrane in this unit if it were yours?" A good one will tell you.