Your RO output TDS has gone up — what that actually means

Does this match your symptoms?

• Output TDS higher than it used to be on the same meter
• Water tastes faintly salty or brackish
• TDS rose suddenly right after a service visit
• Purified and unpurified water now read similarly

Work through the causes in this order

Ordered by how often each turns out to be the real cause — not by what it costs to fix. 2 of these cost nothing to resolve.

1. Most commonYou can fix thisFree to fix

   1. Input TDS has risen — membrane is fine

   How to confirm it

   Measure the TDS at the inlet tap as well as at the purifier outlet, then compute the rejection rate. If rejection is still around 90%, nothing is wrong with your purifier — your supply changed. This happens routinely when a municipal supply is supplemented with borewell water in summer, or when a tanker delivery differs from the usual source.

   The fix

   Nothing to repair. If the new input TDS exceeds your membrane's rated maximum, that is a different matter — a membrane rated to 2,000 ppm fed 2,500 ppm water will not last, and the unit is now under-specified for your supply.

   Part: None

2. CommonTechnician jobUp to ₹400

   2. TDS controller or blend valve opened too far

   How to confirm it

   Did the TDS jump immediately after a service? The TDS controller deliberately blends a little untreated water back in to retain minerals, and it is often left open too wide. The tell is a sudden step change tied to a visit, rather than a gradual drift.

   The fix

   Have the controller readjusted to give 50-150 ppm at the outlet. This is a valve adjustment. You should not be charged for a part.

3. CommonTechnician job₹1,500 – ₹3,500

   3. Membrane genuinely failing

   How to confirm it

   Rejection rate below roughly 85%, computed from a proper input and output measurement, with a membrane more than two or three years old. Rejection is the diagnostic here, not the raw output figure — this is the distinction that decides whether you spend ₹2,500 or nothing.

   The fix

   Replace the membrane, and replace the pre-filters at the same time. Ask what the rejection rate reads after the change; a competent technician will measure it in front of you.

   Part: RO membrane

4. OccasionalTechnician job₹50 – ₹300

   4. Worn O-ring or bypass around the membrane

   How to confirm it

   A perished O-ring on the membrane housing lets feed water bypass the membrane entirely. The signature is a large, sudden drop in rejection on a membrane that is not old — often shortly after a service where the housing was opened.

   The fix

   Replace the O-ring and reseat the membrane. A very cheap part that is frequently misdiagnosed as membrane failure, so ask for it to be checked before you agree to a new membrane.

   Part: Membrane housing O-ring

5. OccasionalYou can fix thisFree to fix

   5. Meter error or measuring the wrong outlet

   How to confirm it

   Check the meter against a known reference, and confirm you are sampling the purified faucet rather than a mineral or alkaline outlet. Units with a separate alkaline dispenser deliberately raise TDS on that outlet, and reading it will alarm you for no reason. Cheap meters also drift, particularly with temperature.

   The fix

   Re-measure with a calibrated meter, at the correct outlet, on water that has been running a few seconds.

   Part: None

When to stop and call a technician

• Rejection rate is below 85% on a properly measured input and output pair
• TDS jumped immediately after a service visit — ask them to return and check the controller and O-ring at no charge
• Your input TDS now exceeds the membrane's rated maximum, which is a specification problem rather than a fault

When replacement beats repair

Costs shown are indicative bands for India, published so you can sanity-check a quote — they are not quotations, and they vary by brand, city and whether the technician is brand-authorised. Last reviewed 2026-08-08.