Many RO purifiers have a small valve called the TDS controller (also called TDS adjuster or mineral valve). Set it right and your water tastes natural; set it wrong and you either get flat water or water that is barely purified. Here is how it works and how to set it.
What does the TDS controller do?
RO removes most dissolved salts, so the output can be very low – 20–40 ppm. The TDS controller lets a small amount of pre-filtered (carbon-filtered) water bypass the membrane and mix with RO water. More bypass = higher TDS. In RO + UV purifiers, this mixed water still passes through UV before reaching the tank.
What TDS should you set?
| Your tap water TDS | Suggested RO output TDS |
|---|---|
| Below 300 ppm | 80–120 ppm |
| 300 – 800 ppm | 70–120 ppm |
| 800 – 1500 ppm | 50–100 ppm (keep bypass low) |
| Above 1500 ppm | Keep the controller almost closed; rely on the minerals/alkaline stage for taste |
The aim is 80–150 ppm for good taste – see our TDS chart. With very high input TDS, even a small bypass adds a lot of salts, so be careful.
How to set the TDS controller: step by step
- Find the controller – usually a small screw or knob on a pipe near the membrane or UV chamber (check your purifier manual).
- Measure tap water TDS and RO output TDS with a TDS meter.
- Turn the screw a quarter turn (anticlockwise usually increases TDS).
- Run the RO for 5 minutes, drain the first glass, then test again.
- Repeat in small steps until you reach your target. Note the final reading.
Signs the TDS controller is set wrong or faulty
- RO water TDS is almost the same as tap water → controller fully open or membrane failed.
- TDS keeps changing a lot day to day → controller loose or blocked.
- Water tastes flat even though TDS controller is open → controller blocked; check with a technician or on WhatsApp.
If turning the controller makes no difference, the issue may be the membrane. Read when to replace an RO membrane.
Frequently asked questions
Is it safe to increase TDS with the controller?
Yes, within limits. Keep RO output in the 80–150 ppm range, and keep it lower if your tap water TDS is very high.
Does the TDS controller affect purity?
Yes – bypass water is not RO-treated, so do not open it too much, especially with borewell water.
My purifier has no TDS controller. What can I do?
A minerals/alkaline cartridge after RO improves taste. See alkaline and mineral filters.
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