RO TDS: what number is right for drinking water

TDS — total dissolved solids — is the one number every RO owner should know. It is also the one most often misunderstood, in both directions.

What it measures

The total of minerals and salts dissolved in the water, in milligrams per litre. It says nothing about bacteria or chemicals; it is a mineral count.

Tricity inlet water

Groundwater across the region commonly reads several hundred and can be higher. That is well above the level where water tastes hard and scales appliances, which is why RO is common here in the first place.

What to aim for at the tap

Somewhere between roughly 50 and 150 is a sensible range for taste and for keeping some minerals in the water. Far below that — 10 or 20 — tastes flat and is not better for you. Far above — say 300 or more from an RO — means the membrane is not doing its job.

Why lower is not better

Water with almost nothing dissolved in it is not more pure in any useful sense. It tastes empty, and there is a reasonable case that some mineral content is preferable. Many units have a mineraliser or a TDS controller for exactly this reason.

Checking yours

A pocket TDS meter costs very little. Dip it in a glass from the RO tap. Then dip it in a glass from the ordinary tap. The gap between the two is what the membrane is achieving. Write both numbers down; the RO number climbing over months is the earliest warning the membrane is wearing.

If your RO output reads close to the inlet, the membrane has failed or a bypass valve is open. That water is not being purified. Stop drinking it and book a visit.