Three glasses to give you one

There is a second pipe under most Indian kitchen sinks. It runs from the purifier to the drain, and while you are filling a glass it is filling the drain.

That is not a defect. A reverse osmosis membrane only works by having water forced against it under pressure, and everything that will not pass through has to be flushed away, carrying the concentrated contaminants with it. No reject line, no RO.

The ratio

A typical domestic RO recovers around 40% of the water it draws. Put plainly: for every glass that reaches you, roughly one and a half go down the drain. Many units in real Indian conditions do worse than that, which is where the familiar three-glasses-for-one figure comes from.

Capacitive deionisation does not force water at a barrier, so it does not need to flush one. In the IAPMO testing the Biofier’s recovery ran from 54.6% to 72.7% across fifteen conditions. At the middle of that range, for every two glasses you drink, roughly one goes away.

What that adds up to

Take a household drinking about eight litres of purified water a day — a family of four, nothing unusual. Over five years that is roughly 14,600 litres consumed.

• At 40% recovery, producing it costs about 36,500 litres. Nearly 22,000 litres go to waste.
• At 70% recovery, waste drops to about 6,300 litres.
• The difference is in the region of 15,600 litres over five years — roughly what fifty trees need in a year.

Does the reject water matter?

It is not poisoned. It is your feed water with the removed dissolved solids concentrated into it, which makes it saltier and harder than what came in. You can use it for floors, flushing or the garden if you can plumb it that way, and plenty of households do.

But most do not, because it means keeping a bucket under the sink for five years. The honest position is that reject water is usually wasted, and the way to waste less of it is to reject less of it in the first place.

Recovery figures: IAPMO India reports IAPMO/ILAB/20800/26-27 and IAPMO/ILAB/20667C/25-26.