Free tool
RO Water Wastage Calculator: How Many Litres Go Down the Drain?
How much water is your RO actually discarding?
Every RO purifier sends concentrate to the drain — that is intrinsic to how the membrane works. This shows what that adds up to for your household, and what collecting it would be worth.
Your household
About 14,821 litres to the drain per year
41 litres a day · 2.0 L discarded per litre purified
Purified: 20 L/day
Total feed water used: 22,121 L/year
Value of the reject: ₹2,223/year
Equivalent: 3.0 tanker loads (5,000 L)
Collected in a canister, that reject would cover about 6 toilet flushes a day — the single easiest way to stop it being wasted, at zero cost.
Typical for an Indian domestic RO. Check your inlet pressure and pre-filter condition — both push this ratio the wrong way as they degrade.
Assumptions: steady-state operation, membrane in reasonable condition, no tank overflow losses. Reject water is your ordinary supply at roughly 2-4× its original TDS — safe for flushing, mopping and pre-rinsing, not for drinking.
Rejection is intrinsic to reverse osmosis rather than a defect in it. The membrane separates water from dissolved salts, and those salts have to leave in a continuous stream of concentrate — that stream also sweeps the membrane surface clean, which is what stops it scaling solid within days. What varies between purifiers is the recovery ratio, not whether there is a reject at all.
That is why the useful question is not 'does my RO waste water' but 'is it wasting more than it should'. A unit at 25% recovery is usually telling you something: inadequate inlet pressure, a failing booster pump, or pre-filters left past their interval, all of which starve the membrane. Fixing inlet pressure alone commonly moves a purifier from 1:3 to 1:2 with no other change.
The second lever costs nothing at all. A 20-litre canister under the purifier fills itself, and for most households it covers the day's toilet flushing and floor mopping outright. It is the least glamorous recommendation on this site and one of the most effective.