Does reverse osmosis waste water?
In short
Yes, and it is inherent to the process: to produce purified water, the membrane must be continuously rinsed by a flow that carries contaminants to the drain. iSpring, the manufacturer of the systems we install, states a ratio of 2 litres to drain for every litre produced; independent testing reports closer to 3 to 1 depending on inlet pressure. For a household using 6 litres of filtered water a day, that is roughly 12 to 18 litres to the drain — about one toilet flush.
Updated
Why some water necessarily goes to the drain
A reverse osmosis membrane does not work like a sieve holding impurities in place. If it did, it would clog within hours. A stream of water continuously sweeps its surface and carries away what it has just rejected.
That rejected water is not lost in the strict sense: it is tap water, more concentrated in minerals and contaminants, going back to the sewer. It is neither polluted nor loaded with chemicals.
Without that rinse, the membrane would foul and need constant replacement — far more costly, in money and in resources.
The real number, in perspective
iSpring states a 2:1 ratio across its reverse osmosis systems. Independent tests published on those same models report ratios closer to 3:1. The gap comes mostly from supply pressure: the lower it is, the more the system rejects.
In practice, a family of four drinking and cooking with 6 to 8 litres of filtered water a day sends between 12 and 24 litres to the drain. A modern toilet uses 4.8 litres per flush, an older one up to 13. An eight-minute shower exceeds 60 litres.
Put differently, a residential reverse osmosis system's daily discharge compares to two or three toilet flushes — not to a tap left running.
How to reduce the discharge
Correct inlet pressure is the first lever, and the cheapest. Below 40 psi, output drops and discharge climbs. We measure pressure during the water test, before any installation.
A booster pump, added when line pressure is insufficient, clearly improves both the ratio and the system's output. It is one of the options that explains the spread in our price range.
Finally, the rejected water can be collected: a bucket under the drain is enough to water plants or wash a floor. It is still tap water, simply more mineralised.
Being straight about this
- We will not claim reverse osmosis wastes nothing: that is false, and anyone with a bucket and a watch can check it.
- The right way to weigh it is against what you already do. If you buy bottled water, producing and shipping those bottles consumes far more water than a system sends to the drain.
- If your municipal water is good and you already drink it without a taste problem, a reverse osmosis system is a comfort, not a necessity. The water test is also there to establish that.
Frequently asked questions
It depends on how much filtered water you use. For 6 litres drunk and cooked with per day, expect roughly 12 to 18 litres to the drain, the equivalent of two or three toilet flushes. iSpring states a ratio of 2 litres rejected per litre produced.