RO wasting too much water: the reject ratio explained

It comes as a shock to many owners: for every litre of drinking water an RO makes, it sends two, three or more to the drain. That is not a fault. It is the process. But the ratio can be worse than it should be.

Why there is reject water at all

A membrane lets water through and holds dissolved solids back. Those solids have to go somewhere, and they are flushed away in a stream of concentrated water. Without that flush the membrane would clog in days.

What is normal

Roughly one part clean to two or three parts reject on hard water. Some newer units do better with a recovery system. If yours is nearer one to five or worse, something is off.

What makes it worse

• A worn membrane — it lets less through, so more goes past
• A blocked or misadjusted flow restrictor on the reject line
• Low inlet pressure, so the membrane is not working efficiently
• Very high inlet TDS, which simply needs more flushing

What to do with the reject water

It is not dirty — it is just harder than the inlet. Collect it in a bucket from the reject line and use it for mopping, flushing or watering hardy plants. A household can save a meaningful amount of water this way, and it costs nothing but a bucket.

Do not tamper with the flow restrictor to reduce waste. Closing it down starves the membrane of flushing and destroys it within weeks. Reducing waste properly means a better membrane or a recovery kit, not a smaller drain.

The visit

Inlet pressure and TDS measured, the ratio checked, the restrictor inspected, the membrane assessed. If the ratio is genuinely poor, you are told whether it is the membrane, the pressure or the water.