The Purifier Lab · Water
How much water does a domestic RO system actually waste?
For a given domestic RO unit on a given feed water, how many litres of reject are produced per litre of product — and how does that ratio change with feed TDS, feed pressure and membrane age?
Results
No results yet. This page publishes the protocol only.
We have written and published the method below, and we have not yet run it. There are no measurements on this page, no charts and no percentage claims, because there is nothing to report.
The protocol is published first deliberately. It means we cannot quietly amend the method once we have seen a result we did not like, and it lets anyone — including manufacturers — tell us the method is wrong before we run it. If you can see a flaw in it, tell us.
The protocol
Collect and measure product and reject volumes simultaneously over a full tank-fill cycle, at recorded feed TDS, feed pressure and temperature, and repeat across feed conditions.
Instrument
Calibrated measuring vessels (±10 ml), inline pressure gauge, TDS meter, stopwatch
Model: To be stated on first published run
Calibration: Vessels checked by mass against a balance. TDS meter checked against a 342 ppm standard.
What is held constant
- Full tank-fill cycle measured from empty to auto-cutoff, not a timed sample — recovery varies through the cycle as back-pressure builds.
- Feed TDS, feed pressure and water temperature recorded at the start and end of every cycle.
- Reject line disconnected from drain into a measuring vessel, with no restriction added or removed.
- Same unit, same membrane, same filters throughout a comparison series.
- Booster pump state recorded, since a unit with and without a pump behaves differently at the same supply pressure.
Steps
- 1
Record feed conditions
Feed TDS, line pressure and water temperature before starting. These three determine the result more than the unit does.
- 2
Empty the tank fully
Draw the storage tank down to auto-restart, so the cycle measured is a complete one.
- 3
Collect both streams for one full cycle
Route the reject into a measuring vessel and the product into another. Run until the system reaches auto-cutoff. Record elapsed time.
- 4
Compute recovery
Recovery = product ÷ (product + reject), expressed as a percentage, with the reject-to-product ratio stated alongside because that is the figure households recognise.
- 5
Repeat across feed TDS
Where possible, repeat on a materially different feed water — municipal against borewell — to show how strongly recovery depends on feed rather than on the machine.
- 6
Publish with conditions attached
A recovery figure without its feed TDS and pressure is meaningless. Every published number carries all three.
What this test cannot tell you
Published with the protocol rather than buried after the results. A test whose limits are not stated is a claim, not a measurement.
- Recovery is a property of the system AND the water. A figure measured on 350 ppm municipal supply does not transfer to 1,400 ppm borewell water.
- Feed pressure is uncontrolled on a mains-fed domestic system and varies through the day. Multiple cycles at different times are needed before a figure is representative.
- Membrane age affects the result, so a comparison between two units is only fair if both membranes are of similar age and condition.
- This measures water volume only. It says nothing about whether the reject is being usefully reused, which is a household plumbing question rather than a machine specification.
Note