Every RO system produces two streams: clean treated water and a stream that carries away the concentrated salts. This second stream is called reject water, concentrate or waste water. It is not drinking quality, but it is not necessarily useless.
Why reject water exists
The membrane holds back dissolved solids. Water has to flush them away continuously, otherwise they would build up on the membrane. Read how reverse osmosis works for more.
Possible uses
- Mopping and cleaning floors.
- Flushing toilets.
- Washing cars or outdoor areas.
- Watering suitable non-edible plants, if salt levels allow. Very salty water can harm plants, so test first.
Do not use it for
- Drinking or cooking.
- Baby formula.
- Watering edible plants unless you have checked the water quality.
Reducing waste
Well-maintained systems waste less water. Replace pre-filters on time and check the membrane; see when to replace an RO membrane. For households that want to limit waste, ask us about suitable residential RO systems and under-sink RO filters.
Practical ways to use RO reject water, and what to avoid
An RO system sends some water to drain along with the dissolved salts it removes from the drinking water. This reject water is not dirty, but it is saltier than your tap water, so it is not suitable for drinking or cooking. Many households collect it and use it for jobs where salt content does not matter.
Collecting reject water makes sense in a hot country where water is valuable. Use a clean container with a lid, empty it daily and avoid leaving it standing, because stagnant water attracts mosquitoes and develops odours. If your system is connected to a drain only, ask whether a collection line can be added during installation.
Sensible uses
- Mopping floors and cleaning outdoor areas.
- Flushing toilets from a bucket when needed.
- Washing cars and watering salt-tolerant plants occasionally. Too much salty water can harm soil and sensitive plants.
- Pre-rinsing dishes or cleaning tools, where the final rinse uses clean water.
Reject water questions
Can I drink or cook with reject water?
No. It has a higher concentration of dissolved salts and is not intended for drinking or cooking.
Does the reject volume mean my RO system is faulty?
RO systems naturally send water to drain. The ratio depends on the model and the feed pressure. If the reject suddenly increases, have the system checked.
Can I reduce reject water?
Some systems include features to lower waste, such as pump-assisted designs. Ask us which type suits your home.
Learn more about TDS in drinking water and ask us about RO systems.
Related reading and products
Related guides
- Industrial RO Plant Sizing Basics: Capacity, Recovery and Pre-Treatment
- When to Replace an RO Membrane: Signs and Typical Lifespan
- How Reverse Osmosis Works: Stages, Waste Water and Membrane Life
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