RO Reject Water Reuse: Safe Ways to Use It at Home, and What to Avoid

RO reject water is safe to reuse for mopping, flushing toilets, washing vehicles and floors, and, with some care, for laundry. It is your own supply water with the dissolved salts concentrated, not sewage. It should never be used for drinking or cooking, including boiling dal or rice, and it should not be piped back into an overhead tank that supplies drinking water.

What RO reject water actually is

An RO membrane splits incoming water into two streams. One passes through the membrane and becomes your purified water. The other carries away what the membrane held back and goes to drain. That second stream is the reject, sometimes called RO waste water or concentrate.

So reject water is roughly your input water with more dissolved solids in it. If your tap water has a lot of calcium, magnesium, sodium or chloride, the reject has more. If the supply also contains something you do not want, such as fluoride, nitrate or arsenic, the reject contains more of that too. It is not dirty in the sewage sense, but it is not cleaner than your tap water either: it is saltier.

Why an RO sends water to drain at all, and how to reduce the amount, is covered in our guide to RO water wastage and how to reduce it. This article is about what to do with the water once it comes out.

Measure it before you reuse it

The single most useful thing you can do is put a TDS meter in a glass of reject water and in a glass of tap water. The comparison tells you how concentrated your reject is and which uses are sensible.

• If your input TDS is modest (comfortably within the BIS IS 10500 acceptable limit of 500 mg/L), the reject is usually still mild enough for most cleaning jobs and often for laundry.
• If your input TDS is high, as with many borewell and tanker supplies, the reject can be very salty. It will leave white marks on floors, glass and cars when it dries, and is less suitable for anything that touches skin, fabric or soil.

If you do not know your input TDS, start there. A low-TDS supply may not need RO at all; see do I need RO if my TDS is below 200.

Safe uses, cautious uses and uses to avoid

Use	Verdict	Notes
Mopping floors	Safe	On high-TDS reject, white marks may show on dark or polished floors; a final wipe with tap water helps.
Flushing toilets	Safe	Pour into the cistern or the bowl. Very salty water can add to scale in the cistern over time.
Washing cars, bikes, balconies, utensils’ first rinse	Safe	Dry vehicles and glass with a cloth to avoid spotting. Give utensils a final rinse in normal water.
Washing clothes	With care	Fine for an initial wash on moderate TDS. Salty, hard reject lathers poorly, so you need more detergent; rinse with normal water.
Bathing	Only if input TDS is modest	On a high-TDS supply, reject water can leave skin and hair feeling dry and rough. Not for babies.
Watering plants	Caution	High-TDS reject can harm many plants over time and salt up the soil, especially in pots. Mix with normal water or alternate.
Drinking, cooking, boiling dal or rice, pets’ water	Never	See the next section.

On plants, the risk is gradual. Salts left behind when water evaporates build up in the soil, and in a pot they have nowhere to go. Leaf tips turning brown and a white crust on the soil surface are signs to stop. Garden beds that get rain are more forgiving than pots on a balcony.

Why never for drinking or cooking

Cooking with reject water feels harmless because boiling kills microbes. But microbes are not the issue. Reject water contains more of every dissolved substance in your supply, and boiling does nothing to dissolved salts. As water evaporates from a pot of dal, the remaining water becomes saltier still.

How to collect it, and why not into the overhead tank

There are two simple ways to collect reject water:

1. A bucket or can under the drain pipe. Ask your technician to route the reject pipe to a spot where a bucket fits, and keep a lid on the container so it does not collect dust or breed mosquitoes.
2. A diverted line to a storage drum. For larger households, the reject pipe can run to a dedicated covered drum near a balcony or utility area, with a tap at the bottom for mopping and washing. Keep the pipe free of kinks and do not let it run uphill, or the purifier’s drain flow can be restricted.

Use stored reject water within a day or two and clean the container regularly. Still water in a warm kitchen can grow algae and smell.

A common idea online is to pipe RO reject water back into the overhead tank. Do not do this if that tank supplies your purifier or any drinking water. Every litre of reject you send back adds concentrated salts to the tank. Over days and weeks, the tank’s TDS climbs, the purifier works harder on saltier water, the membrane and pre-filters wear faster, and the reject gets saltier still. It is a loop that moves in the wrong direction. A separate tank used only for flushing or cleaning is a different matter, and a plumber can advise on that.

Zero wastage and 1:1 recovery: what the claims mean

Recovery is the share of incoming water that becomes purified water. A “1:1 ratio” or “50 percent recovery” means one litre of purified water for every litre sent to drain. Higher recovery means less reject, but it is not free: it depends on input TDS, water pressure, temperature and membrane design, and on very salty water a purifier may need to reject more to protect the membrane.

“Zero water wastage” claims deserve a question. Every RO produces concentrate; that is how it works. A zero-wastage design usually sends the reject somewhere other than the drain, often back into a tank, which brings the TDS problem described above. Ask any seller where the reject goes and what their recovery is at your input TDS, in writing.

Where Boon fits

Boon was founded in 2013 by ex-IIT Kanpur engineers, is backed by the Technology Development Board (Govt. of India), NITI Aayog and Roca, and runs water systems in more than 4,000 organisations. Boon Homie, our wall-mounted RO, is installed by Boon’s own team, who can route the reject pipe to a bucket or a storage container if you want to reuse it. WaterAI shows input TDS, so you know how concentrated your reject is likely to be. This article does not quote a recovery figure for Homie; if recovery matters to you, ask Boon for it at your input TDS before you order, as you should with any brand.

Frequently Asked Questions

Can RO reject water be used for bathing?

Only if your input water has modest TDS. Reject water is your supply water with the dissolved salts concentrated, so on a high-TDS borewell or tanker supply it can leave skin and hair feeling dry and rough. Measure the reject with a TDS meter first. Do not use it to bathe babies, and use ordinary water if you notice irritation.

Can RO waste water be used for washing clothes?

Yes, with care. On a moderate-TDS supply it is fine for the main wash. Very salty or hard reject water lathers poorly, so you may need more detergent, and it can leave fabrics feeling stiff. Rinse with ordinary tap water. Check that your washing machine’s inlet will not draw from a container that could run dry.

Can RO reject water be used for cooking or boiling pulses?

No. Reject water contains more of every dissolved substance in your supply, including anything harmful such as fluoride, nitrate or arsenic. Boiling kills microbes but does nothing to dissolved salts, and as water evaporates the remaining water becomes saltier. Use purified or ordinary tap water for cooking, and keep reject water for cleaning.

Is RO reject water good for plants?

It depends on its TDS. Low-TDS reject is usually fine for hardy plants, especially in garden beds that get rain. High-TDS reject can harm many plants over time, because salts build up in the soil, particularly in pots. Brown leaf tips or a white crust on the soil are signs to stop. Mixing with ordinary water reduces the risk.

Can I send RO reject water back to my overhead tank?

Not if that tank supplies your purifier or any drinking water. Each litre of reject adds concentrated salts, so the tank’s TDS rises over time, the purifier works harder, filters wear faster and the reject becomes saltier still. A separate tank used only for flushing or cleaning is different, and a plumber can advise you on setting one up.

What does a 1:1 recovery ratio mean in an RO purifier?

It means the purifier produces one litre of purified water for every litre it sends to drain, which is 50 percent recovery. Recovery depends on input TDS, water pressure, temperature and membrane design, so the same purifier can recover differently on different supplies. Ask the seller for the recovery figure at your input TDS, in writing, before you buy.