RO reject water: how much is normal, how to cut it, what to do with it

A healthy home RO sends roughly two to three litres to the drain for every litre it puts in your glass. That is not a fault, it is how the membrane stays clean: the salts it holds back have to leave in a stream of water or they would crust on the surface within days. You can reduce the ratio a little, you can make sure the unit is not wasting more than it should, and you can put every litre of reject to good use. What you cannot do is make it zero — and anyone who promises a home RO with no reject water is describing a different product.

Why it exists

Water under pressure meets the membrane. Some passes through and comes out nearly salt-free. The rest, now carrying a higher concentration of salt, runs across the surface and out through the reject line to the drain. That sweeping flow is what keeps the membrane from scaling. Cut the sweep too hard and the salt concentrates, hardness crystallises, and the membrane dies early.

The ratio is set by a small flow restrictor on the reject line and, on many units, by the membrane's own condition and the inlet pressure.

What is normal and what is not

Normal: reject runs steadily while the tank fills, and stops when the tank is full and the unit shuts off. Two to three litres per litre purified on moderate water; a little more on very high TDS water, where the membrane needs more sweeping.

Not normal, and worth a visit:

• Reject running all the time, tank full or not. The float valve or auto-shutoff is not closing. Every hour of this is water straight down the drain.
• Reject running hard but the tank barely fills. The membrane is so fouled or scaled that nearly everything is going to waste, or the restrictor is missing or damaged. This is the low water flow visit on our rate card, ₹350 — membrane flush and pump check.
• No reject at all while the pump runs. The reject line is blocked or kinked. The membrane is being starved of its sweep and will scale fast. Switch off and book.
• Reject ratio that has quietly climbed. If a bucket under the reject line used to fill in an hour and now fills in half that, something changed — usually the membrane, sometimes the restrictor.

A bucket and a clock is all the instrument you need to know which of these you have.

How to cut it — safely

• Keep pre-filters fresh. A membrane fed clean water at full pressure purifies more per litre of reject than a starved one. This is the single biggest lever and it costs nothing extra beyond the filter you were going to change anyway.
• Keep the pump healthy. Low pressure means less water through the membrane and the same amount down the drain. A pump sucking air because the overhead tank runs dry every evening is a Nellore summer problem worth fixing at the tank, not the purifier.
• Have the TDS controller set for your water. On low-TDS municipal supply, the controller can blend some pre-filtered water back in, which means less water has to go through the membrane and less goes to reject. On high-TDS borewell water there is far less room for this.
• Do not let anyone "tune" the restrictor to near zero. It feels like a win for a month; then the membrane scales. On hard water the true cost of a too-tight restrictor is a membrane at MRP.
• Right-size the technology. If your inlet TDS is under 300 on treated supply, a UV+UF unit wastes nothing. An RO on that water is the wasteful choice, not the safe one.

What to do with it

The reject is not dirty water. It is your inlet water with the salts somewhat concentrated, and in a city where summer tanker water is a cost and a worry, it is too useful to pour away.

Use	Suitable?
Mopping floors, washing the yard, the car, the bike	Yes, freely
Flushing the toilet	Yes — a bucket by the cistern or a line to the flush tank
Pre-soaking clothes, washing utensils (first rinse)	Yes, if inlet TDS is moderate; salty reject leaves spots
Hardy outdoor plants, lawn	Yes, alternating with normal water, if inlet is not brackish
Pots, seedlings, vegetable beds	Avoid on high-TDS reject — salt builds up in the soil
Drinking, cooking, pets	No

The practical setup is a drum or bucket under the reject line instead of a direct connection to the drain, emptied into the mop bucket and flush tank. A 20-litre drum catches a day's reject for a typical family. Where the purifier is far from the drain, a reject line run to a tank on the balcony or in the wash area does the same with no daily carrying.

The Nellore angle

Many homes here pump borewell and municipal water into the same sump, and the blend gets harder in summer when the bore is doing more of the work. That pushes the reject ratio up and the membrane life down at the same time. Measuring the sump once a season and having the controller and restrictor checked at the post-monsoon service keeps both in balance. And in a tanker summer, every litre of reject that goes into the flush tank is a litre of tanker water you did not pay for twice.