My RO Wastes Too Much Water: What Is Normal in Delhi?

How to measure your own reject ratio in ten minutes

Do this before you accept anybody’s opinion about your machine. It is free, it takes one bucket, and it gives you a number you can compare again in six months.

• Find the drain tube. It is the thin pipe from the machine that runs to the sink drain or a bucket. Pull it out and put it into an empty measuring jug or a marked bucket.
• Empty some treated water first so the machine starts a full production cycle, then let it run for a fixed time — five or ten minutes is plenty.
• Measure both. Note how much collected in the drain bucket, and how much treated water the unit produced in the same period (draw it off into a second measured container).
• Write the numbers and the date on a slip taped to the machine. The single reading is useful. The comparison in six months is more useful.

Do the same measurement again after any service, especially after a membrane change or after anyone adjusts anything on the drain line. That is how you find out whether a service actually improved the machine.

What reject ratio is normal — and why Delhi is different

There is no single correct number, because the ratio depends on your water and your pressure. What is honest to say is this: domestic ROs typically send more water to the drain than they deliver as treated water, and on high-TDS input the gap is wider.

• Input TDS drives it. The higher the dissolved solids going in, the more sweep flow the membrane needs to stay clear. A unit moved from a low-TDS DJB line to a high-TDS borewell supply will reject more, with no fault at all.
• Inlet pressure drives it. Low incoming pressure means less water is forced through the membrane, so a larger share ends up at the drain. Delhi flats on low floors, gravity feeds from a rooftop tank, and weak building pressure all push the ratio the wrong way. See low water pressure for the wider causes.
• Temperature has an effect. Cold winter water passes through a membrane more slowly than warm water, so the same machine can look worse in January than in June.
• Newer designs recover more. Some units recirculate part of the reject stream back to the inlet to improve the ratio. If you are buying, ask about it — see our choosing a purifier tips.

Because so many things move the number, the comparison that matters is your machine against itself over time, not your machine against a figure in a brochure.

When wasting more water is actually a fault

A ratio that has got noticeably worse, or a drain that runs when it should not, is a real problem. These are the causes, roughly in order of how often they turn up.

• The flow restrictor is worn or wrong. The flow restrictor is a small component in the drain line that deliberately limits the reject flow. If it wears, is damaged, or is replaced with the wrong rating during a service, the drain flow rises and the machine wastes more for the same output. It is a small, inexpensive part — ask for it to be checked whenever the ratio has jumped.
• The auto shut-off is not closing. An RO should stop producing when the storage tank is full. If the float valve or the shut-off valve fails, the machine keeps running and keeps draining around the clock. This is the single most expensive fault in water terms. Our RO not working guide covers the symptoms.
• Choked pre-filters. A blocked sediment or carbon cartridge starves the membrane of flow and pressure, so output collapses while the drain flow carries on. The ratio looks terrible and the cause is a cheap consumable — see when to change your RO filter. In Delhi’s sediment load these choke faster than most manufacturer schedules assume.
• A fouled or scaled membrane. A membrane coated in hardness scale passes less water for the same pressure, which again means a worse ratio. Hard water across NCR makes this the normal end of a membrane’s life rather than an unusual failure.
• A tired pump. If the booster pump is no longer building the pressure it should, recovery drops. A technician can measure this rather than guess.

Can I reuse RO reject water?

Yes, for household jobs that do not involve drinking or cooking, and plenty of Delhi households already do it. Be clear about what it is first: reject water is your input water with the dissolved solids concentrated, so it is harder and higher in TDS than what came out of your tap. That limits what it is good for.

• Good uses. Flushing the toilet, mopping floors, washing the balcony or the car, cleaning outside stairs, and pre-rinsing vessels. These are the highest-volume uses in a typical home, so redirecting reject water at them makes a real dent.
• Poor uses. Watering plants is popular advice but needs care — the salt concentration is higher than tap water, and on already hard NCR water repeated use can build up salts in pot soil. Dilute it heavily if you use it on plants, and do not use it on anything sensitive.
• Never for drinking, cooking or ice, and not in a steam iron or a humidifier where the concentrated hardness will scale things up quickly.
• How to collect it practically. Extend the drain tube into a covered bucket or drum near the unit, and keep it covered so it does not become a mosquito breeding site — that is the failure mode that makes people give up on the whole idea. Empty it daily.

If you are re-plumbing a kitchen anyway, ask the plumber about routing the reject line to a utility area or a flush tank rather than a bucket. It only makes sense during other work, but it removes the daily chore that kills the habit.

How to reduce the waste without buying anything

Some of this is free, some is a service item. None of it requires replacing the purifier.

• Keep the pre-filters current. This is the highest-return action, because a choked pre-filter wrecks the ratio and costs the least to fix.
• Fix the shut-off promptly if the machine will not stop when the tank is full.
• Do not draw off and discard treated water casually. Every litre you tip away costs you the reject water that was made alongside it.
• Sort out the inlet pressure. If your unit is running on a starved supply, the ratio will always be poor. Sometimes the fix is as simple as a fully opened inlet tap or a cleaned inlet filter screen.
• Ask for the restrictor to be checked at service. Very few customers ask, and it is a genuine part of a proper service — see what a service should cover.
• Collect and reuse the rest as above, and pair it with the wider habits in our reducing water waste tips.