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RO plant output has dropped — finding the cause

Aqua Filtration System, Faridabad · 9+ years in water treatment · Updated September 2026

Short answer: Do not start at the membrane. Most output problems originate upstream — a choked cartridge filter, a weak pump, or low feed pressure. Check pressure and flow at each stage in order, and the cause usually becomes obvious within fifteen minutes.

The three readings that matter are flow, pressure and TDS, and it is the combination that tells you the answer.

Read the pattern first

Permeate flow	Feed pressure	Permeate TDS	Cause
Down	Up	Normal	Membrane fouling or scaling
Down	Down	Normal	Upstream — pre-filter or pump
Down	Normal	Normal	Cold water, check temperature
Normal	Normal	Up	Membrane degraded
Up	Down	Up	Chlorine damage or seal failure

The single most useful distinction: flow down with pressure up means the membrane is blocked. Flow down with pressure down means the problem is before the membrane. That one observation eliminates half the possibilities.

Check in this order

1. Water temperature

Start here, because it is free and it explains a great many "problems". Membranes produce noticeably less water in cold weather — flow can fall 20 to 30 percent in winter with nothing wrong at all.

Compare against a reading taken at a similar temperature before concluding anything is faulty.

2. Feed water supply

Is the raw water tank actually full?
Has the borewell yield dropped?
Is the feed pump delivering its rated flow?

If the plant cannot get enough feed water, nothing downstream will help.

3. Cartridge filter

The most common single cause. Check the pressure gauges before and after:

Difference above 1 kg/cm² means it is choked — change it
No gauges fitted? Fit them, they cost about ₹630 the pair

See our cartridge change guide.

4. Media filters

If there is a sand, carbon or softener vessel upstream, check pressure drop across each. A choked media bed needs backwashing — see our backwash guide.

5. High pressure pump

Is it reaching its rated pressure?
Check the current draw against the nameplate
Listen for cavitation — a rattling sound means it is starved of water
Check for internal wear if the pump is old

6. The membrane itself

Only after all of the above. If pressure is up and flow is down with clean pre-treatment, the membrane is fouled or scaled. See our CIP cleaning guide.

If it is the membrane — fouled or finished?

Carry out a proper CIP cleaning:

Performance restored → it was fouling. Now find out why it fouled
Little improvement → the membrane has reached end of life

See our membrane replacement guide.

Do not just close the reject valve

This is the most damaging response to reduced output. Closing the reject valve raises recovery, which appears to restore permeate flow — but it concentrates the reject stream further and accelerates the scaling that probably caused the problem. The plant then declines faster.

See our recovery guide.

Instruments that make this easy

Diagnosis takes minutes with instruments and hours without:

Instrument	Where	Approximate cost
Pressure gauge	Before and after cartridge	₹630 pair
Pressure gauge	Before and after membrane	₹630 pair
Rotameter	Permeate and reject	₹1,100 pair
TDS meter	Feed and permeate	₹5,500

Placement is covered in our flow meter guide.

Prevention

Record six readings weekly — flow (permeate and reject), pressure (feed and permeate), TDS (feed and permeate)
Change cartridges on pressure drop, not on a guess
Check dosing tank levels weekly — and fit level switches
Clean before performance drops 15 percent, not after it halves
Keep the baseline record from when the membranes were new

A plant with a weekly record almost never has an unexplained failure — the decline is visible weeks in advance.

The most common causes, ranked

Choked cartridge filter
Cold weather (not actually a fault)
Antiscalant dosing stopped unnoticed
Feed water supply inadequate
Membrane fouling from poor pre-treatment
Pump wear
Membrane at end of life

Note that the first four are all upstream of the membrane, and all are inexpensive to fix if caught early.

RO plant not performing?

Send your flow, pressure and TDS readings and we will help you work through the diagnosis.

💬 WhatsApp par poochhein 📞 9910646957

Frequently asked questions

My RO output has dropped — where should I start?

Not at the membrane. Check water temperature first, then the feed water supply, then the cartridge filter pressure drop. Most output problems originate upstream and are inexpensive to fix.

How do I tell whether the problem is the membrane or upstream?

Read flow and pressure together. Flow down with feed pressure up means the membrane is blocked. Flow down with pressure also down means the problem is before the membrane, such as a choked filter or weak pump.

Why does output drop in winter?

Membranes produce less water at lower temperature, and flow can fall twenty to thirty percent in cold weather with nothing actually wrong. Always compare against a reading taken at a similar temperature.

Should I close the reject valve to restore output?

No. That raises recovery and appears to restore flow, but it concentrates the reject stream and accelerates the scaling that probably caused the problem. The plant then declines faster.

What is the most common cause of reduced output?

A choked cartridge filter. Fitting pressure gauges before and after it costs very little and shows you immediately when it needs changing, which removes most guesswork.

Read next: See our membrane replacement guide and the CIP cleaning guide.