A few months back my under sink RO unit started being a lot more noisy than usual when making water. It sounds like there is a lot of water gushing through the system and down the drain. The unit is 10 years old and I have never noticed this much noise before. I replaced all the filters and the RO membrane, but the problem persists. I am wondering whether the shutoff valve is getting dodgy - it does shut off OK when the RO water tank is full but then, when someone runs the adjacent city water faucet, the water will start gushing through the RO drain again for quite a while. If I open and close that adjacent city water faucet, sometimes the draining will stop, sometimes not. The noise is getting to be really irritating... what can I do to fix this ? How can I tell if the shutoff valve is bad ? Can I disassemble it to check for defects ? Could something else be the problem ? Here are some photos of the unit:

If it is an abnormal amount of discharge you are hearing, my guess would be that you didn't seat the membrane right, or you're missing an O-ring or a gasket.

Could it be that there is just some residual air in one of the filter chambers that hasn't worked out yet?

Everything looks sealed, and there are no leaks. In any case, the excessive drainage started before I touched the filters... it was one of the reasons I changed everything, to see if a defective filter was the cause. The valve shuts off OK when the storage tank is full, except for when someone runs the adjacent tap (which sets the RO unit draining again). I can't believe there is still air in there - it has been over a month since I changed the filters.

Most likely the automatic shut off valve is the issue. It works by differential pressure, you may also want to make sure the storage tank is not waterlogged.

Also the drain from the R.O may have an air gap in the faucet (small hole near the base) if that gets dirty it will be loud and make the noise you described.

If the membrane was installed incorrectly or the "Brine Seal" is cut or twisted you would notice a high flow of water to the faucet or storage tank. That water will be poor quality if tested by a TDS meter.

Good news is it can be saved

Can I use any old RO auto-shutoff valve, or do I need to replace with this particular valve ? If so, can anyone point me to a source/manufacturer ?

I got one off ebay. White in a figure 8 shape with input/output on different sides. My ro system would run constantly, trickling before i changed it. Make sure if u have a pressure regulator on the feed line that the pressure is high enough to allow the valve to work once your holding tank is full

I hate screwing with mine

Anyone will work. Make sure you install it in the right direction. I've been at this 30 yrs and sometime I fuck it up!

UPDATE: I replaced the Automatic Shutoff Valve, but the noise is still there. Upon closer inspection, I have two more observations:

1) When water is flowing through the filter system, it sounds like air bubbles are blasting through it... there is a continual, quite "popping" sound. This started prior to my replacing the filters and RO membrane a few months back, but the filter change did not cure it.
2) When I pulled the unit out I found no evidence of a flow restrictor on the drain line. I am told most of the new ones don't need them, but mine is old - it dates back to 1998. Should I be concerned ?

The noise is still driving me nuts.

1) Check the wasteline. does the line go to the faucet and then to the drain. I'm thinking the noise is from the airgap faucet.

2) It maybe in the membrane housing.

1) The waste line goes directly from the end of the RO membrane housing to the horizontal part of the under-sink drain (shared by two sinks and a dishwasher). The line enters a fitting at a slight upward angle. It has always been this way.

2) If it is the membrane housing, what should I do about it ? The membrane was replaced months ago, so I would expect any trapped air to have been expelled by now. I see no evidence of a leak, which I would have thought the more likely outcome if a seal was bad. Is there anywhere in the system where there is a vacuum, and where a leaking seal could allow air to be sucked in ?

Mine is from 1998 as well. The flow restrictor is probably in there, mine was.. In the waste line there's a 1 inch barbed piece that acts as a cheapo restrictor. Can't tell it's there without looking in the fitting.

Anyhow, mine would cycle all the time, and i would hear the noise of the drain line running under the sink. Drove me batshit

I played with the pressure reducer a bit, drained the storage tank, and rerouted the lines a bit.  Now it's quiet... I don't even look at it now, in case it gets ornery again

1) Open the faucet on the R.O. system, let it run until the water stops or just drips. Check the pressure tank and make sure the tank is empty.

2) The restrictor only acts as a deivce to put slight back pressure on the membrane by reducing the flow to drain. I can tell you if you have a fast drip to the drain and not a steady stream it is NOT the problem.

3) If the pressure tank empties out shut the faucet off and let it fill all the way up. Then shut the valve off on the pressure tank and seeif the noise is still there.

4) is the R.O. only hooked to a faucet or does it also go to the refigerator ice maker.

OK, I disassembled the RO membrane housing - found no problems. I did discover the location of the flow restrictor in the fitting where the drain line goes into the membrane housing. However, it is clearly not working... when I checked the flow rate to the drain it is about 3 liters per minute (not just a steady stream, but a gush)!!! No wonder there was so much noise. I am guessing the bubbles are due to cavitation as the water passes through the flow restrictor orifice. I have bandaided this excessive flow by installing a John Guest shutoff in-line valve in the drain line and throttling it down to get only about 300ml/minute flow. The noise has gone now .

QUESTION: What should the flow rate down the drain be when RO water is being made ? The ones available at on-line stores appear to range from 150ml/min ip to 1200ml/min, so my restrictor must e bad. The question is, how can it go bad ? It is just a small orifice inserted in the end of the drain line tube !!!

They wear out. Water is flowing through it and eventually make the orfice size larger. Can you get a pic of it. What GPD is your membrane I prob have a flow restrictor I can drop in the mail.

Have you checked your ro drain line?  Mine makes a similar sound when the drain line becomes clogged.  The air gap at the faucet then makes popping, water running noises.  Just a thought.

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