Auto shut-off valve in under-sink RO systems

Hi,
I am preparing to get an under-sink RO system with pressurized storage tank for both drinking and aquarium use, and have a question about the auto shut-off (ASO) valves included in most systems.
When the ASO valve is activated it shuts down the input to the RO cartridge and causes the pressure differential across the membrane to decrease (and maybe even reverse). If residual water remains in the cartridge, will this situation allow any significant amout of impurities to pass from the "dirty" side to the "clean" side of the membrane, especially at night when this condition would last for many hours?
Would it be better to shut off the flow at the brine (dirty) output of the RO cartridge rather than the input? If so, are there any valves that would do this based on the pressure in the storage tank, just like the regular ASO does?
Or am I making a mountain out of a molehill?
Thanks for any info.
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In the land of rec.aquaria.marine.reefs the word of the Lord came to concerned_RO_user and verily he spoke saying:

I have no help to offer but just out of curiosity, I am looking for the same thing with a tank for drinking water. Where do you find something like that?
--

-John

Because it makes things difficult to understand.
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You can get the following - Marc http://www.sparklingfloorservice.com/tank/ro_di.html
Or from AquaticReefSystem http://www.aquaticreefsystems.com/Reverse_Osmosis_4Stage.htm http://www.aquaticreefsystems.com/Drinking_Water_5Stage.htm
You do not really need a resoviour / pressurized tank to have a drinking water. You just simply get slow output. The pressure tank will give you quicker product water + product water directly from your RO.
If you want a pressurized tank and already have a RO [RO/DI], you can get it from above link or from anywhere else like HomeDepot. You just attach that tank to your RO product output water line with a "T" valve. The "T" valve will let you be able to turn off/on the flow when you need it.
CapFusion,...
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If you are overly concerned with TDS creep in your membrane, don't use the first few minutes worth of water. However, if that water is going to be pumped into a tank, it would take a lot longer than a few minutes to purge it all out.
I guess I wouldn't worry about it. Compared to the water coming out of the tap, you are way ahead of the game.
Marc
concerned_RO_user wrote:

-- Personal Page: http://www.sparklingfloorservice.com/oanda/index.html Business Page: http://www.sparklingfloorservice.com Marine Hobbyist: http://www.melevsreef.com
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The issue you bring up is something that can happen after the shutoff activates. You will get a SLIGHT blend of the water from the "dirty" side of the R/O membrane. This blend may be very insignificant. One thing we recommend that our customers do to guarantee the highest quality water possible is to completely drain their resevoir tank once/month. This will remove any lower quality water and replace it with brand new water.
When we test our customer's R/O systems, we drain their tanks before we test it. This is to ensure that we are testing the water straight from the module, instead of a possible blend water sitting in the tank.
--
Mike Ross
Culligan Rochester
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