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Re: [APD] Storing large amounts of RO water & reconditioning ROwater

Hi Gordon,

My apologies if somebody already answered this..

> It is my understanding of how RO filters work, they are incapable of
> taking sodium out of solution, without considerable pressure (I think
> the number is around 700+ psi), requiring massive stainless steel
> membrane housings.   Perhaps that is just to desalinate sea-water?

RO membranes "reject" sodium ions.   The water molecules are "pushed" across
the membrane.  The sodium ions concentrate at the membranes surface.  This
creates a layer that slows down the rate that water can pass through the
membrane.  The more salt in solution, the harder it is to drive the water
across the membrane.

Home RO units do remove sodium.  There is not much sodium most tap waters,
and they use a type of membrane that works very well in low pressure
situations.  And they dump a lot of the feed water.  The amount of
Desalinated water out of the unit is small compared to the waste water.
This all adds up to a low energy separation - hence less pressure.

You are correct industrial desal plants use very high pressures - but they
also want to produce as much "clean" water as they can from the feed water
used.  The sodium (and other ions) are much more concentrated - takes more
energy to separate the water from them, hence the higher pressures.


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