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Re: RO/DI (was APD V3 #1273

Hello Kevin,

Aside from special-purpose RO membranes, optimized for a specific task,
the "typical" RO membrane excludes about 90% of all anions. Among the
common ions it does the worst with silicate (85-90%), best with
sulphate  (~99%) and phosphate (95-99%) because they are large ions,
easily excluded. Nitrate and chloride come in at about 90%.

I would suspect that the promotional DI literature overemphasizes the
~10% NOT removed by RO, in order to sell the DI units (some of which
produce other problems). In rare cases, a post-RO DI column might be
necessary, but definitely NOT needed with a run-of-the-mill potable
water system.



> Date: Sun, 12 Sep 1999 16:30:32 -0400
> From: "Kevin Zippel" <kzippel at wcs_org>
> Subject: Re: RO extraction
> Bob, Here I quote from the Pet Warehouse catalog (admittedly not the
> best source of accurate information):
> "But, unfortunately, the standard R.O. membrane will not remove some
> nuisance chemicals like phosphate, nitrate & silicates.  ....  For this
> reason, you would need to use one of the R.O. units with a final stage
> deionization (DI) cartridge like the Maxxima series.  The DI resin will
> remove any phophate, etc., that passes through the R.O. membrane."
> Is this just BS to sell another product?  Kevin
> >Well, actually, Kevin, an RO unit should remove phosphates, silicates,
> >carbonates, and any other "ates" in your water.  Someone has informed
> you
> >wrongly.  As for the carbon pre-filter, I don't know.  I quit using
> carbon in
> >my box filters years ago, because the stuff that was available then
> would
> >actually add phosphates to my water.
> >Bob Dixon
> >Cichlid Trader List Administrator.
> >
> >> I have a question about what RO filters remove.  I had heard that
> they
> remove
> >> just
> >>  about everything (up to 99%) but can't get phosphates, nitrates, and
> >> silicates.
> >>  One would need a DI unit to extract these.  I tested my tap and RO
> water
> for
> >>  phosphates and nitrates (I don't worry about silicates) and found 0
> nitrates
> >> in
> >>  both, high levels of phosphate in the tap, but none in the RO.  How
> is this
> >>  possible?  Can the carbon pre-filter remove phosphate?  Thanks,
> Kevin
> - --
> ***********************
> Kevin Zippel
> Department of Herpetology
> WCS/Bronx Zoo
> Bronx, NY 10460
> (718) 220 8683 or 5042
> ***********************