Richard Casady wrote:

snip

There are a number of studies showing that RO membranes (which are not
absolute porosity filters, but are spiral wound depth filters)

That explains a lot. I have cut open,[ they make a tool just for that]
a number of pleated paper oil filters. With those they seem to either
pass a particle size, or not. Like any sieve, its all or nothing.

Well, absolute porosity is really a misnomer, even though the term is
used frequently. It's really based on a statistical measure of retention
capability, since all membranes are essentially "mats" of material, not
like you'd expect for, say, sintered metal. And they all depend on
adsorption, impaction, and physical sieving to achieve that porosity rating.

I
have seen filters made from spiral wound string, for fuel, if I
recall, but they didn't make any claims of micron size. That would be
a spiral wound depth filter?

A very common type, yes. A DE pool filter is another example. Depth
filters become *more* effective as they load up, since the accumulated
material provides additional sieving action.

I can see how it could pass some, but not
all, of the same size particles. You could call it an attrition
filter. but clearly it isn't a simple sieve. Photographic filters take
out a percentage, but not all, of the light.I was under the impression
that RO filters were all or nothing,like any sieve.

Unfortunately, all membrane filters have large and small pores, and the
interactions between the mean pore size, the configuration of the pore
pathway (i.e. the more tortuous path provided through the membrane, the
more likely that physical impaction will sequester a particle), membrane
charge, fluid pressure, and fluid velocity, among other esoteric
factors, determines the retention capability of the membrane. So
retention is a statistical measure of performance, rather than an
absolute capability. Depth filters, like wound membranes, have much
larger variability in retention capability, relative to their nominal
pore size, than do most membrane filters.

It is good of you
to post some actual information. A newsgroup with news, of all things.

Well, it's sort of topical at the moment, since I'm currently working on
qualifying a multi-effect still, pure steam generator, and an
ultra-filtration/diafiltration skid.

are not 100% viral retentive, or bacterial retentive (especially for Giardia
oocytes, and certainly not for mycoplasma) when challenged with a
significant upstream population. The prevalence of these organisms (and
almost-organisms) in seawater is, however, extremely low, and a 2 to
3-log reduction (about what the literature seems to support) gives a
very high probability of 100% removal. Safer than tap water, by a long shot.

Can you run the stuff through twice and get the same percentage
reduction for the second pass?

Yes, and no. Since retention is statistical in nature (i.e. the
likelihood of 100% retention is not only directly related to particle
size distribution, but also on upstream particulate concentration), the
retention effectiveness for the second pass would actually be much
greater (with respect to ensuring a clean filtrate) than on the first
pass.

Now, that is if you're talking about dual pass in series. Most
"double-pass" RO systems are designed for water savings, not filtration
effectiveness, and are in a series/parallel configuration where the
rejected water from the first pass goes to the second pass, and that
permeate (filtrate) and is then pooled with the permeate from the first
pass. So the membranes are in series, but water flow is in parallel,
only passing through one membrane, either first pass or second pass.

Someone mentioned arsenic,and the CRC does list it as a component of
sea water. Three to twenty-four parts per billion. Or mg/ton. About
the same as iron. 1970 edition, your milage may vary.

In industrial applications, this isn't an issue, since most systems use
some type of chelating agent or sequestrant that complexes such
materials making them easy to filter. Not real amenable to the cruiser
though, and I don't know off hand how well arsenic is rejected.


As for viral proteins being toxic, the only studies I'm aware of have
been done on the common viral pesticides, where no oral toxicity has
ever been observed - doesn't mean it can't happen, but given how rapidly
protein is denature in the stomach, it's pretty unlikely.

That is the reason for having stomach acid isn't it? That and
dissolving "insoluble" heavy metal salts.

Yep. That it is.

Keith Hughes