Larry, I usually agree with your posts, but I have to respectfully
disagree with these about RO water. I've been "making" and drinking RO
water for several years with no problems nor any ill effects.
I have no affliation with any RO watermaker company other than as a
customer.
On Mon, 20 Apr 2009 04:16:01 +0000, Larry wrote:
AS it was explained to me, these boat RO systems, to be small, use very
high pressure on the membrane, as opposed to large commercial systems
like a public utility would use in a purification plant at low pressure.
Incorrect. The reason high pressure is used is because the salt
concentration of seawater results in an osmotic pressure of about 800
psi. Brackish water generally ranges between 200 and 400 psi.
Home RO systems (and brackish water systems) use different membranes
and lower presures - 30 - 65 psi for home, freshwater systems. Same
for large commercial _freshwater_ plants; sal****er ones use the
higher pressure with seawater membranes.
This high pressure traps the bacteria against the membrane, where I
suppose it's like his little head is stuck in a hole his body can't fit
through, a crude cartoon-of-the-mind's-eye.
Never heard of that. I guess it could happen if feed water flow were
insufficient. Generally a system is set up for 10% recovery. That is
feedwater flow is 10 times freshwater output. 60 GPH feed to get out 6
GPH of potable water. So 90% of the feed water is rushing through the
system, in one end and out the other. This flow rate, along with the
design of the housing and membrane result in optimum "washing" of the
membrane surface.
I think you may be under the impression that the "other end" is closed
off and all the water is forced though the membrane. Not so.
Shutting down with seawater in the system, then not running it for too
long, will result in bacterial growth and eventual "plugging" of the
membrane. The result is reduced product water flow with no "pieces" of
bacteria included.
Now trapped in a high pressure environment, at some point, the bacteria
explodes, releasing its internal load of really small toxins onto the
surface of the membrane where it can, because of its tiny size crude
molecules pass through the membrane with the H2O, contaminating the
outlet water. The key, I'm told, is the high pressure, which rips many
biologicals apart into tiny pieces. I don't see why this is not a
possible scenario and a source of possible sickness for the drinkers.
Doesn't happen. The bacteria normally is not "trapped". The little
bugger is spun around and bounced around, then spit out of the reject
line. BTW, the poor little creature would implode, not explode.
The nearest thing to your scenerio occurs right at startup and lasts
at most 5 minutes. There will be a certain amount of "smelly stuff" in
the water, mostly hydrogen sulfide. That "rotten egg" smell. Running
enough product water, about a gallon, through the system just before
shutting down minimizes this.
This is because seawater (or river water) contains organic material:
plankton, seaweeds and flotsam of all types. After a watermaker has
been turned off, this material soon begins to decompose, both in the
prefilters and the membrane housings. As it does, it breaks down into
a number of chemicals composed of smaller molecules. Some of these
molecules are small enough to pass through the watermaker membrane
along with the product water.
Again, all this is flushed out after a few minutes. It's simply a
matter of rejecting the first few minutes of product water.
Rick Morel
S/V Valkyrie
http://www.morelr.com/valkyrie