Glenn Ashmore wrote:

I am not sure either of you guys has any idea what goes on inside a R/O
pressure vessel.

Well, you'd be wrong there Glen...at least by half.

First of all the normal pressure range for a seawater
system is between 750 and 850PSI.

If you'd followed the thread, you'd know that my response had *nothing*
to do with seawater systems *in seawater*. The post I was responding to
was about purifying lake water using RO followed by UV sanitization.

Seawater R/O membranes are made from
completely different materials and construction from municipal and most
other industrial purification membranes. They can withstand pressures
almost twice that high.

Withstanding higher pressures is no guarantee they won't suffer from
compaction and membrane/o-ring failure over time.

Unless they are damaged by chlorine they don't
leak.

Right. I've seen a number of TFC membranes leak at lower pressures. It
all depends on usage, care and feeding.

The only O-ring that might potentially leak brine into the product is
at the output end of the product tube. Any leakage there would immediately
raise the salt level above the drinkable level.

No salt in *this* discussion.

The other two O-rings seal
the pressure vessel itself and when they start to leak with 800PSI behind
them the result is more than a little bit noticeable.

Now as to squashed bacteria. The process of reverse osmosis is not simply
squeezing water through super small holes. The salt is rejected by the
electrical charge of the salt ions. The charged ions are pushed away from
the membrane surface.


In the process bacterial is pushed away with the
ions.

Sorry, but this does not follow. Bacteria are not pushed away from the
membrane concomitant with ions. In the case of bacteria, its a
mechanical exclusion, or sieving effect.

The seawater flow across the membrane surface is 7 times that of the
product flowing through it. Bacteria being several million times larger
than the salt ions are swept away so they never get a chance to collect on
the membrane in normal operation.

For the most part yes, you have tangential flow. You still have
impaction, and bacteria will embed and adhere to membrane surfaces. How
quickly they proliferate depends on feedwater flowrate, temperature, and
nutrient content.

If you leave the membrane idle for
several days however the bacteria can settle on the membrane and start
reproducing and THAT can cause clogging.

That was my point. I've seen many industrial systems running 24/7 that
develop a significant biofilm. And how many boat systems run 24/7?

What causes scaling is the
accumulation of molecules of minerals with very low charge. A flush with a
mild acid every couple of hundred operating hours will take care of that.

Normally R/O desalinated water is 100% free of bacteria but if you are
drawing from a harbor or other polluted water with a high virus count
statistically some viruses can get through the membrane and that is what UV
post treatment is for.

Normally yes. With any membrane penetration, that's a problem. And UV
is only effective short term, as bacteria can repair themselves within
36-48 hours following UV exposure, and exposure to visible light. Do you
cycle through all your stored water in that time? And UV is ineffective
against larger pathogens such as giardia and cryptosporidia - likelier
to be found in lake waters. In unpolluted seawater, I wouldn't worry
too much about it, but in inland lakes? RO followed by sterile
filtration doubtful (organics and heavy metals dontcha know), RO alone,
no thanks!

Keith Hughes