Re: DI water
Is this the posting you referred to?
by miller at en_ecn.purdue.edu (Tim Miller)
Date: 25 Aug 92
The reason you can drink the RO water that you buy in the grocery
store (or where ever you buy it) is that those machines use cellulose
acetate membranes to perform the reverse osmosis. They are extremely
susceptible to bacteria, and highly tolerant of chlorine. So even
though they remove 90-95% of the ionics and all of the colloids, the
CA membrane passes enough of the chlorine to keep the bacteria from
growing. So your "pure" drinking water still has a few odds and ends
left in it.
Cellulose Acetate membranes are "old" technology. Newer membranes are
thin-film composites, with much higher throughputs and higher
rejection rates (purer product), and higher tolerance of bacteria.
But they have almost no tolerance for chlorine. These are the
membranes that are in most medium and large systems. These systems
will have an activated charcoal filter before the RO to remove the
chlorine. This RO water is highly unsafe to drink, since there is
nothing left to inhibit the growth of bacteria. And some really nasty
bugs can live in the dark on just water and carbon based molecules,
whether it be plastic or your digestive tract.
I cannot imagine any bacteria living in DI water for an extended
period of time, as they would have nothing to eat. Plus, they would
be unable to maintain their internal salt balance, and would
eventually die. I believe the osmotic imbalance would kill them
Yes, there is considerable confusion between RO and DI water, and some
are using the terms interchangeably. A quick search through the APD
archives revealed one poster who claimed to be using 100% DI water
with no additives, but reported a GH of 2 and a KH of 3--no way!!
(unless something was leaching from the substrate or from decorative
There may be enough dissolved salts in 100% RO water to support
aquatic life (albeit a stressful existence), but DI water has
(virtually) nothing. Aquatic life cannot survive in 100% DI water.
Some Na and K is required.