Re: DI water

     Is this the posting you referred to?
     Pure Water
     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.