distilled vs. reverse osmosis
"I'm only asking because the typical system setup on my projects has standard filter cartridges first, then through the RO system to a storage tank, then to the Deionization system and circulated through the building. 18 MegOhms is standard as it leaves the DI unit. Your experience is that the DI and RO units are reversed."
I agree with Dan - this has been my experience too. The RO unit does the bulk of the demineralization, but since it cannot really remove the last traces of ions, the water should go through a deionizing resin bed (for both anionic and cationic) to do the final "polishing". In addition to this, there should be a carbon trap to remove gases and organic compounds, as well as a UV light source to kill any bacteria in the system.
Many larger labs use a circulating system to keep the water constantly moving through the polishing resin beds, the carbon trap, and past the UV light. A large holding tank is used to store a volume of water, as these systems are slow to make deionized water and if there is a large demand, you will need a reservoir somewhere in the system.
Perhaps Brook is mistaking a RO demineralization system with a RO water softener. The softener will remove calcium and iron ions and replace them with sodium. I don't think it does anything with carbonate (someone please clarify this) and so that may be causing the issues seen with certain developers.
distilled vs. reverse osmosis
There are no RO "softening" membranes. The membranes are not ion specific as opposed to the ion exchange resin which can be tailored to either replace the Calcium ion with a Hydrogen ion or a sodium ion. Typically softening is used in home units because it is easier and safer to regenerate the resin with salt.
CArbonate is usually removed by lowering the pH converting it to carbonic acid which subsequently breaks down into carbon dioxide. In the case of home units the carbonate is left in the reject and many times is the cause of scaling and pluggin of the membrane.
distilled vs. reverse osmosis
Dan, my experience is from Germany and a University lab environment, not a production facility. In our lab all the water goes through the DI system first,; as an example that includes the water for the "dishwasher" for the lab glassware. Just a part goes through the Millipore RO system after that to produce the 18MOhm water for the real high purity applications. It is not a continuous circulation system, and that might be the reason its different from a factory unit.
distilled vs. reverse osmosis
John Sexton told us about Ansel Adams experience with a deionzer in his dark room. AA started having problems with his prints. After much investigation, AA called Ilford tech guys in. As soon as they discovered the deion unit, they told AA to disconnect the unit to his darkroom. This solved the problems. Interesting little tid bit!