Sorry for the long absence. Here’s two quick observations about using reverse osmosis water for brewing.
First, I have found that it makes it easier to hit pH targets in the mash with just calcium salt addition (calcium chloride and/or calcium sulfate depending on hopping level) compared to my filtered municipal water, which has some alkalinity. I used to salt that out the alkalinity with phosphoric acid, but in several batches with RO water I have not needed any acid addition to reach a mash pH of around 5.3.
Second, (this is a little tenuous on small sample size), it seems like yeast nutrient might be more helpful with RO water compared to filtered tap water. Not a big surprise, but notable anyway – I’m sure the yeast like their trace elements. Earlier this summer I brewed a saison with RO water (and added calcium salt) and the Wyeast 3724 (DuPont) strain and it was very poorly attenuated despite high temperature. Granted, this is strain notorious for such behavior, but a Kölsch I made earlier had similar fermentation difficulties. I wound up supplementing the saison with a packet of US-04 dry yeast which made the beer acceptably dry, with still good phenolic characteristics from the saison yeast. Most recently I brewed a hoppy wheat beer and for the first time added yeast nutrient to the late boil. US-04 yeast brought it from 1.055 to 1.004 in eight days. Sometime in the future I’ll try the Kölsch and saison yeasts again with nutrient in the RO water and see how they do.