I'm on well water with relatively high iron content. I've never had it tested for brewing because it's not pleasant tasting in it's natural state, so I wouldn't use it anyway. It has a pinkish hue from the iron. All the house water gets run through a softener, which adds sodium, and then through a 5 micron filter. The fridge gets an in-line carbon filter for ice and drinking water. This produces satisfactory drinking and washing water.
I've been buying bulk RO water for brewing from the faucet in front of the grocery store for quite a while. It seems like I might be able to do my own RO setup on the cheap and not have to drive to buy water and the return on investment would be pretty reasonable.
All the multi-stage RO systems seem to have pre-filtering. Some kind of sediment filter to remove a particulates and then carbon for chlorine and other chemicals. Then they seem to have carbon on the other end of the RO membrane too. I can't figure out why you would carbon after the membrane. Based on my already existing whole house setup which does a particulate filter already, I'm thinking I could run a tee off the carbon filter that goes to my refrigerator to feed a stand-alone RO housing with membrane just for my brewing water. That could be done for less than $40 easily.
I'm not a prolific brewer. I typically get to do four batches a year so it's not worth investing a ton of money into. Any major issues with the setup noted above? Would there be a problem with letting the membrane sit idle for months at a time? I could probably be convinced to hook it to a pressure tank to feed the fridge if I need to have it remain relative active. That adds cost, but then I'm getting the "benefit" of RO ice and drinking water. Would that even taste right?