I would tell you, but I forgot the recipe.
jk RO/DI is reverse osmosis with deionization. Usually a multi stage filter that has charcoal, sediment, and particulate filters before entering a reverse osmosis filter than blocks all but the smallest particles then entering a Deionization chamber where most of the remaining particles are absorbed by tiny little spheres made of resin that have an electrostatic charge producing almost pure H2O. You can find several threads on here about the preferred brands. The only ting I suggest is to get on3 75 gallons per day or less due to the larger ones not having as good of membranes. You can pick up a decent RO/DI from Air Water Ice, Spectra pure. Some ok ones on Ebay (Aquasafe) Do a search.