The systems that you buy for household use are quite small. typically they consist of three components. A carbon filter to remove organic material, then the cation and anion exchangers.
If in fact these systems were to bind 100% of intended ions, if you were to hook them up to straight tap water and run them, they would bind....
if you tap water was:
Ca+ 25ppm, Mg+ 10ppm, sodium+ 50ppm (the cations, with 85ppm in total)
Cl- 10ppm, SO4- 50ppm, HCO3- 100ppm (the anions, with 160ppm in total)
.....that means for each liter to pass the system, 85 mg of cation would bind to the cation exchanger, and 160mg of anion would bind to your anion exchanger. Now, if you were to make a 5 gallon batch of all-grain beer, you'd need about 9 gallons of water or so. 9 gallons of water passing an RO system would, if 100% effective, absorb 2.9 grams of cation and 5.4 grams of anion. And thats only looking at the brewing salts of interest. Your water will have more than that.
Basically my point here is that you shouldn't count on your RO system to work if you hook it straight up to tap water. I am sure that it will work for the first gallon or so with decreasing efficiency over time.
I work in a lab. We have an RO system that has three hugh 3 foot canisters. Also, our system is fed with water that has been previously distilled. After 100 liters, our system will have enough calcium in it to screw up some experiments that we run. I work with some professors who teach a class in instrumentation. One of the labs that they do is evaluating the distillation and RO systems in our department by using atomic absorbance (a highly sensitive way of measuring metals such as calcium). Even I was shocked to see how shitty our water system is considering how much money we pay for it.
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