The process of forcing water through a semi-permeable membrane is called Reverse Osmosis. By pushing water through the specialized membrane at a certain pressure, minerals, foreign contaminants, solid substances and large molecules are removed.
A Reverse Osmosis system is often paired with an Ion Exchange Water Softener when on well water. The Water Softener removes calcium, magnesium and iron while the Reverse Osmosis finishes the treatment process by removing contaminants and sediments.
Reverse Osmosis water is slightly acidic with a pH between 5 and 6. Regular tap water is between 6 and 8. Anything under pH 7 is acidic, just like your morning orange juice and coffee.
Here is a list of daily beverage and their pH.
Many of us think we should avoid drinking acidic water but your body is able to maintain your blood pH between 7.35 and 7.45 compared to your stomach's pH of 3 to 5.5. With a balance of vegetables, fruits and natural foods, the pH of reverse osmosis water is not a concern.
The pH of Reverse Osmosis water can be increased using a remineralisation/alkaline filter as a last stage. It will add back some minerals raising the pH closer to neutral. The minerals found in natural water should not be a substitution for the minerals you should be getting in your diet. This is not a medical advice, only facts; if in doubt consult your doctor.