Potable Water in many places contains undesirable impurities like suspended solids and harmful organisms like bacteria and viruses. On the other hand, its chemical composition depends upon its source.
RO water purification systems claim to do remove the suspended impurities. This is done using sand filters also called the pre filters, and killing the harmful organisms by UV light. Next these bring down the total dissolved salts TDS to desirable levels if the water under treatment has excess TDS. Why TDS ? Why not adjust the concentration of individual ions present to the desirable levels?
This they can’t do. These systems only can subtract something from the water composition not make up for the ions concentration which are deficient. As everyone knows, many salts which are essential as micronutrients for our bodies. These are also removed along with the harmful salts. The natural water’s chemical composition is different at different places. Some salts may be present in higher concentration levels. And some desired salts may be in deficit. The ideal drinking water should contain the salts in recommended limits, with no suspended impurities and no bacteria and virus.
But the ionic part is problematic. The RO systems work on bringing down the total dissolved salts (TDS) instead of individual ions. The resultant water has desired TDS but not the desired quantity of individual ions. The systems are not intelligent enough to adjust the concentrations of deficient ions to desired level by addition from outside. Cost will be prohibitively high for all these capabilities.
In conclusion we can say RO systems are as far as the removal of suspended impurities and killing of bacteria and viruses. But on the ionic composition front these are of not much use.
It becomes necessary to compensate this loss, by elevating the intake of these salts through food and other means.