Because water is slightly acidic, as it travels through the rock formations in the ground a small quantity of rock is dissolved into a liquid form, which becomes hard water. But water has other minerals present as well, such as silica. The sum total of all dissolved minerals is known as Total Dissolved Solids, or TDS. TDS is measured in parts per million. The concept of parts per million may be difficult to visualize, but the same measurement can be stated as “milligrams per liter” when you discuss mineral content. If you evaporate one liter of water with mineral content and collect the dried mineral powder, it can be weighed in milligrams. A water sample with 50-70 ppm will contain enough dried mineral content to equal the size and weight of about one generic aspirin tablet.  

Dissolved minerals cannot be removed with mechanical filtration or standard carbon filtration. One treatment that is effective is a Reverse Osmosis system, which removes the small solids and most of the dissolved mineral content from the water supply.