A TDS meter does not detect much difference between unfiltered water and water filtered using a Berkey filter. This is normal, and the reason has to do with what a TDS meter actually measures. According to tdsmeter.com, “Total Dissolved Solids (TDS) is the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of milligrams per unit volume of water (mg/L), also known as parts per million (ppm). TDS is directly related to the purity of water and the quality of water treatment systems, and affects everything that consumes, lives in or uses water – whether organic or inorganic – for better or for worse.”
This statement is both correct and incorrect. It is correct in terms of what a TDS measurement consists of, namely the “total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water”. It is incorrect in its conclusion that TDS is related to the purity of the water and the quality of the water purification system. If the measure of a water purification system were how close it could bring the water to pure H₂O, then it might be correct. But people need minerals to live. So beneficial minerals in your water are actually a good thing. A high-quality water purification system should filter out substances that are harmful to your body, whilst retaining the beneficial elements such as minerals. Black Berkey filter cartridges are designed to retain healthy and beneficial minerals in the water whilst removing unwanted heavy metals such as lead and mercury, as well as sedimentary minerals such as iron oxide and aluminium.
So, if you use a TDS meter, you will find that the reading is roughly the same both before and after the water has passed through the Black Berkey filter cartridges. And that is exactly right. The Black Berkey filter cartridges retain all the beneficial minerals. So, of the substances that a TDS meter will actually detect, the Black Berkey filter cartridges will only remove unwanted heavy metals such as lead and mercury, as well as sedimentary minerals such as iron oxide and aluminium. Therefore, the TDS reading will not change significantly, unless your water contains a substantial amount of heavy metals or sedimentary minerals.
A TDS meter does not measure the level of biological and chemical contaminants (including, for example, chemicals, medicines, hormones, many pesticides and their residues, bacteria and viruses). The most important thing is not to rely solely on a TDS meter to determine whether your water is safe. You can have water with a low TDS reading that still contains harmful bacteria. You may have water with a high TDS reading that is completely safe (e.g. pure, uncontaminated seawater… although you wouldn’t drink it, but that’s only because it’s mineral salt). So make sure you know what a TDS meter actually measures.