Total dissolved solids (TDS) are solids dissolved in water that a normal filter cannot remove. They are mostly salts, which break apart into their constituent ions, (e.g., table salt (NaCl) breaks down to the positively charged Na+ and the negatively charged Cl-) when they are dissolved in water. Sea water has a TDS of about 35,000 parts per million (ppm) and drinking (potable) water is defined as water having less than 1,000 ppm TDS, although the EPA recommends a maximum
concentration for TDS of 500 ppm.