This system consists of both cation and anion ion exchange resin. The water passes through the cation ion exchange resin, where the positive ions are exchanged for hydrogen. Next, the acidic water is passed through the anion ion exchange resin where the negative ions are replaced with hydroxyl. Once through the cation and anion phases, all impurities are now either hydrogen or hydroxyl, which forms water. This process yields what is called deionized water. An acid such as HCl is used to regenerate the cation resin. NaOH is used to regenerate the anion. The waste streams need to be monitored and treated to meet regulated discharge requirements. Water quality is less than 1ppm, so instead of measuring conductivity (expressed as parts per million), water purity is measured in ohms of resistance, and the higher the resistance, the higher the purity. The above serial process from a cation tank to anion tank can yield about 1 mega ohm water. To reach higher levels of about 16 mega ohm water, a mixed resin bed system is needed with multiple stages. Higher levels of purity require recirculation. Materials of construction must be compatible with water purity.