Anti-scalent doesn’t remove the hardness from the water, and instead seeks to prevent the hardness precipitating out using chemical methods. The chemicals work to prevent the formation of large crystals of hardness, nuetralise the positive charge and disperse the particles so that scale does not precipitate.
They are mainly used as pre-treatment for RO or Nano filtration techniques, where some water is constantly bled off to drain, carrying the dissolved hardness with it
An RO membrane has microscopic pores so small that it can reject nearly all the impurities found in water. Nano filtration uses similar membrane technology, but with slightly larger pores. These larger pores allow Sodium and Chloride ions to pass through, but reject most of the larger Calcium and Magnesium ions, softening the water.
Some form of anti-scalent is required to prevent the hardness Ions scaling up the membrane.
A demineraliser works in a similar way to an Ion Exchange Softener, however it uses two pressured vessels in series. The first vessel removes cations (Postively charged Ions such as Iron, Calcium, Magnesium) and the second vessel removes anions (Negatively charged Ions such as Chloride, Bi-Carbonate etc) via Ion exchange.
The first vessel is regenerated with Hydrocloric Acid (HCL) and release’s H+ ions, and the second vessel is regenerated with Caustic soda (NaOH) and release’s OH– ions. The H+ and OH– ions then combine to form H2O (pure water). De-mineralisers remove almost all the contaminents dissolved in water, however they have quite high running costs and harmful chemicals
A Dealk only offers partial softening, by removing the Tempory hardness (That caused by Bi-Carbonate)
A Dealk works in a similar way to the first Cation stage of a De-mineraliser. Postively charged Ions such as Iron, Calcium, Magnesium are removed and H+ ions are released. Removing the Calcium and Magnesium leaves behind the Bi-carbonate, which because of the presence of H+ Ions converts to CO2. The water then passes through a degasser tower, where air is blown upwards through the water and the CO2 is carried away. The final water has slightly lower pH, Lower conductivity and less temporary hardness.