Sodium bicarbonate is a water treatment method used to soften water (removing calcium and magnesium impurities from it) in older water softener systems. The process uses a chemical reaction to convert the calcium hydroxide (or magnesium hydroxide) into calcium carbonate, which isn't water soluble. Softened water usually has elevated levels of dissolved sodium in it compared to standard tap water, which is a minor health concern. Due to the declining costs and greater energy and filtration efficiency of reverse osmosis systems, sodium bicarbonate water softening is slowly being displaced as a water treatment method in homes.
The Chemical Reaction
Sodium bicarbonate is introduced into a filter that water circulates through. The reaction strips dissolved calcium hydroxide out of the water, where the calcium hydroxyl group bonds to the two carbon atoms in sodium bicarbonate, making calcium carbonate and a free sodium ion. The calcium carbonate is then precipitated out of the water flow and stored. A similar reaction also strips out magnesium ions in the water. Because of the nature of the chemical reaction, this method of water softening is less effective when the calcium or magnesium is bonded to sulfur compounds rather than hydroxyl groups.
Because the sodium bicarbonate water treatment method results in a non-soluble precipitate, the water stream typically needs filtration. In triple treatment systems, the sodium bicarbonate treatment is usually the middle treatment in the process, after charcoal and ionic filtering (typically combined into one step) and before placer filtration to remove the precipitate.
Large Scale Applications
While sodium bicarbonate water treatment methods were once common in the household, they have largely been replaced by triple filtration systems, or, more recently, reverse osmosis systems. The primary holdover for the sodium bicarbonate water treatment method is in municipal water plants, where it's used in large batches with industrial scale processes; even so, it's usually used as the first step in a process that ends in reverse osmosis. Because of the nature of the chemical process, there's a threshold of calcium and magnesium impurities below which this process isn't effective.
Municipal Transitions and Salinity Concerns
Many municipalities are offering programs to replace sodium bicarbonate water treatment systems with more energy efficient (and environmentally friendly) reverse osmosis systems. The chief complaints are that the end result of the sodium bicarbonate process is a lot of dissolved sodium, which leaches chlorine atoms from PVC pipes and creates a salt-water waste flow that increases municipal water costs.