Water softening treats hard water, which is defined by high concentrations of dissolved minerals, primarily calcium and magnesium. These minerals cause scale buildup in pipes and appliances and reduce soap lathering. A water softener solves these issues using a process that introduces a small amount of sodium, raising questions for homeowners concerned about dietary intake.
The Mechanism of Ion Exchange
The sodium content is introduced through ion exchange, the operational principle of a water softener. Hard water flows through a tank filled with resin beads saturated with sodium ions (Na+).
The beads have a negative charge, which attracts the positive calcium (Ca2+) and magnesium (Mg2+) ions in the hard water. As the water passes over the resin, the more strongly charged calcium and magnesium ions displace the less strongly charged sodium ions. The resin captures the hardness minerals, while an equivalent amount of sodium is released into the water.
This chemical trade-off successfully removes the problematic minerals, resulting in soft water. The amount of sodium added is directly proportional to the amount of hardness removed from the water.
Calculating the Added Sodium Content
The concentration of sodium added is a direct result of the original hardness level. Approximately eight milligrams (\(\text{mg}\)) of sodium are added per liter of water for every one grain per gallon (\(\text{GPG}\)) of hardness removed. This means the final sodium concentration varies significantly based on the local water source.
For instance, water considered moderately hard (\(10 \text{ GPG}\)) will have about \(80 \text{ mg}\) of sodium added per liter. If the water is very hard (\(20 \text{ GPG}\)), the added sodium will be closer to \(160 \text{ mg}\) per liter. This demonstrates that the harder the water is before treatment, the higher the resulting sodium content in the softened water will be.
Contextualizing Sodium Intake for Health
The sodium added by a water softener is small compared to the amount consumed daily through food. Health organizations recommend that most adults consume no more than \(2,300 \text{ mg}\) of sodium per day, with an optimal limit closer to \(1,500 \text{ mg}\) for certain individuals. The average American consumes about \(3,400 \text{ mg}\) of sodium daily, primarily from processed and prepared foods.
When comparing the added sodium from softened water to common foods, the amount is often negligible. For example, a single slice of commercial bread can contain between \(100\) and \(230 \text{ mg}\) of sodium. A cup of milk typically contains \(100\) to \(130 \text{ mg}\) of sodium.
Even in a very hard water area, such as \(20 \text{ GPG}\), a person drinking two liters of softened water would consume about \(320 \text{ mg}\) of added sodium. This amount is less than the sodium found in two slices of bread, illustrating that the primary source of sodium in the diet remains food, not water. However, individuals on a physician-prescribed, severely sodium-restricted diet, often below \(500 \text{ mg}\) per day, may need to take precautions to minimize this added intake.
Low-Sodium Water Solutions
For households concerned about the sodium content in their drinking water, two primary solutions offer effective alternatives. One option is to use potassium chloride instead of sodium chloride salt in the water softener unit. Potassium chloride operates on the same ion exchange principle, trading potassium ions for the hardness minerals.
While potassium chloride is a viable alternative for softening water without adding sodium, it is typically more expensive, sometimes costing up to three times more than sodium chloride. It may also be slightly less efficient at regenerating the resin, requiring a greater volume of product to achieve the same softness. A more common and highly effective strategy is the installation of a point-of-use Reverse Osmosis (\(\text{RO}\)) system.
An \(\text{RO}\) system is generally installed at a single tap, such as the kitchen sink, and works by forcing water through a semi-permeable membrane. This membrane is designed to filter out dissolved solids, including the sodium ions that were added during the softening process. \(\text{RO}\) filtration can remove \(95\) percent or more of the sodium and is the most thorough way to ensure low-sodium drinking water while still benefiting from soft water throughout the rest of the home.