In the context of water quality testing, the acronym TWH almost universally refers to Total Water Hardness. This measurement is a fundamental indicator of water quality. It affects the taste of drinking water, the longevity of plumbing, and household appliances. Total Water Hardness is a naturally occurring phenomenon with implications for homes and industries.
Defining TWH and Its Chemical Basis
Total Water Hardness measures the concentration of specific dissolved mineral ions in a water sample. The primary minerals responsible are the positively charged divalent cations, mainly calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)). Water becomes hard as it percolates through geological formations, such as limestone and chalk, which are rich in these minerals. Slightly acidic rainwater dissolves these rocks, carrying the mineral ions into groundwater sources and the public water supply.
Hardness is commonly categorized into two types: temporary and permanent. Temporary hardness, also known as carbonate hardness, is due to calcium and magnesium bicarbonates and carbonates. This type can often be partially removed by boiling the water. Permanent hardness is caused by non-carbonate salts, like sulfates and chlorides of calcium and magnesium, which are more difficult to remove through simple heating.
Measuring TWH and Classifying Levels
Water hardness is quantified by expressing the concentration of dissolved minerals as an equivalent amount of calcium carbonate (\(\text{CaCO}_3\)). Common units reported on water test results are milligrams per liter (\(\text{mg/L}\)) or parts per million (\(\text{ppm}\)), which are numerically equivalent. Another unit used in residential treatment is grains per gallon (\(\text{gpg}\)), where one grain equals about \(17.1\) \(\text{mg/L}\) of calcium carbonate.
Water quality professionals use a classification system to interpret these results. The U.S. Geological Survey (USGS) defines water with \(0\) to \(60\) \(\text{mg/L}\) as soft. Moderately hard water falls between \(61\) and \(120\) \(\text{mg/L}\), and water with \(121\) to \(180\) \(\text{mg/L}\) is categorized as hard. Readings above \(180\) \(\text{mg/L}\) are considered very hard.
Health and Infrastructure Consequences
The most noticeable consequence of high TWH is the formation of limescale, a hard, off-white deposit composed mainly of calcium carbonate. This scale builds up on heating elements inside appliances like kettles, water heaters, and boilers, significantly reducing their energy efficiency. A one-millimeter layer of scale can decrease a heating system’s efficiency by up to ten percent, leading to increased energy costs.
Scale accumulation also occurs inside plumbing and water fixtures, restricting water flow and increasing system pressure. This mineral buildup shortens the lifespan of household appliances and can lead to clogs and expensive repairs. Furthermore, hardness minerals interfere with the effectiveness of soaps and detergents, preventing proper lathering and causing soap scum. This requires consumers to use more product to achieve cleaning results.
From a health perspective, Total Water Hardness is not considered a health risk for drinking water. The calcium and magnesium minerals contribute to overall mineral intake, and some research suggests a positive association with bone density or cardiovascular health. However, hard water minerals can interact with skin and hair, potentially stripping moisture and leaving a residue that contributes to dryness and irritation. The World Health Organization does not set standards for hardness but recommends maintaining a minimum level.
Practical Solutions for TWH Reduction
The most common and effective method for mitigating high TWH is the use of an ion-exchange water softener system. This process involves passing hard water through a tank containing resin beads covered with positively charged sodium (\(\text{Na}^{+}\)) or potassium (\(\text{K}^{+}\)) ions. As water flows over the resin, calcium and magnesium ions are captured by the beads, displacing the sodium ions into the water.
This exchange removes hardness minerals, preventing scale formation and improving soap performance. The resin beads eventually become saturated and require regeneration, where a concentrated salt brine solution is flushed through the tank. The high concentration of sodium ions in the brine forces the captured hardness ions off the resin, recharging the beads for the next cycle. Alternative methods include reverse osmosis (RO) systems, which filter out virtually all dissolved solids but are typically limited to single drinking water taps due to cost and slow flow rates.