Water Hardness

What is Water Hardness?

Soft: 0-60 mg/L
Moderately hard: 61-120 mg/L
Hard: 121-180 mg/L
Very hard: >181 mg/L

Total water hardness (TH) in the United States is often measured in parts per million (ppm), grains (64.8 g of calcium carbonate per gallon, gpg), and millimoles of calcium per liter (mmol/L). The most common unit is ppm, which represents one milligram of calcium carbonate CaCO3 in one liter of solution (mg/L). To indicate total hardness, concentrations of dissolved salts of calcium and magnesium are usually combined and expressed as ppm of calcium carbonate.

The advantages of removal of hard ions from industrial and domestic water are many. Hardness salts in water are the primary cause of tube and pipe scaling, which frequently causes failures and loss of process efficiency due to clogging or loss of heat transfer.

"Hard ions" are removed by replacing them with so called "soft ions", such as sodium and potassium, which is achieved in water softening and conditioning systems.

Industrial methods of water hardness determination usually include:

  • Laboratory methods involve EDTA complexometric titrations with various methods of end-point determination, spectrophotometry, and atomic absorption spectrometry;
  • Instrumentation in-line methods include automated colorimetric or potentiometric measurements.

Water treatment to decrease its hardness has significant economical effect. In industrial plants, boilers, and coolers, hard water causes scaling. One-eighth of an inch of scale can affect the efficiency of a water heater by 27% [2]. Hard water negatively affects application of soaps and detergents. For every grain of water hardness, detergent use increases by 2 to 4 % per 1,000 gallons of water used. Hard water also means that more wastewater and impurities are transported to the sewer [3]. Some large manufacturers of household chemicals, such as Reckitt Benckiser, supply to the market water softening products along with detergents and other washing products. This sector is rapidly growing, which in turn increases demand for monitors of water softening. In 2007, textile washing product sales worldwide generated revenues of $36 billion, and this market exhibits steady growth [4]. In beverage industry, water hardness monitoring is important due to its influence on the microbiological robustness of beverages [5].

In addition to traditional sectors for water softening and monitoring products, new trends may lead to increase in interest in the water hardness monitoring. It has been shown that reducing water hardness may increase ability to remove bacteria from broiler chicken skin [6]


[1] US Geological Survey. Water Hardness and Alkalinity. http://water.usgs.gov/owq/hardness-alkalinity.html
[2] Operators Take Heed: Don't Let Your Equipment Go Down the Drain,. Nation's Restaurant News, 2003, vol. 37 (46), p.18.
[3] Pederson, M Hard Water Can Drain Energy and Expenses, Buildings, 2008, vol. 102 (1), p. 22.
[4] Datamonitor Industry Market Research: NA, Jan 15, 2008.
[5] Dworetsky, T. Water Hardness and Beverages, Beverage World, 1999, vol. 118 (1671), pp. 79-81.
[6] Water Hardness Affects Bacteria Removal. Feedstuffs, 2009, 81 (31), p.13.