Potable Water Treatment Newsletter, July, 2012
Water hardness is a property which all of us know the effects of. It wastes soap and synthetic detergents by binding with them to form soap curd. This will dull the colors of clothes washed in hard water. It leaves unsightly soap scum on the bath tub. It streaks glassware and dishes. It causes scale to build up in any appliances or pipes which use water.
For these reasons and more it becomes necessary to lower the hardness in many water supplies. Water supplies in the US range anywhere from 1 grain per gallon to 350 grains per gallon. We generally say that anything over 10.5 gpg is very hard. For reference, one grain per gallon (gpg) is equal to 17.1 ppm or mg/l calcium carbonate.
The two most common sources for hardness are calcium and magnesium and of these calcium is by far the most common. Some other elements can increase hardness but they are not common.
Water softeners are used in domestic and some commercial applications and they are very effective, capable of removing all of the hardness if they are maintain properly. Softeners use the method of ion exchange to remove hardness. They use a resin (small beads which form beds when they are applied to a columnar shaped tank). The beads have a natural affinity for hardness ions and will remove them from a water supply by exchanging them for sodium ions which are present on the resin. When all the capability of the resin to remove hardness ions is used up the resin is regenerated. In this step the resin is treated with a brine solution which is very high in sodium content. The high sodium content causes the hardness ions to leave the resin and be replaced with sodium. Then the bed is returned to service.
When treating the large amounts of water found in water plants ion exhange is simply not practical. The most common method for treating large amounts of water for hardness is lime softening. It seems impossible to remove calcium hardness by adding lime which is calcium hydroxide. The key is in the pH rise caused by lime. At pH 11 and above most of the calcium will precipitate out whether it is from the lime or was present in the water to begin with. Lime softening has the added benefit of clarifying the water. Most if not all of the turbidity present will be removed by the lime as it precipitates. Sometimes a small amount, .25 ppm to .5 ppm, of an anionic polymer is used to enhance the coagulation.
One disadvantage of lime softening is that is will remove alkalinity as well as hardness. Alkalinty has to be put back into the water and this is done by adding CO2 or soda ash. Lime softening does not remove all of the hardness like ion exhange does. It is used to bring the hardness down to a range where it is tolerated by the community, generally 1 to 7 gpg (17.1 ppm to 120 ppm).
Sources of Hardness
Why do some waters have hardness while others do not? The answer can be found in the hydrologic cycle Put simply, ground water (or well water) has percolated through the ground and many times that ground has limestone (which is calcium carbonate) in it. The water which has fallen on the ground through rain or percolated through the ground from a river, lake or other source dissolves limestone in the ground and hence picks up hardness. Many lime deposits contain magnesium carbonate as well and these cause the water to contain magnesium.
The water which falls to the ground in the form of rain collects in rivers, lakes, and other sources has never been exposed to the ground and hence has very little if any hardness. Rain water, for all practical purposes is distilled from the earth and is very pure. This is why the water in lakes and rivers is generally lower in hardness than ground water sources. Rain can become acidic by absorbing CO2 and some acidic pollutants from the atmosphere. When this water percolates through soil it is the acidity which causes the limestone to dissolve.
In addition to picking up material from the atmosphere, rain water runs across land and picks many things including clay, silt, and decayed plant and animal matter. This is where taste, odor, and color found in surface water sources comes from.
It seems that in discussing where hardness comes from and how it is treated, I have gone into a discussion of the differences between ground water and surface water. I guess the two are inseparable and any discussion of hardness must include a discussion of ground and surface waters and why they are different. I hope this article has proven useful to all and as always, I welcome comments or suggestions on what to cover in next month’s newletter.