Professor POU/POE July 2013: Hard water

July 1, 2013

This month’s topic: Hard water

This month’s topic: Hard water

What is hard water, why is it a problem and how can I fix it?

The term hard water generally refers to water that has high mineral content, but specifically water that is high in calcium and/or magnesium ions. Barium, copper, lead, zinc, cadmium and radium can also be components. Hard water forms precipitates of metal carbonates that are very insoluble and it causes problems by forming scale deposits, accumulating in pipe and in cooling systems where they reduce the heat transfer capability. When combined with soap they interfere with lathering and rinsing and cause the precipitate “bathtub ring.” Hard water is probably the most common and noticeable water quality problem that occurs. It has been said that about 80-85 percent of the U.S. has hard water as defined. The good news is that hardness is not known to cause any human health concerns, but it can certainly have aesthetic and economic consequences.

Water hardness is measured as calcium hardness in milligrams of calcium carbonate equivalent as follows:

Soft                         less than 17 mg/L

Slightly hard         17 to 60 mg/L

Moderately hard    60 to 120 mg/L

Hard                       120 to 180 mg/L

Very hard               > 180 mg/L

The term grains/gallon is commonly used in the water softening industry; 17.1 mg/L as calcium carbonate is equivalent to 1 grain/gallon.

Calcium and magnesium are commonly present in water as soluble bicarbonates that are formed when carbonates react with carbon dioxide. The bicarbonate, carbonate, carbon dioxide and water are in equilibrium. Bicarbonate predominates in the pH range of about 6 to 10. Calcium bicarbonate is soluble at about 16.5 g/100 mL around room temperature, but calcium carbonate solubility is only about 0.0013 g/100 mL, so it is clear that it will precipitate when conditions favor its presence. Higher pH and heat cause loss of carbon dioxide and conversion of the bicarbonate to carbonate.

The mineral forms of the carbonate precipitates are hard solids that will deposit and continuously build up on pipe, tanks, boilers and appliances, and they are difficult to remove. Pipes with depositing carbonate scale have reduced water flow and they can eventually clog. Hot water heaters are damaged from the accumulated precipitates, heating coils are coated reducing heat transfer efficiency and increasing energy costs.

The other commonly noticeable effect of calcium and magnesium and related ions in water is that they react with soap that is a large fatty acid molecule and form the metal salt (e.g. calcium stearate) that deposits as soap scum or bathtub ring. These various precipitates from water are not easily removed, and may require an acid wash to clean them from showerheads, faucet aerators and surfaces. Detergents for clothes washing, dishwashing and shampoos are formulated for specific purposes and are much less affected by hard water because they are structurally different from soaps. So, water hardness is at least an annoyance but that can be overcome. It certainly can have major economic consequences in the home and also for industry.

Water softening is, therefore, a common practice both in the home and in larger scale facilities. Precipitative lime softening with calcium hydroxide and lime-soda ash softening are inexpensive unit processes and used by municipal water plants that soften. Those processes also clarify the water and remove contaminants like radium and cryptosporidium oocycts that have health concerns. Removal of dissolved ions (desalination) on a large multimillion gallons per day scale is also accomplished with membranes, particularly high pressure reverse osmosis, as well as by low pressure distillation processes.

The two most common home water softening techniques are cation exchange and low pressure RO. Iron and manganese can also be removed by these techniques. Point-of-entry (POE) home water softening utilizes a cation exchange resin that is neutralized with sodium ions. As hard water passes through the resin, calcium displaces sodium ions from the resin into the water and the resin retains the calcium and other “hard” ions. The resin is periodically regenerated by treating with a concentrated sodium chloride solution brine that goes down the drain. Cation exchange softened water is not necessarily corrosive. These types of softeners have raised some controversy due to the discharging of sodium chloride to the groundwater environment. Some have suggested softening only the hot water system to protect the hot water heater, and reduce soap consumption, and also as an economy measure.

Point-of-use reverse osmosis (RO) softening is another option in the home. RO home softeners are not very efficient and they produce only a small portion of product water, because they operate at low line pressures and therefore can reject to waste on the order of 90 percent of the influent water. However, there is no chemical addition required. RO process water is very corrosive due to its very low dissolved solids content and low alkalinity, so non-reactive contact plumbing is necessary. POE RO softening, which is not common, would require plumbing in the entire house that is corrosion resistant. Large scale RO desalination is much more efficient because it operates at much higher pressure. The permeate water is very corrosive, so it is stabilized by alkalinity and pH adjustment before it is put into distribution.

Some communities have restricted the use of salt based cation exchange softeners because of the discharges of sodium chloride to the environment. There are non-chemical softener devices on the market that are based upon magnetic, electromagnetic, electrostatic, electrodialysis reversal, electrodeionization reversal, chelation, catalytic and mechanical processes. Some of these are softening by ion removal and some are scale prevention approaches. There is controversy and mixed results, and performance certifications are not yet generally available for home applications from organizations like the Water Quality Association and IAPMO. There is one German standard; however, several standards from these organizations are in development and nearing acceptance. There are examples of good performance for some in higher volume industrial applications where there is generally more control over changing conditions and water quality restrictions for use.

I don’t know of any health based regulations or guidelines for calcium hardness in drinking water, but many industrial processes require water that has low mineralization or hardness so pretreatment of process water or cooling water is a major component of many facilities.

Many studies have suggested a benefit of reduced cardiovascular mortality from consumption of hard water. However, it has been concluded that if there is a benefit, it is associated specifically with the magnesium content rather than hardness per se. The removal of essential minerals like calcium and magnesium from water by softening can result in lower lifetime intakes of those essential elements. In addition, health concerns have been raised by some with regard to cation exchange water softeners because of increased sodium intake, especially for people who are salt sensitive hypertensives. Mixtures of sodium and potassium chloride are used for regeneration in some cases. I have heard of at least one softener product that has a cartridge that adds magnesium after the softening stage.

References: Water Processing: Residential, Commercial, Light-industrial. W. McGowan and JF Harrison, technical editor. Water Quality Association, 2000. World Health Organization Guidelines for Drinking Water Quality, 4th edition, WHO Geneva 2011. Chemical Aspects of Desalinated Water, in Desalination Technology: Health and Environmental Impacts. J. Fawell, M Abdulraheem, J. Cotruvo, F. Al-Awadhi, Y. Magara, and CN Ong. ISBN 978-1-4398-2890-8. CRC Press 2010. Non Chemical Devices: Thirty Years of Myth Busting. T. Keister, Water Conditioning and Purification. April 2008. Physical Water Treatment Devices. P. Undesser, WQA.

Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.

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