Hardness and total dissolved solids

Chapter 10 Hardness and total dissolved solids

10.1 Introduction The hardness or softness of water varies from place to place and reflects the

nature of the geology of the area with which the water has been in contact. In general, surface waters are softer than groundwaters. Hard waters are associated with chalk and limestone catchment areas, whereas soft waters are associated

with imperme able rocks such as grani te (Secti on 4.4 ). Water h ardness is a

traditional measure of the ability of water to react with soap to produce a lather, and for most consumers the problems associated with washing, and also the scaling of pipes and household appliances that use water, are the two major factors of concern.

An alternative measure of hardness is total dissolved solids (TDS), which is a measure of the total concentration of ions in water. The TDS in groundwater is often an order of magnitude higher than in surface waters. In aquifers the TDS increases with depth due to less fresh recharge water to dilute existing groundwater and the longer period for ions to be dissolved. The older and deeper the groundwater the more mineral rich the water becomes resulting in quite saline water. High concentrations of salts in groundwaters are often due to over-abstraction or to drought conditions when old saline groundwaters may enter boreholes through upward replacement, or due to saline intrusion into the aquifer from the sea. In Europe conductivity is used as a replacement for TDS measurement and is widely used to measure the degree of mineralization of

groundwaters (Table 4.5 ).

10.2 Chemistry of hardness

Hardness is caused by metal cations such as calcium (Ca 2 þ ), but in fact all divalent cations cause hardness (Table 10.1 ). They react with certain anions

such as carbonate or sulphate to form a precipitate. Monovalent cations such as

sodium (Na þ ) do not affect hardness. Strontium, ferrous iron (Fe 2 þ ) and

manganese are usually such minor components of hardness that they are generally ignored, with the total hardness taken to be the sum of the calcium and

Hardness and total dissolved solids

Table 10.1 Principal metal cations causing hardness and the major anions associated with them

Cations

Anions

Ca 2 þ (calcium)

HCO 3 (hydrogencarbonate)

Mg 2 þ (magnesium)

SO 4 2 (sulphate)

Sr 2 þ (strontium)

Cl (chloride)

Fe 2 þ (iron)

NO 3 (nitrate)

Mn 2 þ (manganese)

SiO 3 2 (silicate)

magnesium concentrations. Aluminium (Al 2 þ ) and ferric iron (Fe 3 þ ) can affect hardness but their solubility is limited at the pH of natural waters, so that ionic concentrations can be considered negligible. Barium and zinc can also cause hardness, but their concentrations in water are normally extremely small. The

majority of divalent cations in water are Ca 2 þ and Mg 2 þ , so hardness is

calculated by measuring these cations only.

There are a number of additional terms relating to hardness. Total hardness is

þ Mg 2 þ ). Calcium hardness is the direct measurement of calcium only. Carbonate hardness is the hardness

the direct measurement of hardness (Ca 2 þ

derived from the solubilization of calcium or magnesium carbonate by converting the carbonate to hydrogencarbonate. This hardness can be removed by heating. Magnesium hardness is the hardness derived from the presence of magnesium and is calculated by subtracting the calcium hardness from the total hardness. Non-carbonate hardness is the hardness attributable to all cations associated with all anions except carbonate, e.g. calcium chloride and magnesium sulphate. Permanent hardness is equivalent to non-carbonate hardness, which cannot be removed from water by heating. Temporary hardness is equivalent to carbonate hardness and can be removed by heating (Flanagan, 1988 ). All hardness values are expressed in mg CaCO

3 l . The classification of waters into hard and soft is arbitrary, with a number of classifications used (Table 10.2 ). It is generally accepted that waters with a

hardness of <60 mg l 1 are soft.

In terms of water quality, hardness can have a profound effect. The hardness of water was originally measured by the ability of the water to destroy the lather of soap, as this is one of the principal problems of very hard water. Although hardness does neutralize the lathering power of soap it does not affect modern detergent formulations. Soft waters are more aggressive than hard waters, enhancing the corrosion of copper and lead pipes. Above a hardness of

150–200 mg l 1 scaling becomes a problem, although this is only the case with calcium hardness or temporary hardness. The hydrogencarbonate is removed

during heating to form calcium carbonate, which forms a thick scale over the

10.3 Standards

Table 10.2 Two examples of classifications used for water hardness Classification A

Classification B

Concentration

Concentration

Degree of hardness 0–50

(mg 1 1 )

Degree of hardness

Moderately soft

Moderately hard 150–250

Slightly hard

Hard 250–350

Moderately hard

Excessively hard

Very hard

surface of pipes, boilers and, of course, kettles and the heating elements in washing-machines and dishwashers. The chemical reaction that occurs is

2HCO 3 !H 2 O þ CO

2 2 þ CO 3

Hydrogencarbonate

CO 3 2 þ Ca 2 þ ! CaCO 3

Calcium carbonate

The greater the degree of carbonate hardness the more severe the problem

becomes, although a moderate level of hardness (150 mg l 1 ) is useful as it

forms a protective film of calcium carbonate over the inside of pipes, preventing the leaching of metals and corrosion.

In the USA TDS is used in place of hardness. It is a broader classification than hardness that measures the total amount of inorganic salts and the small amount of

organic matter present. Traditionally TDS is calculated gravimetrically by drying water at 180

C and measuring the residual in mg l 1 . The main constituent

cations are calcium, magnesium, sodium and potassium associated with the anions carbonate, hydrogencarbonate, chloride, sulphate and nitrate. However, TDS is closely correlated with hardness. The alternate method of calculating TDS is by using conductivity and multiplying the conductance by a specific factor, usually between 0.55 and 0.75. This conversion factor varies with the type of water source, increasing with anion concentration, although once determined gravimetrically it remains fairly constant for a specific water source. At TDS concentrations

>500 mg l 1 excessive scaling occurs (WHO, 2003 ).

10.3 Standards Hardness is an important factor in the taste of water, although at above

500 mg l 1 the water begins to taste unpleasant. The World Health Organization

Hardness and total dissolved solids

(WHO, 1984 ) set a maximum recommended concentration of 500 mg l 1 in drinking water on aesthetic, not health, grounds. In the second revision (WHO, 1993 ) again no health-related standard was felt necessary but a guideline of

200 mg l 1 was suggested to avoid scale deposition in distribution systems. In the latest revision the WHO ( 2004 ) has made no health-based guideline for hardness but acknowledges that the degree of hardness can affect acceptability to the consumer in terms of taste and scale deposition. They indicate that scale deposition will occur in plumbing, especially when the water is heated, at

concentrations >200 mg l 1 , and that at <100 mg l 1 the water has a low buffering capacity and can increase corrosion in pipework. In the USA, where TDS is used in place of hardness, the National Secondary Drinking Water Regulations set a non-enforceable maximum contaminant level of 500 mg l 1 for TDS to prevent taste and scaling problems.

The current EC Drinking Water Directive does not set any specific standards for hardness. In the earlier Directive a standard was set for water that had been softened, where a minimum hardness concentration of 150 mg l 1 was required.

In the earlier Directive hardness was expressed solely in terms of calcium ions.

So 150 mg l 1 as CaCO

3 is equivalent to 60 mg l as Ca þ . To convert ions to CaCO 3 equivalent, multiply mg l 1 of Ca 2 þ by 2.495, and Mg 2 þ by 4.112 to

give mg l 1 as CaCO 3 equivalent. Alternatively:

Hardness ¼ mg 1 1 of each ion ·

3 ðmg CaCO 1 1 Equivalent weight of each ion

Equivalent weight of CaCO

3 Þ: So, for example, for 30 mg l 1 of the ion calcium (Ca 2 þ ) the hardness is

calculated as:

30 mg 1 1 of Ca 2 þ · 50 =20:04 ¼ 74:9 mg CaCO 3 1 1

where the equivalent weights for the most important ions are Ca 2 þ

20.04, Mg 2 þ

12.16, HCO 3 6.1 and SO 4 2 48 and the equivalent weight of CaCO 3 is 50.

Concentrations of calcium in natural waters of

2 þ l 1 are common and > 200 mg Ca 2 þ l 1 rare. High levels of magnesium in water supplies are far

2 more uncommon with concentrations 1 >100 mg Mg þ l unusual. The TDS of natural waters range from 30 to 6000 depending on the solubilities of the

minerals within the catchment. Most surface waters have a TDS <500 mg l 1

compared to

<1000 mg l 1 for groundwaters.

10.4 Health aspects It was observed as early as 1957 that the occurrence of cardiovascular disease in

the population was related to the acidity of water (Zoetman and Brinkman, 1976 ). Since that time a considerable number of studies, mainly in the USA, have indicated a correlation between hardness or TDS and mortality, especially

10.5 Conclusions

cardiovascular disease. A study carried out in the UK and reported in 1982 confirmed that mortality from cardiovascular disease was closely associated

with water hardness, with mortality decreasing as hardness increased. The effect was present for both stroke and ischaemic heart disease, but not for non- cardiovascular disease. The difference in mortality rates between soft water

(around 25 mg CaCO l 1 3 ) areas and hard water areas ranged from 10% to 15%.

This is in general agreement with an earlier figure of a 0.8% decrease in

mortality with every 10 mg CaCO l 1 3 increase in hardness up to 170 mg

CaCO

3 l (Lacey, 1981 ; Powell et al., 1982 ). Hardness is not usually associated with cancer, yet a number of studies have identified just such a relationship. Turner ( 1962 ) studied a number of counties in England and Wales and found that gastric cancer was inversely associated with water hardness. Similar results were reported from a study involving 66 cities in Canada (Wigle et al., 1986 ), while a weak inverse association between total cancer mortality and water hardness in 473 US cities was reported by Morin et al. ( 1985 ). In contrast elevated stomach and female breast cancer mortalities have been reported in hard water areas in a study of 80 British towns (Stocks, 1973 ). Most of the research on health effects in relation to the mineral content of water has been done using hardness, although the results of these studies equally apply to TDS.