Engineering Chemistry 1–CHARACTERISTICS and HARDNESS of Water

WATER TREATMENT

Water is our lifeline, played a key role not only in the history of countries, but in religion, mythology and art. Water has always been perceived as a gift from the Gods.So it rained from the heavens. Water provides the earth with the capacity of supporting life. An organism does not have to be told how important water is to their existence. An amphibian knows to lay its eggs in water or else there will be no new born. Even flies know to lay their eggs in fresh water. The only organism that does not understand the importance of water is humans especially in industrialized countries.

Although water covers more than 70% of the Earth, only 1% of the earth’s water is available as a source of drinking. Yet our society continues to contaminate this precious resource.

Water is known as a natural solvent. Before it reaches the consumer’s tap, it comes

into contact with many different substances including organic and disease – producing contaminants

that may be present in the water. Although disinfection is an important step in the treatment of potable water, they are used to prevent disease, can create byproducts which may pose significant health risks. Today, drinking water treatment at the point-of - use is no longer a luxury, it is a necessity.

Occurrence:

Water is the only substance that occurs at ordinary temperatures in all three states of

matter: Solid, Liquid and Gas. As a so33.3.33lid, ice, it forms glaciers, frozen lakes and rivers, snow, hail and frost. It is liquid as rain and dew, and it covers three- quarters of the earth’s surface in swamps, lakes, rivers and oceans. Water also occurs in the soil and beneath the earth’s surface as

a vast groundwater reservoir. As gas, or water vapour, it occurs as fog, steam and clouds.

Water purification:

Impurities are removed from water by seeming, sedimentation, filtration, chlorination or irradiation. Aeration removes odours and tastes caused by decomposing organic matter, industrial wastes and some gases. Various salts and metals cause hardness in water. Hardness may be removed by boiling, by adding sodium carbonate and lime or by filtering through natural or artificial zeolites. Water is also purified by processes such as desalination, reverse osmosis, electrolysis etc.,

Characteristics of water

Water hardness

Hardness = the amount of dissolved salts in water.

dH

ppm (or) mg / lit CaCO3

Soft

0.3

0-50

5

dH - hydrogen Power

pH = the concentration of hydrogen ions in water

CHARACTERISTICS OF WATER

As per the suggestion given by World Health Organisation (WHO) and by Indian Council of Medical Research (ICMR), the following are the important characteristics of potable water.

1. It should be clear, colourless and odourless.

2. It should be cool and pleasant to taste.

3. It should be free from harmful bacteria and suspended impurities.

4. It should be free from dissolved gases like CO2, H2S, NH3, etc., and poisonous minerals like lead, arsenic, manganese, etc.,

5. Hardness should be less than 500 ppm.

6. Chloride ion content should be less than 250 ppm.

7. Fluoride ion content should be less than 1.5 ppm.

8. Total Dissolved Solids (TDS) content should be less than 500 ppm.

9. pH of the potable water should be 6.5 – 8.5.

Acidic

Neutral

Alkaline

pH=0-7

7

pH=7-14

Chemical Characteristics of water

The most important chemical characteristics of water are its acidity, alkalinity, hardness and corrosiveness. Chemical impurities can be either natural, man made (Industrial) or be deployed in raw water sources by enemy forces.

Some chemical impurities cause water to behave as either an acid or a base. Since either condition has an important bearing on the water treatment process, the pH value must be

determined. Generally the pH influences the corrosiveness of the water, chemical dosages necessary for proper disinfection and the ability to detect contaminants.

Hardness

Hardness is caused by the soluble salts of calcium, magnesium, iron, manganese, sodium, sulphates, chlorides and nitrates. The degree of hardness depends on the type and amount of impurities present in the water. Hardness also depends on the amount of carbon-di-oxide in solution. Carbon-di-oxide influences the solubility of the impurities that cause hardness.

The hardness caused by carbonates and bicarbonates is called carbonate hardness. The hardness caused by all others (chlorides, sulphates, nitrates) is called non-carbonated hardness.

Hard water

Water which does not produce lather with soap solution, but produces white

precipitate (scum) is called hard water.

In other words, water that contains mineral salts (an calcium and magnesium ions)

that limit the formation of lather with soap.

This is due to the presence of dissolved Ca and Mg salts.

3

Soft water

Water, which produces lather, readily with soap solution is called soft water. This is due to the absence of Ca and Mg salts.

Water that is not hard (ie., does not contain mineral salts that interfere with the formation of lather with soap).

Hardness of water

How to detect hardness?

Hardness of water can be detected in two ways.

· When the water is treated with soap solution, if it prevents lathering and forms white scum, the water contains hardness.

· Water containing hardness, gives wine red colour with Eriochrome Black –T indicator.

The total water hardness (including both Ca2+ and Mg2+ ions) is read as parts per million (ppm) or weight / volume (mg/L) of Calcium Carbonate (CaCO3) in the water. Although water hardness usually measures only the total concentrations of calcium and magnesium (the two most prevalent, divalent metal ions), iron, aluminum and manganese may also be present at elevated levels in some geographical locations. The predominant source of magnesium is dolomite (CaMg (CO3)2).

Types of hardness

Depending upon the types of dissolved salts present in water, hardness of water can be classified into two types:

· Temporary Hardness

· Permanent Hardness

Temporary Hardness (or) Carbonate Hardness (CH) (or) Alkaline Hardness

Temporary hardness is caused by a combination of calcium and magnesium bicarbonate ions in the water. It can be removed by

· boiling water

· by the addition of lime (Ca(OH)2)

Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon coving.

Ca (HCO3)2 à CaCO3 ↓+ H2O + CO2

Mg (HCO3)2 + 2Ca(OH)2 à Mg (OH)2↓+ 2CaCO3↓ +2H2O

Permanent Hardness (or) Non – Carbonate Hardness (NCH) (or) Non – alkaline Hardness

Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulphates and /or chlorides which become more soluble as the temperature rises. Despite the name, permanent hardness can be

removed using water – softener or ion-exchange column, where the calcium and magnesium ions

are exchanged with the sodium ions in the column. It can be removed by

· Lime – Soda process

· Zeolite process

CaCl2 + Na2 CO3 → CaCO3↓ +2Nacl

(Soda)

CaSO4 + Na2Ze →CaZe + Na2SO4

Zeolite =(Na2 Al2 Si2 O8. X H2O)

Hard water causes scaling, which is the left- over mineral deposits that are formed after the hard water had evaporated. This is also known as lime scale.

Total Hardness

The sum of temporary hardness and permanent hardness.

Table 1 :1 Molecular weights of some hardness producing salts.

Hardness

producing salt

Molecular

weight

Hardness producing salt

Molecular

weight

Ca(HCO3)2

162

MgSO4

120

Mg(HCO3)2

146

MgCO3

84

Mg(NO3)2

148

MgCl2

95

Ca(NO3)2

164

CaCl2

111

CaCO3

100

Ca2+

40

CaSO4

136

Mg2+

24

Expression of hardness in terms of equivalents of CaCO3

The concentration of hardness producing salts are usually expressed in terms of an equivalent amount of CaCO3. CaCO3 is chosen as a standard because,

i) Its molecular weight (100) and equivalent weight (50) is a whole number, so the

calculations in water analysis can be simplified.

1

ii) It is the most insoluble salt, that can be precipitated in water treatment. If the concentration of hardness producing salt is x mgs/lit. then

2

Example

If the concentration ( or) weight of CaSO4 is 43mgs/lit, then weight equivalent to

Units of Hardness

1. Parts per million (ppm)

It is defined as the number of parts of CaCO3 equivalent hardness per 106 parts of water.

2. Milligrams per litre (mg/lit)

It is defined as the number of milligrams of CaCO3 equivalent hardness per 1 litre of water.

3. Clarke’s degree (oCl)

It is defined as the number of parts of CaCO3 equivalent hardness per 70,000 parts of water.

4. French degree (oFr)

It is defined as the number of parts of CaCO3 equivalent hardness per 105 parts of water.

Relationship between various units

1ppm = 1 mg/lit = 0.10 Fr = 0.070 Cl