hard ness

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What is water?

• H2

O + “dissolved matter”

• 90%+ of even the strongest beer

• The “Universal Solvent” due to its polar nature

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Hardness

• Origin debatable: originally used to describe water

with a high mineral content. “Hard” to lather up soap

• Total water hardness is the measure of the

bicarbonate, calcium, and magnesium ions and is

measured in two ways Temporary hardness

permanent hardness

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Water Softening

I. Introduction

Reasons to Soften1. Reduce Soap Consumption

2. Improve Aesthetics of Water

3. Hot Water Heaters last longerReasons not to Soften

1. Expensive Process

2. May be less healthy3. Competes with health related costs

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LIME-SODA PROCESS:

•

In this method, the soluble calcium andmagnesium salts in water are chemically

converted in to insoluble compounds, by

adding calculated amount of Lime andSoda. CaCO3 and Mg(OH)2 so

precipitated, these precipitates are filtered

off.

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Softening of Water

If water contains more than 50 parts of

hardness per 1,00,000 parts, it is generallyaccepted that softening is desirable.

Softening can be quite expensive, and the use

of softened water where it is unnecessary iswasteful.

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Removal of temporary hardness

• Temporary hardness is removed by boiling, butthis is impractical in daily use. The carbondioxide can, however, be extracted from the bicarbonate by the action of an alkali, calciumhydroxide being commonly used.

The reaction is as follows:

Ca(HC03)2 + Ca(OH)2 → 2CaC03 + 2H20

• Thus the whole of the temporary hardness due tocalcium is precipitated as calcium carbonate.According to the equation, 100 parts of temporaryhardness require 74 parts of calcium hydroxide or 56 parts of calcium oxide.

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• The reaction follows a slightly different course withmagnesium carbonate.

The first stage is the conversion to magnesium

carbonate:Mg (HC03)2 + Ca(OH)2 →MgCO3 + 2CaC03 + 2H20

• The reaction, in this case, however, does notsufficient to soften the water because magnesium

carbonate is sparingly soluble in water.• A second molecule of calcium hydroxide must

therefore be added to precipitate the insolublemagnesium hydroxide:

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• A second molecule of calcium hydroxide musttherefore be added to precipitate the insolublemagnesium hydroxide:

Mg C03+ Ca (OH) 2 →Mg (OH)2 + CaC03

• Thus, each molecule of magnesium bicarbonate present requires two of lime for complete

precipitation, and every part of temporaryhardness due to magnesium,

• expressed as calcium carbonate requires 2 x 56 =112 of quicklime (CaO). Water will often contain

dissolved carbon dioxide, which will combinewith some of the lime added for softening:

• Co2 + Ca(OH)2 → CaC03 + H20

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Removal of permanent hardness

Permanent hardness is removed by converting

the calcium and magnesium sulphates intocarbonates by the action of sodium carbonate:

CaSO4+ Na 2C03 → Na2SO4+ CaC03

MgSO4+ Na2C03 → Na2SO4+ MgC03 The calcium sulphate is thus removed ascalcium carbonate, an equivalent quantity of sodium sulphate being left in solution.Magnesium sulphate would be converted intomagnesium carbonate and this would require precipitation (to calcium carbonate) by lime.

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If, however, enough lime be added to precipitateall the magnesium present, the magnesiumsulphate is converted into calcium sulphate

which, in turn, is precipitated as its carbonate:

MgSO4+ Ca(OH)2 → Mg(OH)2 + CaSO4

It is apparent that in order to calculate the

quantities of lime and sodium carbonate necessaryfor softening any given volume of hard water, wemust know:

• i) The temporary hardness

• ii) The hardness due to magnesium

• iii) The permanent hardness and

• iv) The free carbon dioxide.

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Caused By Quicklime Req.

Sodium bi

carbonate

Req.

Calcium bicarbonate

Magnesium bicarbonate

Calcium sulphateMagnesium sulphate

0.56

-

0.56-

-

-

-1.06

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• The formula provide a convenient method for determining the required amounts of lime andsodium carbonate.

• As water hardness has generally been expressed intenns of grains per gallon, the formulae conform tothese units.

•

Quantity of quicklime required per 1,00,000gallons = 0.56 (Ht + Hm) lb, and

• Quantity of dry sodium carbonate = 1.06 x H lb.

• Where, Ht represents temporary hardness

• H p represents permanent hardness

• Hm represents hardness due to magnesium,

• Each in terms of grains per gallon of water.

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ADVANTAGES OF LIME-SODA PROCESS:

1. It is very economical

2. If this process is combined with sedimentation withcoagulation, lesser amounts of coagulants shall beneeded.

3. The process increases the pH value of the treated-water; thereby corrosion of the distribution pipes is

reduced.4. Besides the removal of harness, the quantity of

minerals in the water is reduced.

5. To certain extent, iron and manganese are also

removed from the water.6. Due to alkaline nature of treated-water, amount of

pathogenic bacteria in water is considerably reduced.

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DIS-ADVANTAGES OF LIME-SODA PROCESS:

1. For efficient and economical softening, careful

operation and skilled supervision is required.2. Disposal of large amounts of sludge or insoluble

precipitates poses a problem. However, the sludge

may be disposed off in raising low-lying areas of the

city.

3. This can remove hardness only up to 15 ppm, which

is not good for boilers.

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ZEOLITE PROCESS

In this process the hard water is passed through a

bed of zeolite, which is hydrated sodium aluminosilicate. The zeolite exchanges reversibly its sodium ion

for hardness producing calcium and magnesium ions in

the water

When the zeolite bed is exhausted (i.e. it is nolonger able to cause ion exchange), the supply of hard

water is stopped and the zeolite is treated with 10%

brine solution which regenerates the exhausted bed.

Special synthetic resins are also used in place of a

zeolite in this ion-exchange water softening process

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