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Chapter: 11th Chemistry : Environmental Chemistry

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Quality of drinking water

Now a days most of us hesitate to use natural water directly for drinking, because biological, physical or chemical impurities from different sources mix with surface water or ground water.

Quality of drinking water:

 

Now a days most of us hesitate to use natural water directly for drinking, because biological, physical or chemical impurities from different sources mix with surface water or ground water.

Institutions like WHO (World Health Organisation) at world level and BIS (Bureau of Indian Standards) and ICMR (ICMR: Indian Council of Medical Research) at national level have prescribed standards for quality of drinking water. Standard characteristics prescribed for deciding the quality of drinking water by BIS, in 1991 are shown in Table.15.3


Table 15.3 Standard characteristics of drinking water


 

Characteristics and Desirable limit

 

I     Physico-chemical Characteristics

i)    pH : 6.5 to 8.5

ii)   Total Dissolved Solids (TDS) : 500 ppm

iii)  Total Hardness (as CaCO3)        : 300 ppm

iv)  Nitrate : 45 ppm

v)   Chloride         : 250 ppm

vi)  Sulphate         : 200 ppm

vii) Fluoride          : 1 ppm

 

II   Biological Characteristics

i)    Escherichia Coli (E.Coli) : Not at all

ii)   Coliforms : Not to exceed 10 (In 100 ml water sample)

 

Fluoride:

Fluoride ion deficiency in drinking water causes tooth decay. Water soluble fluorides are added to increase the fluoride ion concentration upto 1 ppm.

The Fluoride ions make the enamel on teeth much harder by converting hydroxyapatite, [3(Ca3(PO4)2.Ca(OH)2], the enamel on the surface of the teeth, into much harder fluorapatite, [3(Ca3(PO4)2.CaF2].

However, Fluoride ion concentration above 2 ppm causes brown mottling of teeth.

Excess fluoride causes damage to bone and teeth.

 

Lead :

Drinking water containing lead contamination above 50ppb can cause damage to liver, kidney and reproductive systems.

 

Sulphate:

Moderate level of sulphate is harmless. Excessive concentration (>500ppm) of sulphates in drinking water causes laxative effect.

 

Nitrate:

Use of drinking water having concentration of nitrate higher than 45 ppm may causes methemoglobinemia (blue baby syndrome) disease in children.

 

Total dissolved solids (TDS):

Most of the salts are soluble in water. It includes cations like calcium, magnesium, sodium, potassium, iron and anions like carbonate, bicarbonate, chloride, sulphate, phosphate and nitrate. Use of drinking water having total dissolved solids concentration higher than 500 ppm causes possibilities of irritation in stomach and intestine.

 

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