Types of Impurities in water | Quality of Water – Water Supply

Quality of Water

Quality of water is the degree of goodness of physical, chemical & biological characteristics of water in all aspects.

The following are important requirements of water for domestic use.

  1. It should be colorless & sparkling clear. It must be free from solids in suspension & must not deposit sediment on standing.
  2. It should be good taste, and free from odor.
  3. It should be reasonably soft.
  4. It should be plentiful & cheap.
  5. It should be free from pathogens.
  6. Free from objectionable dissolved gases & have a sufficient quantity of dissolved oxygen.
  7. Should be free from harmful salts & minerals.
  8. It should be free from radioactive substances, poisonous metals, etc.

Impurities in Water

An undesirable substance contained in water in any form is called impurities in water. On the basis of the characteristics of impurities, the following impurities are present in water.

A) Physical Impurities

Those impurities affect the physical characteristics of water such as color, odour, taste & turbidity.

Eg:- Sand, silt, mud, clay, etc.

B) Chemical Impurities

Chemical substances present in the water affects the chemical characteristic of water such as PH, Hardness, Alkali, etc.

C) Biological Impurities

  • The bacteriological impurities affect the bacteriological characteristics of water.
  • Eg: Pathogenic (salmonella) & nonpathogenic
  • These impurities cause disease to humans.

On the basis of the state of presence, impurities may be classified as,

Types of Impurities in water | Quality of Water

A) Suspended Impurities

  • These are those impurities that normally remain in suspension due to the same specific gravity as that of water.
  • These impurities make the water turbid.
  • Can be removed by filtration & sedimentation.
  • Example of these impurities is sand, silt, clay, algae, fungi, etc.

B) Dissolved Impurities

Dissolved impurities are not visible.

These impurities are large in amount because water is a very good solvent & can dissolve all the salts with which it comes in contact.

The dissolved impurities may be organic compounds, inorganic salts, liquids or gases, minerals, etc.

These impurities make bad taste, hardness & alkalinity.

The concentration of dissolved impurities is measured by weighing the residue after evaporation of the water sample from a filtered sample. It is measured in ppm or mg/L.

These impurities can be removed by precipitation, adsorption, distillation, chemical method, or aeration method.

C) Colloidal Impurities

These are the small, nonvisible with the naked eye, and electrically charged particles, which remain in continuous motion.

  • Their size is 10-3mm to 10-6mm.
  • Organic matters containing bacteria and sources of epidemics are examples of colloidal impurities.
  • A Colour test is done for measuring the concentration of these types of impurities
  • These particles can be removed by neutralizing charge by the addition of chemicals resulting increase in the weight of impurities and their sedimentation.
  • They cause the color in water & these impurities cannot be removed by the ordinary filter.

Hardness of water

The characteristic of water which prevents the formation of sufficient lather or foam with soap is called the hardness of the water.

The hardness of water is due to the presence of bicarbonate, sulphates, chlorides & nitrates of calcium & magnesium.

Effects of hardness

  • It leads to the modification of colors in dyeing.
  • Cause corrosion & incrustation of pipelines & fittings.
  • Wasteful consumption of soap while bathing & washing.
  • Make food tasteless, effect to our digestive system.

These are two types of hardness.

  1. Carbonate or temporary Hardness (CH)
  2. Non- carbonate or permanent Hardness (NCH)

1) Carbonate Hardness (CH)

It happens due to the presence of carbonate & bicarbonate salts of calcium & magnesium.

It is removed by boiling or mixing the lime in water.

           Ca (HCo3)2 —————- Ca Co3 +H2 o + co2

           Mg (HCo3)2 ————— Mg Co3 + H2o+ Co2

2) Non-carbonate hardness (NCH)

  • It happens used to the presence of sulfate, chlorides & nitrates of calcium & magnesium.
  • It can’t be removed by simple boiling.
  • It can be removed by a special method of water softening (permit process or ion exchange process)

Hardness is expressed in term of the equivalent amount of CaCo3 as,

Total hardness is the sum of carbonate hardness and non-carbonate hardness.

                             TH = CH + NCH

Its unit is mg/Lit or ppm or 0clark [10clark =14.25]

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