Water, either from surface source or underground source, contains number of impurities in varying amounts. Impurities are classified as follows

• Suspended impurities.
• Colloidal impurities

• Dissolved impurities

  • Suspended Impurities : These impurities are in the form or suspension/dispersion of solid particles which can be removed by sedimentation or filtration. Suspended impurities include clay, algae, fungi, organic and inorganic matters. These macroscopic impurities cause turbidity in the water. Hence, it is customary to measure the concentration of suspended impurities by its Turbidity.

  • Colloidal Impurities : These impurities are in the form of very finely divided dispersion of particles in water. These impurities being very small, cannot be removed by ordinary filters. These are not visible to naked eye. All colloidal impurities are electrically charged and repel one another and hence remain in continuous motion and do not settle. Therefore, their removal is very difficult. Colloidal organic matters present in water are often associated with bacteria which may become the chief source of epidermics. Colloidal impurities often impart colour to water. Hence, their concentration is measured by colour tests. The size of the colloidal particles ranges between 1 micron to 1 millimicron.

  • Dissolved Impurities : These impurities may be in the form of solids, liquids or gases. This type of impurities may contain organic and inorganic compounds. The concentration of total dissolved solids is usually expressed in mg// or p.p.m.