Filtration - Types of filtration and filter media

Filtration
The process of passing water through beds of granular material (Ex: sand) is known as filtration. Filtration may help in removing:

• colour
• odour
• turbidity and
• few pathogenic bacteria from water.

The two types of filters commonly used for treating municipal supplies are:

• Slow sand gravity filter
• Rapid sand gravity filter 

Types of filters

• Slow sand gravity filters are used for removal of impurities and bacteria from water.
• As the name suggests, they operate at a very low rate, require large areas and are expensive

Due to the above mentioned drawbacks, slow sand filters have become obsolete and are replaced with rapid gravity filters.

Theory of filtration
Filters purify water by four mechanisms as discussed below:

• Mechanical straining:
• Flocculation and sedimentation
• Biologicalmetabolism and
• Electrolytic changes

Mechanical straining
Under this process, suspended particles present in water and are bigger than the size of voids in the sand layers, cannot pass through and get arrested. The resultant water will be free from the particles. Such particles are removed in the upper sand layers. The arrested particles including coagulated flocs form a mat on top of the bed which further helps in filtering out impurities
Flocculation and sedimentation
 The  void spaces in the filter medium act as tiny coagulation-sedimentation tanks where the colloidal matter gets arrested as a gelatinous mass that attracts other fine particles. These fine particles settle down in voids and can be removed

Biological metabolism
Certain microorganisms and bacteria that are present in the voids of the filter medium require organic impurities (algae, plankton) as food for survival. These organisms utilise organic impurities and convert them into harmless compounds by the process of biological metabolism. The harmless compounds so formed at the top surface is called schmutzdecke or dirty skin. This layer helps in absorbing and straining out impurities.

Electrolytic changes
The purifying action of filter is explained by the theory of ionization. As per this theory, a filter helps in purifying the water by changing the chemical characteristics of water. The sand grains of the filter media and the impurities carry electrical charges of opposite nature. When they come in contact with each other, they neutralise each other thus changing the character of water and make it purer.
When the electrical charges of sand particles get exhausted, they have to be restored by cleaning the filter.

Filter media

1. Sand (fine or coarse) is used as filter media.(effective size: D10)
2. Gravel is used as filter media below sand (coarsest: 20 to 60mm & finest 3 to 6mm)
3. Anthrafit (made from anthracite coal and is a type of coal-stone)
4. Activated Carbon
5. Catalytic Carbon
6. Kinetic Degradation Fluxion (KDF)
7. Mixed media
8. Reverse Osmosis membrane
9. Activated Aluminum
10. Manganese dioxide