Mechanism of Filtration

Filtration can be defined as a solid-liquid separation process in which solids are separated from the suspension by passing through a porous medium that accumulates the solids but allows the passage of fluids. The suspension of solid and liquid which is to be filtered is known as slurry. The porous medium used to filter the solution is known as a filter medium. The accumulated solids are referred to as filter cake. The clear liquid passing through the filter is filtrate. The term clarification is used when the amount of solids in a liquid is not more than 1% w/v. The rate of filtration is defined as the volume of filtrate collected in unit time.

Filtration Process
Fig: Filtration Process

Application of Filtration:

  1. During the manufacturing of sterile products, it is necessary to remove the smallest particle. Therefore, the air is filtered through HEPA filters (high-efficiency particulate air filters) to get sterile air.
  2. The solution, suspension, eye drops, elixirs, etc. should be free from suspended solids. Filtration is a major step during the production of such formulations.
  3. Filtration methods are used in the home to clarify potable water
  4. Filtration is necessary to separate substances of different chemical composition
  5. On an industrial scale, filtration is used for the dewaxing of oils.
  6. The filtration technique is used to treat sewage and waste-water treatment.

Mechanism of Filtration:

  • several mechanisms. It has been The filtration process involves identified that straining is the main mechanism that is operative in the elimination of solids in suspension during the filtration. Other mechanisms include impaction, interception, and adhesion, etc.
 Chemical & Physical adsorption
 Biological growth

  Figure: Steps involved in the mechanism of filtration 
  1. Straining: The particles which are larger than the pore space of the filtering medium are strained out mechanically. While particles smaller than the pore space are trapped within the filter.
  2. Sedimentation: The particles within the filter settle on the filtering medium.
  3. Impaction: Heavy particles do not follow the streamlined flow.
  4. Interception: The particles are removed during contact with the surface of the filter medium
  5. Adhesion: The particles adhere to the surface of the filter medium as they pass
  6. Flocculation: It occurs within the interstices of the filter medium.
  7. Chemical Adsorption (ie. Bonding, chemical interaction) and Physical adsorption (i.c. Electrostatic force, van der wall force): Once a particle has come into contact with the surface of the filter medium or with other particles, one of these mechanisms, chemical or physical adsorption or both may occur.
  8. Biological growth: The biological growth inside the filter reduces the pore volume and improves the removal of particles with any of the previous removal mechanisms

Type of Filtration:

  • Based on the mechanism, filtration is classified as
  1. Depth filtration: In this method, the removal of suspended material from the liquid suspension is performed by passing the liquid through a filter bed composed of a granular or compressible filter medium. The material used for the filter bed is a packed bed of sand, anthracite, or other granular media. Solids (particles) get attached with gradient density structure to the media by adsorption by physical restriction. This method is used in the treatment of surface waters for potable water supply.
  2. Surface filtration: Surface filtration involves the removal of material suspended in a liquid using sieving. In this method, the liquid passes through a thin septum (ie, filter material). Materials that have been used as a filter septum include woven wire cloths, cloth fabrics of different fabrics, and a variety of synthetic materials
  3. Membrane Filtration: Membrane filtration is a separation process that uses a semipermeable membrane. It consists of two parts:
  • permeate containing the material that passes through the membranes, and
  • retentate includes species being left behind.
  • Membrane filtration can also be classified in terms of the size range of the permeating species, the rejection mechanisms, the driving forces employed, the chemical structure and composition of the membranes, and the geometry of the construction. The most important types of membrane filtration are microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and Reverse osmosis (RO).
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