Clarification & Filtration (Pharmaceutics) notes

Terms used in Filtration

  • FILTRATION may be defined as the “Separation of solids from a fluid by means of a porous medium that retains the solids but allows the fluid to pass.
  • CLARIFICATION: When solid does not exceed 1.0%, filtration is termed as clarification & filtrate is the primary product.
  • STRAINING: The object of filtration is to remove large visible particles using a coarse filtering medium (muslin/cotton wool).
  • ULTRA-FILTRATION: Separation of inter-micellar liquid from solid by use of high pressure on a semi-permeable membrane
  • SLURRY/FEED: Suspension of solid in liquid to be filtered
  • FILTER MEDIUM: Porous medium used to retain/entrap the solids
  • FILTER CAKE/RESIDUE: Accumulation of solids on the filter medium
  • FILTRATE/EFFLUENT: Clear liquid passing through the filter medium
  • CAKE FILTRATION: If the recovery of the solid is desired, the process is called cake filtration.
filtration, slurry, filter medium, filter cake, filtrate
Terms used in Filtration

Difference between clarification & filtration


FILTRATION may be defined as the “Separation of solids from a fluid by means of a porous medium that retains the solids but allows the fluid to pass.


CLARIFICATION: When solid does not exceed 1.0%, filtration is termed as clarification & filtrate is the primary product.

Darcy’s Theory (Rate of Filtration)

Darcy’s Theory (Rate of Filtration)
 V= Volume of filtrate
K= Permeability coff.
A=Area of filter bed
∆P= Pressure difference on liquid & below the filter medium
η= Viscosity of liquid
l= thickness of filter cake

Factor affecting the rate of filtration

  • Factor affecting the rate of filtration
    1. Filtration
    2. Viscosity
    3. Surface area of filter media
    4. Temp.
    5. Particle size
    6. Pore size of filter media
    7. Thickness of cake
    8. Nature of solid materials (porosity of filter cake)


    1. Filter paper (Grade-Coarse, medium & fine)
    2. Glass wool (Corrosive liquids)
    3. Cotton wool
    4. Asbestos(Al. silicate): Corrosive liquids, Alkalinity of filtrate)
    5. Sintered glass: borosilicate glass, Parenteral, corrosive liquid & Oxidising agents)
    6. Membrane filter
    7. Filter cloths


Substances that reduce the resistance of the filter cake & increase the filtration. (Added to the liquid slurry to be filtered)

Filter aids Should form a porous cake & Insoluble in liquid

Filter aids forms a fine surface deposit- prevent solid from contacting & plugging the supporting filter medium.

Filter aids added by 2 methods:

  • Pre-coating &
  • Body mix

List of Filter aids used in Pharmaceutical preparations

CarbonCarbonFiltering strong alkaline solutionsCoarse grade only & Expensive
CelluloseCelluloseHigy purity & Excellent chemical resistanceExpensive
Diatomaceous earthSilicaWide range; Used for very fine filtrationSlightly soluble than diatomites in acid & alkali
AsbestosAluminosilicateGood retention on coarse screensMore soluble than diatomites in acid & alkali
PerliteSilica & AluminosilicateWide range; not capable of finest retentionMore soluble than diatomites in acid & alkali
Filter aids used in Pharmaceutical preparations


mechanism, filtration, Impingement,
Impingemet m echanism of Filtration
  1. Straining like Sieving = pores are smaller than the particles so that particles are retained on the filler medium.
  2. Impingement = Streamlined strike the cylinder- triangular ridge
  3. Entanglement = particles are smaller than the pores; so Particle became entangled
  4. Attractive forces:- the potential difference is used; Particles may collect on a filter medium as a result of attractive forces.

Filtration Equipment

  1. Filter leaf
  2. Filter press – plate & Frame
  3. Meta-filter
  4. Membrane filter
  5. Sintered filter
  6. Rotary filter
  7. Filter candle

List of Filtration Equipment used in Pharmaceutics

Filtration equipmentAdvantageDisadvanctageRemarks
Filter leafRemoval of Cake: Reverse of Airflow or Immersing the filter in a waterBatch process/Non-continuoususe for slurry up to 5 %
No. of leaves are connected to the common outlet (to provide large surface area) = Sweetland filters
Filter press – plate & Frame
(2 units – plate & frame press)
– Large volume can be handled/filtered.
– Provide a large surface area for filtration
-Filtering media can be used repeatedly by washing
-Surry – max 5% & produce dry cake
-Bears up to 20 bars (withstand)
– Batch process / Not continuous process
– Slum upto 5%
– Costlier – labour is required for washing & replacement of filter cloth
– Leakage between plate may take place
Frame-Receive/inlet; Plate-Support filter cloth & outlet
(Edge filter -Corrosive Liquid)
Backflushing = Removal of cake
Withstand High pressure & corrosive liquids
-Clarification of syrups elixir & parenteral solution (more viscous liquid)
– Withstand High pressure & corrosive liquids
Membrane filter (Made of cellulose nitrate & cellulose acetate, polyamide, polysulfone or silver)-Thickness 50 – 150 micrometer(120 micrometer Average)
-No. of pores: 400 – 500 pores per cm2
– Uniformity in pore size
-Required Rigid Perforated Support for membrane filter
-Pre-filtration required to prevent clogging of a membrane
-Can not be used for filtration of – organic solvents (alcohols, Keton as, esters, CHCl3)
-Used for sterilization of both Aq & Non-Aq preparation Liquids
-Disposable, so No cross-contamination but costly
Sintered filter (Made of Borosilicate glass)
Rotary filter-can Handle – Conc. Slurry (15-30%)
-Continuous process – (has system for removing the cake that is formed so suitable for concentration slurry
Filter candle (Made of porcelain or kieselguhr)Tendency to adsorb liquid-Used for sterilization of solutions
Filtration Equipment


membrane pore size 0.001-0.01  µm under high pressure; removing dissolving molecules on the size of membrane size & retain macromolecules & pass certain size molecules.

Integrity of membrane filter check by:

o   Bubble point test

o   Diffusion test

o   Forward flow test

Pore size of membrane filter (micrometer)

  1. 0.2:-  All Bacteria
  2. 0.45:-    All coliform group of Bacteria
  3. 0.8:-    All airborne particles
  4. 1.2:- Non-living particles which are dangerous is in fluids
  5. 5:-    All cells from body fluids
  6. 0.010 to 0.10:-   Can remove virus from water & air

Note: Method of choice for sterilization of solutions that are chemically & physically unstable under heating condition:- Filtration- Sterile filtration (Membrane, sintered & HEPA filter).

Cake filtration:

o   Filter press

o   Rotary drum vacuum (continuous process)

Rotary filter

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