2. Definition
Filtration:
It may be defined as a process of separation of
solids (particles) from a fluid by passing the
same through a porous medium that retains the
solids but allows the fluid to pass through.
Clarification:
When solid are present in very low
concentration, i.e., not exceeding 1.0% w/v, &
filtrate is the primary product , the process is
called clarification
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5. Need of filtration
1. Production of sterile products:
HEPA filters or laminar air bench
Membrane filters
2. Production of bulk drugs (recovery of desired solids)
3. Production of liquid dosage (improved appearance)
4. Effluents and waste water treatment
5. Separation of suspended solid particles from air by using HEPA filters & pre filters
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6. Mechanism of filtration
1. Straining: Similar to sieving, i.e., particles of larger size can’t pass through smaller pore size
of filter medium.
2. Impingement: Solids having the momentum move along the path of streaming flow and
strike (impinge) the filter medium. Thus the solids are retained on the filter medium.
3. Entanglement: Particles become entangled in the masses of fibers (of cloths with fine hairy
surface or porous felt) due to smaller size of particles than the pore size. Thus solids are
retained within filter medium.
4. Attractive forces: Solids are retained on the filter medium as a result of attractive force
between particles and filter medium, as in case of electrostatic filtration
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7. Types of filtration
1-Surface/ screen filtration
It is a screening action by which pores or holes in
the medium prevent the passage of solids
Mechanism : straining and impingement
For this, plates with holes or woven sieves are
used
The size of particles retained is slightly higher than
the mean pore size of medium
Efficacy is defined in terms of mean or maximum
pore size.
E.g. cellulose membrane filter
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8. Types of filtration
Depth filtration
In this slurry penetrates to a point where the
diameter of solid particles is greater than that of the
tortuous void or channel
Mechanism : Entanglement
The solids are retained with a gradient density
structure by physical restriction or by adsorption
properties of medium
The size of particles retained is much smaller than
the mean pore size of medium
E.g. Ceramic filters and sintered filters
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9. Theory of filtration
The flow of liquid through a filter follows the basic rules that govern the flow of any liquid
through the medium offering resistance.
The rate of flow may be expressed as
The rate of filtration may be expressed as volume per unit time (dv/dt)
Driving force = pressure differences (pressure upstream – pressure downstream )
Resistance is not constant. It increases with an increase in the deposition of solids on the filter
medium so, filtration is not a steady state
The rate will be greatest at the beginning of filtration because the resistance is minimum as
the deposition increases resistance to flow increases
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10. Theory of filtration
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After formation of filter cake, its surface acts as filter medium and solids continuously deposit
adding to thickness of the cake
12. Theory of filtration
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Darcy’s Equation
Poiseullie's law assumes that the capillaries found in the filter are highly irregular and non-
uniform.
Therefore, if the length of capillary is taken as the thickness of bed, a correction factor for
radius is applied so that the rate is closely approximated and simplified by Darcy’s law as
This model relates not only to filter beds or cakes but also applied to depth filter
13. Theory of filtration
The factors that governs the rate of filtration
are,
1. Surface area
2. Viscosity &Temperature
3. Pressure difference
4. Properties of solid
5. Resistance of cake and filter
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14. Filter Media
The surface upon which solids are deposited in a filter is called the “Filter medium”
Properties of ideal filter medium:
It should
1. have sufficient mechanical strength
2. Be chemically inert
3. have high retention power for the solids without plugging at the start of filtration
4. be capable of delivering a clear filtrate at a suitable production rate
5. Absorb negligible amounts of dissolved material
6. Sterile filtration imposes a special requirement since the pore size must not exceed the
dimension of bacteria or spores
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15. Filter Media
Filter media are available in different materials and forms.
Selection of filter media depends on followings:
1. Size of particle to be filtered
2. Amount of liquid to be filtered
3. Nature of product to be filtered
4. Purpose of filtration
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16. Material used as filter media
1- Membrane Filter:
is a common filter medium for microfiltration & ultra filtration
Made up of cellulose, Polyvinyl chloride, Nylon ,Teflon, poly sulfone
and other cellulose derivatives.
It has different grades, qualities & pore size such as coarse, medium
and fine
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17. Material used as filter media
Advantages:
1. Bacteria are removed
2. Absorption of medicament is negligible
3. In every new operation, a new disc is used
4. quite rapid
5. Don’t liberate particles to the filtrate
Disadvantages:
1. Fine pores may easily clogged
2. Soluble in certain organic solutions e.g.
ketones and esters
3. fragile ( partially overcome by use of
monofilament nylon as a supporting web
within the membrane structure)
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18. Material used as filter media
2-Woven material
Made up of wool, silk, metal or synthetic
fibers (rayon, nylon etc.)
These include
• a- wire screening (woven wire cloth)
• b- fabrics of cotton, wool, nylon
Woven wire cloth: made from stainless
steel is
• durable,
• resistance to plugging and
• easily cleaned or sterilized
Cotton: is most common & used as primary
medium in plate and frame press & as
fabricated bags for straining
Nylon: is superior for pharmaceutical use,
since it is
• unaffected by mold, fungus or bacteria ,
• has negligible absorption properties &
• extremely strong as compared to cotton cloth
Both are Suitable for coarse straining in
aseptic filtration
The choice of fiber depends on chemical
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19. Material used as filter media
3-Non-Woven material
Include felt, bonded fabric and Kraft paper
A-Felt:
Is a fibrous mass free from bonding agents
& mechanically interlocked to yield specific
pore diameter that have controlled
retention
• Function as depth media
• High flow rate with low pressure drop is the
primary characteristic
• Used where gelatinous solutions or fine
particulate matter is involved
B- bonded fabric:
made by binding the textile fabrics with resins,
solvents & plasticizers
Not used pharmaceutically because of
interaction with additives
C-Kraft paper:
Is of pharmaceutical standard
• Controlled porosity
• limited adsorption characteristics
• Low cost
• Used in plate & frame filters and horizontal
plate filters
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20. Material used as filter media
4-Cotton Wool:
• Commonly used
• Small tough of cotton wool placed in the neck of
funnel
5-Glass wool:
• Use for filtering highly corrosive chemicals
• May contaminate the filtrate with glass fibers
6-Asbestos:
• Also used for filtering the corrosive liquid
• impart alkalinity to the filtrate
• Alkaloids may get absorbed on them
• May contaminate the filtrate
7-Sintered Glass:
Consist of Pyrex glass powdered & sifted to
produce granules of uniform size ---heated to
fuse
Liquid pass through the interstices between the
particles that form a connective system
• Used for filtering parenteral preparations
• Useful for filtering the corrosive liquid and
oxidizing agent
• Don’t shed particles
• Don’t absorbs any liquid
• Can be easily washed
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21. Material used as filter media
8- Cartridge units
• convenient & economical
• available in pore size of 100 μm to even less than 0.2 μm
• Can be either surface cartridges or depth type cartridges
Surface cartridges: are corrugated and resin treated papers and used in hydraulic lines
• Ceramic cartridges and porcelain filter candles are examples
• Can be reuse after cleaning
Depth type cartridges: Made up of cotton, asbestos or cellulose
• are disposable items, since cleaning is not feasible.
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22. Filter Aids
“The substances which when added to the liquid to be filtered, reduce the resistance of the
filter cake and increase the filtration”
The objective of filter aid is to prevent the medium from becoming blocked and to form an
open, porous cake, hence, reducing the resistance to flow of the filtrate
Characteristics of filter aids:
• Chemically inert and free from impurities
• Suitable particle size for retention of solids
• Able to remain suspended in liquids
• Porous structure rather than dense, so permits formation of pervious cake
• Recoverable
• Free from moisture
Examples: Diatomite (Keiselguhr) ( obtained from natural siliceous deposits)Perlite , it is an aluminium
silicate, Cellulose, Asbestos, charcoal, talc, bentonite , etc
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24. Filter Aids
Handling of filter aids
Filter aids may be applied by
1) Pre- coating technique:
by forming a pre-coat over the filter medium by suspending in a liquid until filter aid is
uniformly deposited
2) Body- mix technique:
• A small proportion of the filter aid (0.1- 0.5 %) is added to the slurry to be filtered. This
slurry is recirculated through the filter until a clear filtrate is obtained, filtration then
proceeds to completion
• Minimizes equipment requirement and cross contamination potential
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