Filtration is a physical, biological or chemical operation that separates solid matter and fluid from a mixture with a filter medium that has a complex structure through which only the fluid can pass

1. FILTRATION
Md. Saiful Islam
BPharm, MSc
North South University
Fb Group: Pharmacy Universe

2. Filtration
0 Filtration may be defined as the separation of an insoluble solid
from a fluid by means of a porous medium that retains the solid
but allows the fluid to pass. The term ‘fluid’ includes both
liquids and gases.
0 For proper understanding of filtration technique the following
terms is essential:
Filter medium: - The porous membrane which is allows
the liquid to flow but retains solid.
Slurry: - Mixture or suspension of solid and
liquid to be filtered.
Filter cake: - Solids accumulated on the filter.
Filtrate: - clear liquid passing through the filter.

3. Pharmaceutical importance of filtration
0 Filtration is frequently the method of choice for sterilization of
solution that are chemically or physically unstable under heating
condition.
0 Sterile filtration of bulk drug solution prior to an aseptic
crystallization process eliminates the possibility of organisms being
occluded within crystals.
0 Solid /liquid filtration-
- It improves the appearance of solution.
- It removes potential irritant, e.g. Eye drop.
- Certain operations, such as the extraction of
vegetable drugs, may yield a turbid product with a small
quantity of fine suspended colloidal matter,
this can be removed by filtration.
- Detection of microorganisms present in liquid. This
method can also be used to assess the efficiency of
preservatives.

4. 0Solid gas filtration is used in the removal of suspended
solid material from air in order to supply air of the
required standard.
0Solid gas filtration can remove microbes from air of
areas where sterile products are being manufactured.

5. Factors Affecting Filtration
Filtration is affected by the characteristics of the slurry,
including:
1- The properties of the liquid , such as density, viscosity, and
corrosiveness.
2- The properties of the solid, for example, particle shape,
particle size, particle size distribution, and the rigidity or
compressibility of the solid.
3- The proportion of solids in the slurry.
4- Whether the objective is to collect the solid, the liquid ,or
both.
5- Whether the solids have to be washed free from the liquid or a
solute.

6. Rate of Filtration:
0 All other things being equal, the objective of the operation is to filter the slurry as
quickly as possible.
0 The factors affecting rate of filtration are related by Darcy’s law and may be
expressed as:
0 dV / dt = KA P /µ l
0 where,
0 V= volume of filtrate, t = time of filtration , K = constant for the filter
medium and filter cake , A = area of filter medium , P = pressure drop across the
filter medium and filter cake , µ = viscosity of the filtrate , and l = thickness of cake.
Factors affecting rate of filtration
1- Permeability coefficient: The constant (K) represents the resistance of both the
filter medium and the filter cake. As the thickness of the cake increase, the rate of
filtration will decrease. Also the surface area of the particles .the porosity of the cake,
and rigidity or compressibility of the particles could affect the permeability of the
cake.
2- Area of filter medium: The total volume of filtrate flowing from the filter will be
proportional to the area of the filter. The area can be increased by using larger filters.
In the rotary filter, the filter cake is removed continuously, giving, in effect, an infinite
area for filtration.

7. 3- Pressure drop; The rate of filtration is proportional to the
pressure difference across both the filter medium and filter cake.
The pressure drop can be achieved in a number of ways:
0 Gravity: A pressure difference could be obtained by maintaining
a head of slurry above the filter medium. The pressure developed
will depend on the density of the slurry.
0 Vacuum: The pressure below the filter medium may be reduced
below atmospheric pressure by connecting the filtrate receiver to
a vacuum pump and creating a pressure difference across the
filter.
0 Pressure: The simplest method being to pump the slurry into
the filter under pressure.
0 Centrifugal force: The gravitational force could be replaced by
centrifugal force in particle separation,

8. 4- Viscosity of filtrate:
0 It would be expected that an increase in the viscosity of the filtrate will
increase the resistance of flow , so that the rate of filtration is inversely
proportional to the viscosity of the fluid.
0 This problem can be overcome by two methods:
0 a- The rate of filtration may be increased by raising the temperature of the
liquid, which lowers its viscosity. However, it is not practicable if
thermolabile materials are involved or if the filtrate is volatile.
0 b- Dilution to increase flowability.
5- Thickness of filter cake;
0 The rate of flow of the filtrate through the filter cake is inversely
proportional to thickness of the cake. Preliminary decantation may be useful
to decrease the amount of the solids.

9. Mechanisms of Filtration
The mechanisms whereby particles are retained by the filter are of significance only in
the early stages of liquid filtration, as a rule. Once a preliminary layer of particles has
been deposited, the filtration is affected by the filter cake, the filter medium serving
only as a support.
1. STRAINING - The simplest filtration procedure, in which, like sieving, the pores are
smaller than the particles so that the latter are retained on the filter medium.
2. IMPINGEMENT – As a flowing fluid approaches an object such as a cylinder, the
flow pattern is displaced and the solids will strike the cylinder and be collected on
its surface.
3. ENTANGLEMENT – If the filter medium consists of a cloth with nap or a porous
felt, then particles become entangled in the mass of fibres. Usually the particles are
smaller than the pores so that it is possible that impingement is involved.
4. ATTRACTIVE FORCES – In certain circumstances, particles may collect on a filter
medium as a result of attractive forces. The ultimate in this method is the
electrostatic precipitator, where large potential differences are used to remove
particles from air streams.

10. Filter Media
0 The surface upon which solids are deposited in a filter is called the
“Filter medium”
0 Properties of ideal filter medium:
0 1- It must be capable of delivering a clear filtrate at a suitable
production rate.
0 2- It must withstand the mechanical stresses without rupturing or being
compressed.
0 3- No chemical or physical interactions with the components of the
filtrate should occur.
0 4- It must retain the solids without plugging at the start of filtration.
0 5- Sterile filtration imposes a special requirement since the pore size
must not exceed the dimension of bacteria or spores.

11. Classification of filter media (material)
0 1- Woven filters: these include a- wire screening . b- fabrics of cotton,
wool, nylon.
Wire screening e.g. stainless steel is durable, resistance to plugging and
easily cleaned.
Cotton is a common filter , although, Nylon is superior for
pharmaceutical use, since it is unaffected by mold, fungus or bacteria and
has negligible absorption properties.
0 2- Non- woven filters: Filter paper is a common filter medium since it offers
controlled porosity, limited absorption characteristic, and low cost.
0 3- Membrane filters: These are basic tools for micro-filtration, useful in the
preparation of sterile solutions. These filters are made by casting of various
esters of cellulose, or from nylon, Teflon, polyvinyl chloride. The filter is a thin
membrane with millions of pores per square centimeter of filter surface.
0 4-Porous plates: These include perforated metal or rubber plates, natural
porous materials such as stone, porcelain or ceramics, and sintered glass.

12. Filter Aid
0 Usually, the resistance to flow due to the filter medium itself is very
low, but will increase as a layer of solids build up , blocking the pores
of the medium and forming a solid cake.
0 The objective of the filter aid is to prevent the medium from getting
blocked and to form an open, porous cake, therefore reducing the
resistance to flow of the filtrate. The particles must be inert, insoluble,
incompressible, and irregular shaped.
0 Filter aids may be used in either or both two ways:
0 1-Pre- coating technique: by forming a pre-coat over the filter medium
by filtering a suspension of the filter aid .
0 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.
0 The following filter aids may be used:
0 Diatomite ( Kieselguhr ) , obtained from natural siliceous deposites.
0 Perlite , it is aluminium silicate.
0 Cellulose and Asbestos.

13. Classification of Filters (machinery)
Filters may be organized into three classes on the
basis of external force namely---
1.Gravity filters.
2.Vacuum filters.
3.Centrifuge filters.
4.Pressure filters.
1. Gravity filters:
Filters that rely solely on gravity and generate low
operating pressures are called gravity filters.
For example— Sand filters, Cellulose fibers, Bag filters,
Porous ceramics, Tray and frame filters etc.
Advantages: Disadvantages:
-- Easy to use. -- Low filtration rate.
-- Simple and cheap. -- Use on large scale is limited.

14. 2. Vacuum filters:
Vacuum filters are filters which are associated with a vacuum
system to filtrate a large amount of slurry.
For example – Rotary vacuum filters, Drum filters etc.
Advantages: Disadvantages:
- Automatic and continuous operation. - A complex operation.
- Very low labour costs. - Equipments are costly.
- Larger capacity. - Washing and drying are
- Cake thickness can be controlled. not efficient.
3.Centrifuge filters:
The filters on which centrifugal force is applied during filtration
process is known as centrifuge filters.
For example: - Perforated basket centrifuge filter.

15. 4. Pressure filters:
Pressure filters are filters that feed the product to the
filter at a pressure greater than that which would arise from
gravity alone.
For example: Meta filters, Plate and frame filter press, Disc
filters, Pressure Leaf filters, Pre-coat pressure filters.
Advantages:
1. It possesses considerable strength and high pressure
can be used with no danger of bursting.
2. It is very economical.
3. There is scope to provide excellent resistance to
corrosion and avoid contamination of the product, e.g.
Metafilters.
4. It is possible to remove very fine particles and to
sterilize a liquid using this filter.

16. Filters can be classified also based on the operation of the
filtration:
1. Filters used in Continuous filtration where discharge
of filtered solids and filtrate are separated in a
uninterrupted way.
2. Filters used in discontinuous filtration where
discharge of filtered solids is intermittent but filtrate is
removed continuously. The operation should be
stopped to collect the solids.

17. Classification based on the nature of filtration:
1. Cake filters: - Remove large amounts of solids.
2. Clarifying filters: - Remove small amounts of solids.
3. Cross-flow filters:- Feed of suspension flows under
pressure at a fairly high velocity
across the filter medium. The
feed flow travels
tangentially across the surface of
the filter, rather than into the filter.

18. Selection of filtration equipment
There are a number of product related factors that should be
considered when selecting a filter for a particular process.
These include:
◘ The chemical nature of the product:
Interactions with the filter medium may lead to leaching
of the filter components, degradation or swelling of the filter
medium or adsorption of components of the filtered product
on the filter. All of these may influence the efficiency of the
filtration process or the quality of the filtered product.
◘ The volume to be filtrated and the filtration rate required:
These dictate the size and type of equipment and the
amount of time needed for the filtration process.

19. ◘The operating pressure needed:
This is important in governing the filtration rate and
influences whether a vacuum filter is appropriate. High
operating pressures require that the equipment be of
sufficient strength and that appropriate safe operating
procedures be adopted.
◘ The amount of material to be removed:
This will influence the choice of filter, as a large 'load'
may require a filter where the cake can be continuous
removed.

20. ◘ The degree of filtration required:
This will dictate the pore size of membrane filters or
the filter grade to be used. If sterilizing is required
then the equipment should itself be capable of being
sterilized, and must ensure that contamination does
not occur after the product has passed the filter .
◘The product viscosity and filtration temperature:
A high product viscosity may require elevated
pressures to be used. The incoming formulation can
be heated, or steam-heated jackets be fitted to the
equipment. Care should be taken to ensure the
equipment seals etc. can operate at elevated
temperatures.

21. Industrial filter
Filters used in the industry to provide a large area of
filtration are taken as industrial filter.
These includes: 1. Filter press
2. Rotary filter
3. Filter leaf
4. Edge/Meta filter
5. Cartridge filters

22. 1. Filter Press
There are two basic form of filter press:
◘ Plate and frame press
◘ Washing Plate and frame press
According to wide application in pharmaceutical practice
only the plate and frame press method is used

23. Plate and frame press
This press is made up of two types of units-
a) plates
b) frames
Filter cloth is used as filter medium between the two units.
The frame opens with an inlet. The plate has an outlet for the
filtrate.
THE OPERATION
The slurry enters the frame from the feed channel. The filtrate
passes through the filter medium onto the surface of the
plate. The solids form a filter cake in the frame. The filtrate
then drains down the surface of the plate and escapes from
the outlet.
Filtration is continued until the frame is filled with filter cake,
when the process is stopped, the frame is emptied and the
cycle re-started.

24. Plate and frame press
contd.
Channels for the slurry inlet and the filtrate outlet can be arranged
by fitting eyes to the plates and frames. This has the advantages
that the filtrate from each plate can be seen and, in the event of a
broken cloth, the faulty plate can be isolated and the filtration
continued with one plate less.
The thickness of the cake can be varied by using frames of
different thickness and, in general, there will be an optimum
thickness of filter cake for any slurry, depending on the solids
content of the slurry and the resistance of the filter cake.
As filtration proceeds, the resistance of the cake increases and the
filtration rate will decrease. At a certain point it will be preferable
in terms of the overall output of the process, to stop and empty
the press rather than to continue filtration at a very low flow rate.

25. Plate and Frame Press

28. 0For the control of viscosity, the plates may
incorporate heating or cooling units.
0Plate and frames may be made of various-
a) Metals ( resistance to corrosion)
b) Non metals ( e.g. reinforced plastics)
0Certain varieties of wood are also used.
0Plate and frame presses may be of considerable
size, with 10 to 100 filtering surfaces.

29. 2. Rotary filter
0Rotary filter is an important industrial filter
0It may be up to 2m in diameter, 3.5m in length with a
filtration area of around 20m2
0 The most common form of it, as is used in
pharmaceutical industry, is rotary drum filter.
Advantage of rotary filter
0It is automatic and continuous in operation, therefore
the labour cost is very slow.
0It has a large capacity (20m2).
0Variation of the speed of rotation enables the cake
thickness to be controlled.

30. Disadvantages of rotary filter
1.The rotary filter is a complex piece of filter equipment, with
many moving parts. In addition to the filter itself, ancillary
equipments such as vacuum pumps, vacuum receivers and
traps, slurry pumps and agitators are required.
2. It is very expensive.
3. The cake tends to crack due to the air drawn through by the
vacuum system, so that washing and drying are not efficient.
4. Being a vacuum filter, the pressure difference is limited to 1
bar and hot filtrates may boil.
5. The rotary filter is suitable only for straight forward slurries. It
is less satisfactory, when the solids form an impermeable cake
or will not separate cleanly from the cloth.

31. Operation of rotary filter
0At the beginning of the operation, the cylinder rotates
slowly in the slurry which is kept agitated. Vacuum is
applied and it draws filtrate into the septa. Then filter cakes
are deposited on the filter cloth.
0The slurry sticks to the surface of the drum from the pick-
up zone and its liquid content drained by vacuum in the
drainage zone.
0This is followed by the washing zone (with water).
0Retaining the vacuum connection, the cake is isolated from
water, which is drained in the drying zone to produce partial
dryness.
0Finally, the cake is removed. Scraper is aided by
compressed air forced into the septa to loosen the cake
deposit.

32. Fig: Rotary filter

33. Application of rotary filter
0Most suitable for continuous operation on large
quantities of slurry, especially if the slurry contains
considerable amounts of solids, that is, in the range of 15
to 30 per cent
Pharmaceutical application:
0Collection of calcium carbonate, magnesium carbonate
and starch.
0Separation of the mycelium from the fermentation liquor
in the manufacturing of antibiotics.

34. 3. Filter leaf
Filter leaf is the simplest form of filter.
THE OPERATION
In use, the filter leaf is immersed in the slurry and a
receiver and vacuum system connected to the filtrate
outlet. The method has the advantage that slurry can be
filtered from any vessel and the cake can be washed simply
by immersing the filter in a vessel of water. Removal of
cake is facilitated by the use of reverse air flow.
The leaf filter is most satisfactory if the solids content of
the slurry is not too high, 5 % being a suitable maximum.

36. In industry an alternative
method is to enclose the
filter leaf in a special
vessel into which the
slurry is pumped under
pressure. A number of
leaves are connected to a
common outlet, to provide
a large area for filtration
e.g. Sweetland
filter.

37. Sand Filter
A. Slow Sand/Bio-Sand Filter
The slow sand filter is the oldest type of large-scale filter. In the
slow sand filter, water passes first through about 36 inches of
sand, then through a layer of gravel, before entering the under
drain.
The sand removes particles from the water through adsorption
and straining.
A slow sand filter contains biological activity and is therefore often
referred to as a bio-sand filter. As micro-organisms such as bacteria,
viruses and parasites travel through the sand, they collide with and adsorb
onto sand particles. The organisms and particles collect in the greatest
density in the top layers of the sand, gradually forming a biological zone.
The biological zone is not really a distinct and cohesive layer, but rather a
dense population that gradually develops within the top layer of the sand.
The population of micro-organisms is part of an active food chain that
consumes pathogens (disease-causing organisms) as they are trapped in
and on the sand surface.

38. B. Rapid Sand Filter
The rapid sand filter differs from the slow sand filter
in a variety of ways, the most important of which are
the much greater filtration rate and the ability to
clean automatically using backwashing.
The mechanism of particle removal also
differs in the two types of filters - rapid sand filters
do not use biological filtration and depend primarily
on adsorption and some straining.

40. Clarification of Filters
Clarification is the process by which finely divided solids and
colloidal materials are separated from liquids without the use of
filters.
Purpose:
●The process is employed to remove suspended oil from
aqueous solution. Example: Aromatic water
● To remove the undesirable solids that interfere with the
transparency of such natural products. Example: Honey or fruit
juice extraction
Clarifying agent:
The clarification process generally carried out by adding a
clarifying agents such as paper, pulp, talc.
Albumin and gelatin are obtained from natural source and
polyamine substances, synthetic in nature are also used as
clarifying agents.

41. THANK YOU