Filtration sterilization involves passing liquids or gases through filters with pores too small for microorganisms to pass through, removing them without exposure to heat. Common filter materials include asbestos, diatomaceous earth, porcelain, and membranes. The most used filter has 0.22 micrometer pores, retaining bacteria but not necessarily viruses. Filtration is useful for heat-sensitive substances like antibiotics and intravenous solutions.

1. Filtration Sterilization: Types, Mechanism and Uses
Filtration is the preferred method of sterilizing heat sensitive
liquid and gases without exposure to denaturing heat. Rather
than destroying contaminating microorganisms, it simply
removes them. It is the method of choice for sterilizing
antibiotic solutions, toxic chemicals, radioisotopes, vaccines,
and carbohydrates, which are all heat-sensitive.
The liquid or gas is passed through a Alter, a device with pores
too small for the passage of microorganisms, but large enough
to allow the passage of the liquid or gas. These Alters are made
of different materials;

2. •
Materials Name of the Alter
Asbestos pad Seitz Alter
•
Diatomaceous earth
•
Procelain
•
Sintered glass disks Cellulose
•
Borosilicate glass Aber
•
Clay, mud
•
Berkefeld Alter
•
Chamberland-Pasteur Alter
•
Sintered glass Alter Membrane Alter
•
HEPA Alter
•
Candle Alter

3. 1cm
Human cell
Mitochondria
("0,6 to 0,8urn)
Bacteri
(''0.5 to 5um)
ON A
(~nm)
Red blood cels
(-Sum)
Bacteriophages
(~20 to 200nm) (~100 to 800nm)
Viruses
Atoms
Atoms
(~l-5 Angstrom)
1mm
100um
10nm
10urn
100nm
10nm
1nm
0.lnm

4. Relative size of human cells, bacteria and
virus (Image source: Patrice D Cani)
The selection of Alters for sterilization must account for the size range
of the contaminants to be excluded. The most commonly used Alter is
composed of nitrocellulose and has a pore size of 0.22^m. The size of
the bacteria ranges from 0.3 to 0.5 ^m whereas the size of the viruses
ranges from 20 nm to 0.36 ^m. Thus a Alter of 0.22^m retains all
bacteria and spores but not all viruses.
Solutions of intravenous use are made pyrogen-free using filtration. Heat
sterilization of such solutions may kill the organisms but heat-resistant
endotoxins (lipopolysaccharide of the gram-negative bacteria) may still
remain and cause fever.

5. Contents
0.1 Working Mechanism of Filtration
Sterilization 0.2 Depth Filters 0.2.1 HEPA filter 1
Membrane filters
1.0. 1 Advantages of Filtration Sterilization
1.0. 2 Limitations of Filtration Sterilization
Working Mechanism of Filtration Sterilization
Filters work by physically trapping particles larger than the pore size
and by retaining somewhat smaller particles via electrostatic
attraction of the particles to the Alters. Besides porosity, other
factors also influence the efficiency of filtration, they are:
■ electric charge of the filter
■ electric charge carried by the organisms
■ nature of the fluid being filtered

6. Filtration of liquids is accomplished either by pulling the solution
through a cellulose acetate or cellulose nitrate membrane with a
vacuum (i.e, by applying negative pressure in the filter paper) or by
forcing the solution through filter paper by imposing positive pressure
above the fluid.
Filtration of air is accomplished using high-efficiency particulate air
(HEPA) Alters designed to remove organisms larger than 0.3 ^m from
isolation rooms, operating rooms, and biological safety cabinets.
Depth Filters
A depth Alter is a fibrous sheet or mat made from a random array of
overlapping paper or borosilicate (glass) fibers. The depth Alter traps
particles in the network of fibers in the structure.

7. Uses
1. Filter sterilization of air in industrial processes
2. Forced air heating and cooling systems used in houses contains
simple depth filter to trap dust, spores, and allergens
3. Use in biosafety applications such as biosafety cabinets.
Biological safety cabinets contain a filter known as high-efficiency
particulate air (HEPA) filter, which is a type of a depth filter.

8. HEPA filter
A typical HEPA Alter is a single sheet of borosilicate glass fiber that
has been treated with a water-repellent binder. The Alter, pleated to
increase the overall surface area, is mounted inside a rigid, supportive
frame. HEPA Alters come in various shapes and sizes, from several
square centimeters for vacuum cleaners to several square meters for
biological containment hoods and room air systems.
Control of airborne particulate materials with HEPA Alters allows the
construction of "clean rooms" and isolation rooms for quarantine, as
well as specialized diagnostic/research laboratories. HEPA filters
typically remove 0.3 pm test particles with an efficiency of at
least 99.97% including most microorganisms, from the airstream.

9. Continuous sheet of filter medium Filter frame
Filter sheet of randomly arranged fibres
Interception
Impact
Diffusion
Aluminium
separator
HEPA Filter Image source: Ladyofhats

10. Membrane filters
Membrane Alters are the most common type of Alters used for liquid
sterilization in the microbiology laboratory. Membrane Alters are
composed of high tensile strength polymers such as cellulose acetate,
cellulose nitrate, or polysulfone. Membrane Alters are prepared as
circular membranes of about 150pm thickness and contain millions of
microscopic pores of uniform diameters; the size of which is adjusted
based on requirements, during the polymerization process.
Porosities of membrane Alters range from 0.1 pm to 10pm and the most
commonly used membrane Alter has the pore size of 0.22pm and 0.45pm.
The membranes are held in special holders and often preceded by depth
Alters made of glass Abers to remove larger particles that might clog the
membrane Alter. The solution is pulled or forced through the Alter and is
collected in previously sterilized containers.

11. Uses of membrane filter
■ Sterilization of Auid materials (pharmaceuticals, ophthalmic solutions,
antibiotics, and other heat-sensitive solutions in laboratories and
industries
■ Identification and enumeration of microorganisms
Advantages of Filtration Sterilization
■ Less capital intensive
■ Suitable for heat-sensitive liquids (infusions, vaccines, hormones, etc).
■ Large volume of liquids can be filtered reasonably fast

12. Limitations of Filtration Sterilization
■ Only liquids and gases can be sterilized by this process
■ Filters are expensive to replace, especially nano-filters
■ Inherent limitations of materials used in filters affect the efficacy
of this process i.e, breakage of glass filters, rupture of the membrane
filter and absorption of the filtrate by Sietz filter
■ Clogging may occur