Methods of filtration - advantages & disadvantages

 

 

Liquid Filtration – Blood

 

1.                   Depth Filtration

a)                   Process that traps contaminants both within the matrix and on the surface of the filter media

b)                  Mechanism is one of impaction and subsequent adsorption of particles on to the filter media

c)                   Do not have a defined pore size

d)                  Efficiency of the filter increases as the flow path becomes longer or narrower; efficiency is maximised if the blood passes through a greater bulk of material - providing a larger number of tortuous pathways through which the blood must past and a larger wettable surface for particle adsorption

e)                   Composed of random mats of eg packed fibres (Dacron wool) or porous plastic foam

f)                    Rely on the density & the thickness of the mats to trap particles, and generally retain large quantities of contaminants within the matrices

g)                  Advantages

i)                    Lower cost

ii)                   High throughputs

iii)                 High particulate-holding capacity

iv)                 Protects final filters

v)                  Removes variety of particle sizes

vi)                 High flow rates

h)                  Disadvantages

i)                    Nominal rating

a)                   Ability of the filter medium to retain the majority of the particulate at and greater than the rated size. Operating pressure & concentration of the contaminant have a significant effect on the retention efficiency of the filter

ii)                   Media migration potential

a)                   Shifting of the filter medium under stress

iii)                 Particulate unloading at high differential pressures

2.                   Sieve Filtration

a)                   Process that traps contaminants larger than the pore size on the top surface of the membrane

b)                  Operate by direct interception  (physical retention)

c)                   Contaminants that are smaller than the rated pore size may pass through the membrane but may be captured within the membrane by adsorption

d)                  Generally polymeric (woven polymer threads) filters approximately 120 microns thick with a narrow pore size distribution

e)                   To increase capacity, the surface area for the blood to pass through is enlarged - done by pleating screen filter, along with screen supports to gain a large surface area in a minimal volume

f)                    Advantages

i)                    Absolute rating

a)                   retention of challenge particle

ii)                   No channelling or bypass

iii)                 Can be bacterial & particle retentive

iv)                 Very short contact time between blood & foreign surface

v)                  Low extractables

vi)                 Sterilisability

vii)               Integrity testable

g)                  Disadvantages

i)                    Lower flow rates than depth media

ii)                   Lower particulate holding capacity

 

 

Liquid Filtration - Clear fluids

 

1.                   Membrane filtration

a)                   Clear fluids (crystalloids & colloids) do not contain cellular elements, therefore filtration can be taken to a much lower level

b)                  Includes removal of submicron particles & microbes

c)                   Formed rigid structures, appear like paper

d)                  Have a relative depth for microbes

i)                    Eg: 0.2 micron filter thickness is approximately 250 x diameter of bacteria

e)                   Mechanism of direct interception  & adsorption

f)                    Increased resistance to flow

i)                    Resistance to flow is improved by:

a)                   Increasing the density of pores

b)                  Increasing the surface area (pleating)

g)                  Used in prebypass & cardioplegia filtration

 

 

Gas Filtration

 

1.                   Three physical methods of filtration

a)                   Direct interception

i)                    Removal of larger particles such as dust

ii)                   Not so effective for smaller particles such as bacteria

b)                  Inertial impaction

i)                    Smaller particles (eg bacteria) are able to pass within the media but have sufficient inertial mass to impact upon the filter fibres when the gas flow moves around the internal structure of the filter

ii)                   Similar to impaction/adsorption filtration in depth filters

c)                   Diffusional interception

i)                    Very small particles (eg viruses) are subjected to Brownian motion - under a directional flow the particles an oscillating path offering a much greater apparent diameter to the medium

ii)                   On entry to the filter medium they will be intercepted upon the surface & retained by Van der Waal’s forces

2.                   Pore sizes not usually given as effective filtration predominantly occurs via the mechanisms of inertial impaction & diffusional interception

3.                   Gas filtration filters tend to be hydrophobic

a)                   If water wets a filter medium that is ‘wettable’

i)                    A liquid channel will form preventing normal gas filtration mechanisms occurring thereby allowing passage of contaminants

ii)                   Sharp rise in resistance to gas flow

b)                  Hydrophobic material used:

i)                    Keeps liquid on one side of the filter

ii)                   Resistance to flow occurs only when a large percentage of the filter surface area is occluded

iii)                 Filtration efficiency is not affected even when wetted