Randy Wilkins Millipore

FUNDAMENTALS OF STERILE FILTRATION

Selection

and Implementation

Millipore Corporation

Randy Wilkins

Technical Consulting Manager

Agenda

Selection criteria and techniques

• Vmax test

– Principles

– Practical application

– Benefits and limitations

Filter system sterilization

• Considerations for successful SIP

Integrity testing

• Current regulatory perspectives

• Practical solutions

Four Step Sterile Filter Selection

1. Materials Compatibility– Chemical compatibility

– Sterilization compatibility

– Adsorption considerations

2. Retention

– “Sterilizing-grade” – Confirmed with specific process fluid and

operating parameters

• Generally 0.2 or 0.1 um rated

3. Flow Rate

– Can be estimated from vendor flow/dP data

4. Capacity

– Understanding how flow/dP changes during processing

Cake formation Complete pore Gradual pore

plugging plugging

Capacity – Plugging Mechanisms

Factors Controlling Plugging

Concentration

Particle

to Pore

Size Ratio

HighLowLow

High

1

The filtration mechanism depends on:

– Particle to Pore Size Ratio

– Particle Concentration

Gradual Pore Plugging

Complete

Pore

Blocking

Cake Filtration

Monolayer

Adsorption

Complete

Pore

Blocking

Caking

Filtration

Gradual

Pore

Plugging

Plugging Models

<<1 <1 ~1 >1

Particle/Pore size Ratio

• For particle & pore size distributions– One mechanism may dominate hydraulics-apply simple model

– Mechanism may change over filtration-apply simple model to section of data

– Mixed mechanism hydraulics-apply more complex plugging models

• For identical particles & uniform pores– Different models depending on where particles are deposited

Plugging Model Behavior

• Resistance or permeability

changes

– Adsorption - no change

– Gradual - delayed change

– Complete - slight delay, then

rapid change

– Cake - steady change

Plugging Models

1.02.03.04.05.06.07.08.09.0

10.0

0 200 400 600 800 1000

L/m2

R/R

o Adsorption

Gradual

Complete

Cake

Plugging Models

Cake Formation

– Particles accumulate on filter

surface

– Hard, non deformable particles

Complete Pore Blocking

– Particles completely block pore

– Soft deformable particles

Gradual Pore Blocking

– Predominant in biologics

– Particles block a portion of a pore

– Both particle types

Mathematical Models

Cake Formation

– t/V = CV + D

Complete Pore Blocking

– V=E(1 - e-kt)

Gradual Pore Blocking

– t/V = At + B

A,B,C,D,E & k are constants

t = time, V = cumulative volume

Filter Plugging Models - Vmax

Mechanism of filter plugging:

d2t/dV2 = k(dt/dV)n

where:

t = filtration time

V= cumulative volume at time t

k = constant whose dimensions are dependent on n

H.P. Grace, "Structure and Performance of Filter Media,"

AICHE Journal 2(3), 307-336 (1956)

In practical terms:t/v = t/Vmax + 1/Qiwhere: Vmax = maximum volume that can be filtered at time infinity

Qi = instantaneous initial flow

Vmax: test procedure

Test Operation:– Select test filter type for

application

– Select test system & scaledown device

– Wet/vent test filter with buffer or product

– Gently add representative feed solution to reservoir

– Filter at constant pressure

– Record cumulative filtrate weight or volume vs time

– Stop after either set time, set L/m2 throughput, or set %flow decay

Air SupplyScaledown

filter

BalanceFeed Reservoir

Collection

Reservoir

Vmax analysis

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 10 20 30 40 50 60

minutes T

min

/mL T

/V

CGW1 media 13.8 cm2

SHF buffer 13.8 cm2

Vmax: data analysis

• Plot data as t/V vs t

– Gradual pore plugging model gives t/V=t/AVmax+1/AQi

• Use linear least squares to generate a best fit line

• If poor fit (r2 < 0.99) or negative slope

– Eliminate suspect points (initial wetting or buffer flush out transients, scale jumps,…)

– Re-run, run longer (higher % flow decay)

– Try fitting another plugging model

• If good fit (r2 > 0.99), i.e. CGW1

– t/V=0.000209t+0.00979 min/ml

– Extract model parameters:

• Vmax= 1/slope/area= 0.000209 ml-1/13.8 cm2

= 3470 L/m2

• Note Vmax -> as slope->0

• Qi= 1/intercept/area=1/0.00979 ml/min/13.8 cm2 = 4440 LMH

Application of Vmax

• Ideal for rapid screening of membrane and membrane combinations

• Useful for initial system sizing estimates

– Three process scenarios or cases are usually relevant:

• Case 1: Batch Volume to be filtered

• Case 2: Batch Volume to be filtered at a maximum allowable process

time

• Case 3: Batch Volume to be filtered in a process time with a specified

minimum allowable flow rate

– Largest surface area that fulfills all process requirements is selected

– Compare area, Amin, to available filter configurations & select smallest

configuration that meets/exceeds Amin this is Aconfiguration

– Suggested minimum safety factor ~ 20%

Vmax: Sizing Equations

)/( max

min

test

B

AV

VA

Case 1. Only VB (Batch Volume is given; No batch time, minimum flow)

Btesti

B

test

B

tAQ

V

AV

VA

)/()/( max

min

Case 2. VB (Batch Volume) and tB (Batch time) are given

Case 3. VB (Batch Volume), tB (Batch time) & Qmin (minimum flow rate) are given

minmaxmin

min

)/()/(1

AAV

V

AAQ

Q

test

B

testi

• Eq. gives the minimum area required; No safety factor is included

• Ensure that Amin leads to respectable batch times

• Using Trial & Error, Solve Eq. For ‘Amin’

• Select the largest ‘Amin’ from 1., 2., & 3.

CAUTION!!: DO NOT use with Non-

Plugging Streams (Vmax >1000).Assume a

batch time and use equation under 2 below

Vmax Magnitudes

Vmax Range

• >1,000 l/m2

• 200 l/m2 to 1,000 l/m2

• <200 l/m2

• Low Plugging

– Primarily flux based sizing

– Buffers, simple media

• Moderate Plugging

– Vmax critical range

– Protein solutions, some

media

• High Plugging

– Cell harvest

– Serum, hydrolysates

Vmax - Summary

• Ideal for rapid screening of multiple membrane candidates

– Minimum of time and feed required

• Ideal for determining effectiveness of prefilter candidates

• Good tool for initial filter system sizing

– Attention to dP, filter area, and feedstream consistency helps

minimize scaling error

• Does not tell you anything about filtrate quality

– Indirectly Vmax, with a tighter filter, on the filtrate is a measure

of filtrate quality

• Only applies to gradual pore plugging model

Pmax & Tmax: test procedure

• Application:– High permeability filters (depth,

open prefilters)

– Filter plugging or filtrate quality limits performance

• Test Operation:– Select test filter type for

application

– Select test system & scaledown device

– Wet/vent test filter with buffer or product

– Gently add representative feed solution to reservoir

– Filter at constant flow

– Record volume, pressure drop & filtrate quality (e.g. NTU, 0.2 um Vmax) vs time

– Stop after test end-point (time, L/m2 throughput, psid pressure drop, filtrate quality)

Pump

Scaledown filter

BalanceFeed Reservoir

Collection

Reservoir

Quality measure

Pressure

Centrate Clarification

0.027m2, 110 LMH

0

5

10

15

20

25

30

0 50 100 150 200 250

minutes time

psid

0

5

10

15

20

25

30

35

40

45

50

NT

U

A1HC psid

A1HC NTU

Scaling up

Rapid

scaledown

testing

• Batch testing

• Vmax, Pmax, Tmax

• Filter screening

• Extrapolate process &

rough sizing

• to time, throughput

• to variable

flows/pressures

• to series operation

• Sensitivity and

variability testing

• Include scaling &

safety factors

Simulation

scaledown

testing

Simulation

pilot testing

Engineering

runs at scale

• Full filtration process

• Time, throughput

• Variable

flows/pressures

• Single/series

• Monitor ∆P for each

filter, flow rate,

throughput (turbidity

for depth filters)

• Validate batch

testing

• Update process &

sizing

• Flow & capacity

• Pilot test: area,

feed volume

• Full Filtration process

• Prepare to replace

filters that plug

• Attention to all steps

(flush, SIP, integrity)

• Monitor ∆P for each

filter, flow rate,

throughput (turbidity for

depth filters)

• Validate scaledown

• Updated process &

sizing

• Flow & capacity

• Area

Vmax References

System Design Considerations

Sterilization and Integrity Testing

Sterilization Options

Autoclave Steam-in-place Gamma

Post sterilization

assembly

Likely None Can vary from

multiple steps to

none

Operation Automated Can vary from

fully manual to

fully automated

Vendor

Validation Extensive Extensive Vendor

Filter damage Extremely

remote

Design/operation

dependent

Materials

dependent

Options for Process Operation

Sequence

Set up &

Install Cartridges

Flush/WetTest Integrity

Air Blow Down

P < 5 psi

SIP

ProcessFlush/Wet

Test Integrity

Operation Sequence Options

• SIP > Wet > IT

– Advantages

• Identify filters damaged during SIP before operation

– Constraints

• Filter must wet easily after SIP

• Wet > IT > SIP

– Advantages

• Perform IT under non-sterile conditions

– Constraints

• Must be able to SIP wet filter

Successful SIP Considerations

• Steam pressure and differential pressure must be

controlled to assure sterility and prevent damage to filter

– Filters must be resistant to hydraulic and thermal stress

– Integrity failures equal:

• Lost time – 2 to 4 hours for re-installation and re-SIP

• Potential contamination resolution – varies widely from

thousands to potentially millions of dollars

• Process system SIP is a time-consuming operation

– Being able to SIP multiple system components together

saves time

• Minimizes potential for sterility breaches

• Save labor time

SIP Options

Filter + tank - separate sterilization

•Sterilize filter and tank

separately or consecutively

•Sterile boundary

•Long

•More equipment

•Complex

P1

T1

Transfer

line

V1

V2

V3

P3

P4

T3

Air/N2

P2

Vent

filter

Steam

Steam

Multi-Round Experiments –

Forward or Reverse SIP

Dry Forward SIP

Express SHC CHGE73TS3 Multi-Round 3 x 30"

Filter Wetting, Integrity Test, Blow Down, and

"Dry" Forward SIP at 121 oC, 20 minute Exposure Time

0

20

40

60

80

100

120

140

0 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300 315

Time (Min)

Tem

pe

ratu

re (

oC

)

0

10

20

30

40

50

60

70

80

90

100

Diffe

ren

tia

l P

ress

ure

(p

sid

)

TC- A 01 TC- A 02 TC- A 03 TC- A 04 TC- A 05 TC- A 06 TC- A 07 TC- A 08

TC- A 09 TC- A 10 Delta P

Wetting

Integrity

Test

Blow Down SIPSpore

InstallationExposure

Wet Forward SIP

Express SHR with Prefilter CHVE73TS3 Multi-Round 3 x 30"

Filter Wetting, Integrity Test, and "Wet" Forward SIP at 121 oC, 20 minute Exposure Time

0

20

40

60

80

100

120

140

0 15 30 45 60 75 90 105 120 135 150

Time (Min)

Tem

pe

ratu

re (

oC

)

0

10

20

30

40

50

60

70

80

90

100

Diffe

ren

tia

l P

ress

ure

(p

sid

)



Integrity

Test

Wetting SIPSpore

Installation

Exposure

SIP OptionsFilter + tank - Reverse steaming

P1

T1

P3

P2

Vent

filter

Liquid

filter•Easy, maximum simplicity

•Reduced number of drain & valves

•Clean steam required

Requires a robust

filter

Dry Reverse SIP


Filter Wetting, Integrity Test, Blow Down,

and "Dry" Reverse SIP at 121 oC, 20 minute Exposure Time

0

20

40

60

80

100

120

140

0 15 30 45 60 75 90 105 120 135 150 165 180

Time (Min)

Tem

pe

ratu

re (

oC

)

0

10

20

30

40

50

60

70

80

90

100

Diffe

ren

tia

l P

ress

ure

(p

sid

)



Wetting Integrity Test Blow Down Spore

Installation

SIP Exposure

Wet Reverse SIP


Filter Wetting, Integrity Test, and "Wet" Reverse SIP at 121 oC, 20 minute Exposure Time

0

20

40

60

80

100

120

140

0 15 30 45 60 75 90 105 120 135 150 165 180

Time (Min)

Tem

pe

ratu

re (

oC

)

0

10

20

30

40

50

60

70

80

90

100

Diffe

ren

tia

l P

ress

ure

(p

sid

)



Wetting Integrity

Test

Spore

InstallationSIP

Exposure

SIP Cycle Summary

Forward, Dry

– Best option for minimum filter stress

Reverse, Dry

– Best option for simultaneous SIP of filters and tanks

Forward, Wet

– OK if steam introduction is carefully controlled

Reverse, Wet

– Not recommended

Post Sterilization/Pre-use

Integrity Testing

• Regulatory references

– FDA Aseptic Process Guidelines, 2004

– EMEA Annex 1, 2008

• Practical Challenge

• Sterile Barrier Options

– Process equipment

– Catch Can

– Millidisk Barrier Filter

• Redundant filter system testing

• Cost/Benefit Considerations

Regulatory Guidance - FDA

• FDA

– 2004 Aseptic Processing

Guidelines



Regulatory Guidance - EMEA

• EMEA

– Annex 1

– 2008



Regulatory Guidance Summary

• Documents & Guidance provide general, not practical guidance

– Vendors and PDA Technical Report 26, ‘Sterilizing Filtration of Liquids” are good sources of practical guidance

• EU / EMEA says less but appears far more skeptical than FDA

• White Paper – for each company on qualification and validation of aseptic processes is very prudent.

Practical Challenges

Remove

– Condensate

– Wetting Liquid

– Test Gas

Maintain

downstream

– Sterility

– Atmospheric

pressure (test)

Class BClass C

Filling Line

SterilisingProductFilter

A

Steam WaterGasCondensate

Current Practice

• Fluids are sent to downstream equipment

– Vented process tank or filling manifold

– Good option when product wet filter

testing is used

• Catch can with a sterile vent filter

– Not easy to handle

– Separate autoclaving

– No filter blow-down

– One vent filter to integrity test

– Limited wetting volume

to filling

An Alternate Solution

• Millidisk Barrier Filter technology facilitates

– Integrity testing of product filters

• in-situ

• post sterilization

• before use

– Sterile equipment

• Draining (residual condensate)

• Cooling

• Drying

• Venting (maintain atmospheric pressure on the sterile side…)

…Without breaching sterility

Millidisk Barrier Filter Design

One disk pair

3 Hydrophilic membranes

1 Hydrophobic membrane

Gas

Liquid

Condensate

Using the Millidisk Barrier Filter

After SIP, cool downwith compressed airEliminate condensate, steam & air through barrier filter

1

Open

Closed

SIP

Steam

Once cool, wet the sterilizing-grade filter and direct the effluent to drain through the Barrier Filter.

2 Wetting

Water

1 bar max

Integrity test of liquid filter

3

Vent test gasthrough Barrier Filter

Testing

Atmospheric Pressure

Using the Millidisk Barrier Filter

Dry the liquid filter prior to processing. Vent gas through the Barrier Filter.

After integrity of Barrier Filter is confirmed, start processing.

Product

Integrity test Barrier Filter off-line,IPA 70/30 bubble point test.

5 a4 Blowdown

5 b

Process

Compressed air

Redundant Sterile Filtration

P3

P2

P1

T

MilliBarrier

Aervent

Post Sterilization/Pre-use IT Options

Advantages Disadvantages

Process

Tanks

• No additional equipment • Product IT specs needed

• Entire system requires cleaning and re-prep in

case of filter failure

• Lost product

• No post-SIP extractables flush

Catch-Can • Use water test specs

• Extractables flush

• Flush volume limited

• Additional equipment to assemble and sterilize

• Likely no allowance for re-test

• Vent filter must be tested

Barrier Filter • Water test specs

• Extractables flush

• No limit to flush volume

facilitates re-test if necessary

• Easy Filter blow-down

• Additional equipment to assemble and sterilize

• Additional filter to test

• Limited flow rate (limit is 30-inch filters)

Many Examples

Barrier Filter technology currently in

use in a wide range of processes

Post SIP/Pre-use Integrity Testing

Cost/Benefit Considerations

Costs –

• Additional Capital Costs

– Hardware

– Hardware validation

• Processing costs

– Product to drain

– More filters to buy, prep and test

– More time for filter testing

– More cost associated with false IT failure

• Added Risks

– Sterility compromised by additional

manipulations around the sterilized

system

• Frequency unknown although

expected to be low in a well

designed and operated system

Benefits –

• Simple EU regulation compliance

• Avoid processing with SIP damaged filter

– Value of lost product

– Cost of re-work (if allowed)

• GCC analysis suggests SIP related

filter damage occurs about 1/15000

batches

–For all processes worldwide

–Actual for site would be predicted from

site specific history

• Redundant filtration essentially

eliminates this risk assuming one

filter is validated for sterilization

Conclusion

• Designing successful sterilizing filtration systems requires

– Knowledge of filter/fluid interactions

• Retention and plugging mechanisms

• Sizing techniques and applicability

– Sterilization methods

• Selection based on materials and facility

• Understand limitations for successful SIP implementation

– Integrity testing

• Understand risk/benefits of various options

– Including regulatory requirements

• Vendor experience is critical for efficient implementation

Acknowledgements

• Herb Lutz

• Kerry Roche-Lentine

• Richard Morin

• Maurice Phelan