What do you need to know about Depth Filters and Membrane Filters?

Functionality of Depth Filter and Membrane Filter

• is fundamentally different
• accordingly, an application-specific use must always be evaluated

Depth Filter

In this article, we refer to depth filters whose main component is cellulose and not polypropylene or glass fibre.
They have a filter thickness of approx. 4mm as a single-layer version or 8mm in the double-layer version.
In the laboratory, they are initially tested with 47mm discs until they are finally used in production in 16" modules/capsule design or as filter sheets with up to 1200x1200mm.

The main advantage of this product:

• large inner surface area
• high dirt holding capacity
• if necessary, available loading capacity

The manufacturers of such depth filter media have the appropriate products in their portfolio for different pharmaceutical processes.

It is by no means easy to compare different manufacturers due to the undefined absolute retention. Tests with a downstream membrane provide information on the performance of the product. If this is not possible, filtration parameters should be used to compare different products.

We at vipur are happy to support you with our decades of expertise and filtration equipment in this area.

Membrane Filter

Membrane filters, on the other hand, are so-called surface filters with almost no dirt holding capacity, i.e. they block up quickly under load.

The main advantage of this product:

• regulatory retention definition in relation to defined germs
• can be tested for integrity using a filter test device

The scalability of membrane filters is very large for almost all suppliers and ranges from 25mm syringe filters to 30" filter elements/capsules.
The most common membrane materials are PES (polyethersulfone), PVDF (polyvinylidene fluoride) or PTFE (polytetrafluoroethylene).
It is very easy to compare different manufacturers due to the defined retention. The most common test method used here is the so-called Vmax test.

What Depth Filters and Membrane Filters teach us

Depth filters have a very high dirt holding capacity and should protect the downstream system as well as possible when used as a pre-filter.

Membrane filters are pure surface filters and can therefore only be used for the filtration of media that have already been treated accordingly. Their retention rate correlates with the bacterial load. They can be checked for integrity using a filter test device.

Would you like to know more? We will be happy to provide you with detailed information on the subject of "Depth Filter versus Membrane Filter" for download on our website.

See you next time - we look forward to further filtration.

Have a nice day,
the vipur Team

Hydrophilic versus Hydrophobic Membrane

You want to filter something with a membrane - but don't quite know which membrane you should use to achieve the desired results? In addition, each membrane has its own special properties, hence our tip for you:

The first important question is:

What do you need to filter - liquid or gas media? 

There are two main types of membrane:

• hydrophilic membranes → filtration of liquid substances

• hydrophobic membranes → filtration of gas substances

What is there to say about them in brief?

Hydrophilic membranes

Hydrophilic membranes are “more or less easily” permeable to water and, as already mentioned, are therefore used for liquid filtration.
Their structure usually differs depending on the material. The most common at present are: 

• Polyethersulfone membranes (PES)
• Polyvinylidene fluoride membranes (PVDF)

Depending on the pore size, they are used for bioburden reduction through to virus filtration.

Hydrophobic membranes

Hydrophobic membranes are generally impermeable to water. As a rule, because there are of course various exceptions here too. Normally, PTFE membrane filters are mainly used for sterile gas filtration.

• Polytetrafluoroethylene membranes (PTFE)
  
  They are characterized by high strength and high chemical compatibility. In practice, this means that PTFE membrane filters can be used for three-digit steam sterilization cycles with up to 145°C/30min in the direction of flow and against the direction of flow. By reducing the surface tension, these membranes become hydrophilic and can be used, for example, for the sterile filtration of aggressive liquids.

Here is an important note:

Please note that wetting of the hydrophobic membrane with liquid must be prevented.

What are the most common sources of error/possible problems?

Since many PTFE sterile air filters are used for fermenter supply and exhaust air, the formation of condensate and/or foaming of the fermentation medium is a given. 

Possible solutions:

• In the event of condensate formation (= wetting of the hydrophobic PTFE membrane with liquid), the use of a filter housing heater is a relatively simple solution. 
• If the fermenter frequently foams over, causing the hydrophobic PTFE membrane to block, a polypropylene pre-filter may be a solution. 

Another membrane type would be hydrophobic PVDF. However, this is only used peripherally due to the limited supply and, above all, the lower air flow rates.

Hydrophilic or hydrophobic membranes - the difference is as simple as that, but there are certain points to consider when using them.

We are happy to provide you with this information for download on our website.

See you next time - we look forward to further filtration.

Have a nice day,
the vipur Team