A guide to sterile filtration solutions

When working in a sensitive industry, like pharmaceuticals, and food and beverage, its important conditions are as clean as possible. Therefore, companies making products for such sectors can benefit from sterile filtration. In this article, we'll explain more about how this works, along with filter pore sizes and types. We'll also briefly cover how integrity testing relates. Read on to get started.

When it comes to the disinfection or sterilization of liquids there are two common ways. These are heat treatment or pasteurization and sterile filtration. Sterile filtration removes unwanted microorganisms and makes or keeps the fluid sterile. Compared to pasteurization sterile filtration requires less upfront costs and doesn’t have any impact on sensitive components as flavors or dyes. The latter can be affected by heat treatment or pasteurization.

This is because sterile filtration removes contaminants in liquids with porous membrane filters. The pores in the filter's membranes prevent any contaminant larger than the size of the pore from passing through. The smaller the pore, the more effective the filter is at completely blocking contaminants - like unwanted organisms.

To better illustrate what's pointed out above, consider that bacteria in liquids usually range from 0.2 μm to 0.5μm. To sterile filter such a liquid a “sterile grade 0.22µm” membrane filter is used. Means, the membrane of this filter element consists of millions of pores with diameters between 0,2µm and 0.4µm and a typical maximum of around 0.28µm.

When using such a sponge-like membrane, nearly 100% of all bacteria are blocked. It’s worth to mention, that the name “sterile grade 0.22µm” is not referring to the pore size but to the microorganism retained. And according to the “FDA Guideline on Sterile Drug products produced by Aseptic Processing” from 2004 a sterile grade filter element is supposed to retain 10Exp7 microorganisms of the type of Brev. Diminuta (ATCC 19146) per cm² of the filtration surface. And Brev. Diminuta is 0.22µm or larger in diameter.

The membranes used for such sterile-grade filter elements are commonly made of Polyethersulfone (PES), Polyvinylidene fluoride (PVDF), or Nylon. They have a thickness between 100µm and 150µm and are usually covered on both sides by a support layer made of Polypropylene (PP) or Polyester (PET).

Whenever it comes to the sterile filtration of liquids it is recommended to consider some critical aspects:

·         Chemical Resistance

Before you decide for a specific brand or product, make sure, that the materials used in your filter element are compatible with the fluid you want to filter. Additionally, make sure, the materials are also compatible with the solvents you use for cleaning purposes.

·         Integrity Tests

As with any process filtration filter, sterilizing filters need to be integrity tested. Such an integrity test is recommended to be run before a sterile filtration process starts. By doing this you make sure, your filter elements are integer and reveal no damages or leakages. When you run e.g., a Bubble Point Test and the result is equal to the values provided in the documentation you can be sure that the retention of microorganisms corresponds to the stated efficiency.

·         Validation

For the sterile filtration of liquids, it is recommended to validate the filter elements for the reproduceable removal of specific microorganisms important for the application. Means, the filter element supplier provides information on the retention efficiency towards some standard microorganisms (e.g., Saccharomyces cerevisiae). But to be on the safe side in your sterile filtration process, it is recommended to test the filter element for the reliable removal of microorganisms critical for your product.

·         Sterilization

Whenever you plan to use your filtration equipment more than once sterilization of the filter elements becomes mandatory. By applying steam at 121°C on your filter elements for 30 minutes you make sure that all retained microorganisms have been killed and the filter element might be used again for the next sterile filtration. You may run a steam sterilization inside an autoclave which is a very gentle method and preserves the cartridges. But you may also go for an inline steam sterilization where the cartridges stay inside the filter housing and the housing is steam sterilized as well.

Aside from the aspects mentioned above you might also consider some other relevant criteria:

·         Clogging

Premature clogging of filter elements during a sterile filtration is always a vexation resulting in unwanted machine downtime and a costly cleaning process. A proper pre-filtration and a subsequent SDI test might help to estimate the service lifetime of a membrane in a sterile filtration process. It might also be a smart idea to monitor the dp of your final filters and to estimate the remaining service lifetime / volume via some calculation program.

·         Regeneration

Crucial for the service time of a filter element is a proper cleaning on a regular basis. The selection of suitable cleaning agents depends on the type of contamination and the type of filter element. Usually in the product documentation you may find a broad list of chemicals and the resistance of your filter element towards them. A correct way how to properly clean filter elements will be published shortly.

·         Optimized Service Lifetime

It might sound implausible but, in many cases, a premature exchange of filter elements will save you a lot of money (running costs) and massively lower your CO₂ footprint. It’s just an interaction between energy costs, costs for new filter elements and labor costs. We experienced an average cost reduction of 35% and CO₂ reduction around 67% for each filter installation optimized. Please contact us to learn more.

We hope the information contained in this article provides a good overview about sterile filtration. If you'd like more information about any topic covered, our team is happy to point you in the right direction. We're also available to discuss the full range of products we offer for process filtration. Get in touch with us today.