Optimizing Throughput in Sterile Filtration

Venting and flux rate calculation. How to improve your sterile filtration throughput and optimize your process.

July 22, 2021

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Over the last few weeks, Pall’s Filtration Nation blogs have introduced a series of Sterile Filtration Scientific Briefs to address some of the important but often-overlooked fundamentals of sterile filtration, and to explore some of the critical success factors that can make or break your filtration protocol. To recap, in Part 1, prefiltration was examined, Part 2 addressed sterile filtration and integrity, and Part 3 focused on differential pressure monitoring . Part 4, below, will delve into improving sterile filtration throughput.


Throughput, or filtration rate, is a key parameter that we always want to optimize for in sterile filtration. Filters by their very nature retard the flow rate of the fluid passing across them. They are almost always the bottleneck step in any process, the step that determines the overall rate of the entire process. Hence, we can improve the speed of the overall process by paying attention to the throughput of the sterile filtration step and increasing it as much as we can without risking damage to the membrane.


The trick to optimizing sterile filtration throughput is to understand a pair of often overlooked and in general, poorly understood parameters: flux and venting. The Scientific Brief, linked at the end of this blog, provides a concise guide to understanding flux and venting, detailing how to use them to make your sterile filtration process run as fast as possible. What follows is a quick introduction to both.


“Clearing the air” about filtration venting


Simply put, filtration venting is a technique for removing unwanted gas bubbles from the filtration apparatus. Left in place, these bubbles tend to collect on the upstream side of the filter, partially occluding it and effectively reducing the surface area of the filter with obvious negative effects on the rate at which the process fluid can pass through. There are a number of different methods for venting a filter, and the appropriate one to use will depend on the exact filter you are using. Whether or not it has a venting valve (or valves) and whether or not the fluid being filtered is hazardous to human health.


Understanding and using the flux rate

Flux rate in filtration is a measure of the rate at which fluid will permeate a membrane. Flux rate associates the flow rate of a given filter, typically stated in liters or millilitres per hour, with the surface area of the filter in question, typically stated in square centimetres or meters. Hence flux is often stated in units of liters per hour per square meter. Knowing the maximum flux rate for the specific filter you are using will allow you to tune your sterile filtration process, ensuring your flow rate (pump speed) is set close to, but not higher than the flux rate of the filter. So-doing will maximize the flow rate through the filter whilst ensuring the filter membrane is not damaged by a too high flow rate. 


Taken together, these two parameters, flux rate and filtration venting give you everything you need in order to maximize the rate of the sterile filtration process, and to do so in the confidence that you are still operating within the specifications of your specific filter.


The full Scientific Brief on sterile filtration throughput goes into much more depth and details how to make your sterile filtration process run as fast as possible. You can find this brief on our Scientific Resource Hub , along with a host of other concise, highly focused briefs dealing with all the major issues, concerns, and challenges of sterile filtration.


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