Filter Selection Guide

The scientific filtration market consists of a large variety of applications, materials, methodologies, restrictions, limitations, and products. If you are relatively new to these processes, the sheer number of options can be daunting, to say the least. However, when you break down the individual requirements of your specific application with a few basic guidelines, the options start to pare down quickly.

Let’s start with a few high-level segmentations to get the selection process moving.

1. Macro, Micro, Ultra, and Nano Filtration

Before you get started, you should have a good idea of what you are filtering and what you hope to achieve with the filtration process. With that base understanding, you can segment your process into either macro, micro, ultra, or nano filtration. These are just fancy ways to classify particle size, helping to narrow down the right filter type.

Each of these terms, while related to the size of particles, also has numerical values that help fine-tune the process. There is some overlap between these segments, but they serve as a good starting point.

Macrofiltration: The process of removing particles that are typically visible to the naked eye. These are generally larger than 10µm.
Microfiltration: The process of removing suspended solids, bacteria, and larger colloids. The typical pore size range is 0.1µm to 10µm.
Ultrafiltration: The process of separating macromolecules, viruses, and proteins from a solution. The pore size range is generally 0.001µm to 0.1µm.
Nanofiltration: The process of removing divalent ions, low-molecular-weight organic compounds, and smaller viruses. This is a pressure-driven membrane process that separates particles based on both size and charge. The pore size range is typically 0.001µm to 0.01µm.

Below is a chart illustrating the relative sizes of various particles to help you better visualize the filtration scale.

Particle Size Chart

Particle Size Range	Example Particles
0.001µm - 0.01µm:	Welding Soot, Combustion Fumes, Carbon Dots, Colloidal Gold, Diesel Exhaust
0.01µm - 0.1µm:	Parvovirus, Poliovirus, Tobacco Smoke, Ultrafine Oil Mist, Cerium Dioxide
0.1µm - 1.0µm:	Poxviruses, Bacteria, Atmospheric Dust, Lead Dust, Asbestos Fibers
1.0µm - 10.0µm:	Coal Dust, Mold Spores, Mist and Fog Droplets, Fine Wood Dust
> 10.0µm:	Red Blood Cells, Pollen, Fine Sand, Polystyrene Beads, Penicillium Spores

Filtrate, Retentate, Clarification, and Solids Recovery

After understanding what you are looking for and its relative size, you can begin to think about the filtration process itself. Do you want to remove your target particles from the solution, or do you want to remove everything but your target?

Clarification: If you want to remove unwanted pollutants or particles, you will use a clarification process. In this case, the desired product is the filtrate (or permeate)—the solution that passes through the membrane.

Solids Recovery: If your desired result is to recover the target particles from a solution, you will take an approach that includes solids recovery. In this process, you will pass your solution through a membrane, retaining your target particles on the membrane itself. The resulting collection of particles is your retentate (or filter cake), which is the desired product.

Chemical Compatibility

In many scientific, chemical, or pharmaceutical applications, it’s possible to encounter chemicals that may react with filtration materials. To prevent a hazardous reaction or the destruction of key samples, it’s important to understand that some filtration media offer exceptional chemical resistance, while others may decay or degrade if exposed to an incompatible solution.

With millions of possible chemical compounds and solutions, it would be impractical to provide a comprehensive list of compatible uses for each membrane type. It is highly recommended to have several resources at hand to verify compatibility before performing any filtration process that could compromise the health and safety of laboratory personnel or the efficacy of your samples.

It's best to cross-reference any chemical properties with trusted online resources, Material Safety Data Sheets (MSDS), and chemical compatibility charts. For example, the Material Compatibility Chart on our website provides a basic overview of which membrane materials are compatible with various acids, bases, ketones, alcohols, esters, organic oxides, and other substances.

Volume and Load Characteristics

Next, we’ll examine the total volume of material to be filtered and the particle load.

Volume: If your application requires the processing of a large volume of material, you would be better served by a filtration product that allows for a high flow rate and has a pleated filter membrane. The pleated membrane effectively increases the available surface area, allowing more material to pass through. These filters are typically used in large formats that include hollow filter tubes, capsule filters, and cartridge filters. For applications with lower volume requirements, you’ll find many different formats, such as filter funnels, membrane sheets, syringe filters, and centrifugal filters.

Load: The particle load of your solution is a key factor in filter selection. If your solution is heavily loaded, your application may require multi-stage filtration. Prefilters help to remove larger particles before the solution reaches your primary filter, which prevents premature fouling and extends the usable life of the final membrane. In some cases, such as with depth filters, prefiltration stages may be layered on top of the final membrane to provide an all-in-one solution for heavily loaded samples.

Filter Media Characteristics

This list will give some basic examples for use with a few of the most common filtration media.

Cellulose Acetate (CA): Known for its high flow rate and low protein binding, making it ideal for biological samples.
Polypropylene (PP): These are strong, chemically inert, and resistant to a wide range of chemicals. They are often used for general-purpose filtration.
Polytetrafluoroethylene (PTFE): A highly chemical-resistant and hydrophobic (water-repelling) material, making it perfect for filtering organic solvents and gases.
Polyvinylidene Fluoride (PVDF): These membranes are a versatile and widely used type of filter media. They are known for their high mechanical strength, good flow rates, and broad chemical compatibility. PVDF membranes are available in both hydrophobic and hydrophilic forms, which makes them suitable for a wide range of applications.
Nylon (NY): Naturally hydrophilic and strong, Nylon is used for a variety of applications, especially those involving alcohol and other organic solvents.
Glass Fiber (GF): Often used as a pre-filter because it can hold a large amount of particulate matter without clogging.

Selecting the right scientific filter requires a systematic approach that considers your application's specific needs. By first identifying the particle size range (macro, micro, ultra, or nano), you can narrow down the potential filter types. Next, determine if you are performing a clarification process to collect the liquid (filtrate) or a solids recovery to collect the particulate matter (retentate). Always verify chemical compatibility between your solution and the filter material to ensure safety and prevent damage. Finally, select the appropriate filter format and membrane material based on your required volume and particle load. By following these steps, you can confidently and efficiently choose the correct filter for your application.