Page 12 - Filtration Resources

In 2015, the US Food and Drug Administration (FDA) signed off on new legislation to finalize the Food Safety Modernization Act (FSMA), which requires food and beverage manufacturers to be more proactive to minimizing food-borne contamination from microorganisms. The FDA now places more effort on the individual companies to test food products according to the new guidelines in various parts of the production process. These processes can be ideal places to draw fluid samples for bacterial capture and subsequent analysis. The use of membrane filters to evaluate bioburden in these types of liquids is the standard means of recovering and quantifying potentially harmful microorganisms. Does this sound like it makes for more work for the food processors in our communities? It might, but Sterlitech Corporation can help!

According to the new regulations, high-risk food producers will be inspected more frequently, will have to maintain more detailed records, and establish clear food safety plans.

 Q: What is Cross Flow Velocity?  

A: Cross flow velocity (CFV) is the linear velocity of the flow tangential to the membrane surface and is reported in [m/sec] or [ft/sec]. CFV affects the hydrodynamic conditions in the cell, and as a result affects the fouling rate and formation of concentration polarization at membrane surface and is calculated by dividing the volumetric flow rate [lpm or gpm] in the flow channel by the cross sectional area [m² or ft²] of the flow channel.  

Q: How is CFV calculated in Sterlitech’s bench-scale test cells?  

A: Example: Calculate CFV in the CF042 cell  

• Flow channel cross sectional area: Channel depth x Channel width* = 0.23 x 3.92 cm
• Flow rate: 1 l/min = 1/60000 m³/s
• CVF = (1/60000 m³/s) / (0.0023 x 0.0392 m)= 0.18 m/s

*Contact us for more information about channel width in CF016 and Sepa cell

Q: How is CFV calculated when shims or feed spacers are inserted in the flow channel?  

A: Adding shims to the flow channel reduces the depth of

Water, oils, and solvents sometimes need to be filtered. But when you see filters labeled as hydrophilic, hydrophobic, and now oleophobic; what does all this mean?  Sterlitech currently offers membrane disc and sheet filters in 10 different polymer types and 4 inorganic filters. 2 of the polymer types are subdivided into 4 subgroups based on surface chemistry alone.

Filters listed as hydrophilic, which has its origins in the Greek language and means water (hydro) loving (philos), love to get wet! Hydrophilic filters will easily pass water or water-based solutions such as dairy, river water, seawater, cell culture solutions, buffers, beverages, and many more. The filters best suited to handle these solutions are silver, ceramic, glass fiber, cellulose acetate, mixed cellulose ester (nitrocellulose), nylon, polyester, polycarbonate (PVP-treated), polyacrylonitrile (PAN), and PVDF. They can be used for almost any application that needs particle removal, clarification, cell

Sterlitech’s polycarbonate and polyester track-etch membrane filters are a unique product that find use in numerous applications. They possess clean cylindrical pores that transverse the membrane surface from one side to the other. As a result of this unique feature, the pore density must be much lower compared to almost all other standard thin-film membranes such as Nylon, PTFE, PVDF, and more. Particulates either pass through the pores in track-etch filters, or remain on the surface. Almost never do they become embedded within the pore’s interior. But what happens when these filters are needed for your project and the particles being filtered occlude the opening of the pores? This effect is called pore blinding; and you may ask “But what can be done?”

Fear not brave scientist! In most applications, the track-etch filters will work fine on their own. But if a difficult particulate is clogging up your filter too soon, consider use of our Polyester Drain Discs. These discs act as a pre-filter

Filtering difficult biological solutions? Normally, using a thin membrane filter can pose serious questions:

• “How much liquid can I get through the filter?”
• “When will it clog?”
• “What if my solution is very critical to the project?"

Sterlitech Polyethersulfone (PES) membrane filters have a unique asymmetric pore structure that allows the user to take advantage of this feature; PES can be its own pre-filter and dynamically increase throughput!

Hold a sample piece of this material up to the light at an oblique angle, and you will see a glossy/shiny side, and a more matte/dull side. This sided-ness feature (see Figure 1) is helpful in that the more open area (matte side) has larger entry pores compared to the glossy side; and it should face into the incoming liquid stream for the best results with particulate-heavy liquids. If you were to view an SEM image of the matte side, you may quickly notice the pores seem much larger than the stated pore size, this is normal! Each pore’s ability to

Q: What is membrane pre-conditioning and why is it recommended to pre-condition new flat sheet membranes prior to use in the bench scale filtration cells?

A: Membrane pre-conditioning is a filtration step generally done at a pressure equal to or higher than the test pressure using deionized water as the feed. During this process membrane pores are wetted and the membrane structure may go though compaction or swelling. These changes in the membrane affect both the permeate flux and the rejection values. Therefore, it is recommended to pre-condition the membranes prior to use in the bench scale filtration cells to ensure the membrane performs according to the specs provided by the manufacturer, in terms of both permeate flux and rejection values.

Q: How do you know if membrane is pre-conditioned?

A: It is recommended to filter deionized water through the membrane until the permeate flux reaches a relatively steady value. At this point deionized water can be replaced with feed solution and

This document gives general guidelines for cleaning Nanostone RO or NF elements in the ST, ST+, ST++ and DT-Module-Configuration. During normal operation the membrane elements can become fouled by constituents in the feed water. The most common foulants include mineral scale, biological, organic, colloidal particles, and metal oxides. The nature of the fouling is dependent on the system design and feed water. Proactive cleaning is necessary to reduce the potential for irreversible fouling. If the membranes are heavily fouled, the cleaning effectiveness may be reduced and the performance may be permanently impacted. At a minimum, RO/NF elements should be cleaned when:

• Normalized permeate flow drops 10-15%
• Normalized permeate quality decreases by 10-15%
• Normalized pressure drop increases 10-15%.

Notes:

• Ensure that the all water used in the cleaning process is chlorine free
• Ensure the direction of the flow during cleaning is the same as during normal operation
• Consult with Nanostone prior

Over the years, we have received a number of questions about the optical clarity of our polycarbonate (PCTE) membrane filters.  Optically clear membranes are of importance when viewing plankton from the Antarctic under a microscope, manufacturing porous PDMS chambers, epifluorescent microscopy, and numerous other applications studies or even unmanned biology missions into space!   All of our polycarbonate membrane filters are made by the same process, track-etching, but the clarity of our filters varies based on both the use of the base film (amorphous or crystalline– we use both) as well as the pore size and density.  To shed some light on (or through) the matter of our filters' optical clarity we put together a handy infographic that you can check out at the bottom of this page. 

Amorphous polycarbonate has no defined molecular arrangement within the film, and as such tends to show greater clarity (in an otherwise untreated film) when compared to crystalline PCTE.  Crystalline

Defining a Pore Size and Sterile Filtering; 0.2 Micron vs. 0.22 Micron.  What’s the difference?

If you were to spend a little time perusing Sterlitech’s selection of membrane disc filters, one thing we’re very proud of might just jump out at you: we have a lot of pore sizes. So many that you might wonder if it’s a little excessive that we carry both 0.2 and 0.22 micron pore sizes. After all, both are used to sterilize fluid passed through them. Can the tiny difference of 0.02 microns really change a filter’s performance characteristics that much?

To answer that question, we must first take a look at one of the methods used to test a filter’s performance: the bubble point test1. Standard tests to verify a filter’s stated pore size usually entail a bubble point test. This test pushes air under pressure through a submerged membrane (either in water or alcohol) to the point where air bubbles first begin to come through the filter membrane2. The largest pore, or pores, in the membrane will