Page 12 - Filtration Resources

What is silt density index (SDI)?
Silt density index (SDI) estimates the quantity of suspended solids and colloids inside of water. SDI is measured following the ASTM D19.08 Standard Test Method for Silt Density Index (SDI) of Water, using a 0.45 micron membrane. SDI provides information about the fouling potential of water treatment equipment, including membrane filtration systems, and therefore is commonly used in their design and choice.

Is SDI a reliable fouling propensity parameter? How is it used?
SDI is a simple and helpful tool extensively used in pilot or large-scale treatment plants as a standard test to verify the fouling potential of RO and NF membranes. However, using SDI to estimate the fouling intensity of water treatment equipment has some limitations. Even though SDI is measured using a dead-end filtration unit, hydrodynamic conditions in dead-end filtration units are not representative of the hydrodynamic conditions of NF/RO membranes. Hydrodynamic conditions in membranes

When particles of a known and defined size are removed via filtration, it is fairly easy to choose the right filter by simply selecting a pore size smaller than the particle. But what happens when you need to filter a very dirty solution that contains an innumerable amount of particle sizes? If a pore size too small is chosen, a cake layer of debris will form on the membrane surface and foul it completely. If too big of a pore size is selected, then a large portion of the solids will pass right through the filter. Because it could be easily overdone, it’s usually not a good idea to stack multiple filters in the same filter holder and process it all at once. So what is an effective solution to this problem?

Enter the world of pressure drop! Back in 1856, French hydrogeologist Henry Darcy figured out the physics behind fluid flow in a filter medium while he was working in Dijon, France (yes, also the little town that gave us great mustard). Henry figured out that in order to push a liquid

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