Nanofiltration (NF) Membranes

In general, caustic solutions (for example 0.01 to 0.05% aqueous sodium hydroxide solutions) are used for organic foulants and acidic solutions (for example 0.1 to 0.2% aqueous HCL solutions) are used for inorganic foulants. It is important to perform the caustic cleaning first to remove organic foulants since some organics can be irreversibly linked to the membrane at low pH. Please refer to the membrane cleaning article at https://www.sterlitech.com/blog/post/membrane-chemical-cleaning for more information.

Yes, we can supply the flat sheet membranes in the correct disk size for your stirred cell. Please contact us at [email protected] for pricing and availability.

Q. How do I store used flat sheet crossflow membranes?

A. Used flat sheet membranes must always remain wet, including during storage.  Membranes that are allowed to dry will irreversibly lose water permeability.  To prevent microbial growth, the membranes can be stored in a 0.5% solution of formaldehyde.  Alternatively, the membranes can be stored in a 1.0% solution of sodium metabisulfite (SMBS).  To maintain efficacy, the SMBS solution should be replaced monthly.  Or the membranes can be stored in UPDI water that is replaced weekly.

Yes, we are able to offer some of the flat sheet membranes as roll stock. Please contact us at [email protected] to inquire about pricing and availability.

Flow characteristics for a spiral element and flat sheet membrane are significantly different and are really only comparable on a qualitative basis. Variability in the membrane manufacturing process, differences in water composition, test procedures, and test equipment, used in a factory, laboratory, or elsewhere, will impact water flux results.

We do not offer complimentary samples of the flat sheet crossflow membranes. However, most customers find the standard pack quantities to be quite reasonable for initial evaluations. For customers that want to evaluate several different membranes, we do offer custom variety packs. And, in some instances, we may be able to offer custom packs containing smaller sheet quantities. Please contact us to inquire about pricing and availability.

Prior to use, the membranes should be stored in a climate controlled environment, away from sunlight and heat, while sealed in the original packaging. We recommend using the membranes as soon as practical after receiving them. However, most of the flat sheet membranes may be stored for up to one year without affecting performance. The exceptions are the GE cellulose acetate membranes (CE and CK) and the Aquaporin FO membrane which should be used within 6 months.

Q. How do i store used forward osmosis (FO) flat sheet membranes?

Post use, the membrane should be rinsed with water after testing and stored in clean water if the storage duration will be less than 24 hours.  If the membrane is rinsed with water and expected to be stored for longer than 48 hours, it is recommend to use a 1% sodium metabisulfite (SMB, chemical formula Na2S2O5) to prevent bacterial growth from occurring. Periodically inspect the membranes for biological growth. If the preservative solution is cloudy you need to replace it.  Renew the preservative solution every month. 

Yes, you may attempt to reuse flat sheet membranes. However, you may find it difficult to achieve a leak free seal. The cell body o-rings necessarily compress the membrane during installation and the physical action of separating the membrane from the o-rings during removal may cause damage. This damage can impede that ability to achieve a leak free seal when the membrane is reused.

Unfortunately, allowing a previously wetted crossflow membrane to dry usually results in permanent reduction of water permeability. Nevertheless, you can attempt to re-wet the membrane and use it. First, soak the membrane in a 50% alcohol solution for at least 15min (ethanol, methanol, or IPA would be acceptable). Next, install the membrane in the test cell and operate the cell with purified deionized water as though you were pre-conditioning the membrane. After about 30min, the permeate flux should stabilize to whatever rate the membrane is now capable of.

Used flat sheet membranes must always remain wet, including during storage. Membranes that are allowed to dry will irreversibly loose water permeability. To prevent microbial growth, the membranes can be stored in a 0.5% solution of formaldehyde. Alternatively, the membranes can be stored in a 1.0% solution of sodium metabisulfite (SMBS). To maintain efficacy, the SMBS solution should be replaced monthly. Or the membranes can be stored in UPDI water that is replaced weekly.

Yes, for flat sheet membranes that will be used with water or aqueous solutions, we recommend that you pre-condition the membranes prior to conducting your separation experiments. Pre-conditioning helps ensure that the membranes perform as expected. Also, pre-conditioning removes preservatives and other residuals from the membranes. Purified deionized water is used to pre-condition the membranes. For all flat sheet membranes except the forward osmosis (FO) membranes, install the membrane in the test cell and apply the purified deionized water feed at the normal operating rate and pressure for the membrane. Allow the system to operate with the purified deionized water feed until the permeate flux stabilizes at an expected value. You can now replace the purified deionized water feed with your feed solution and begin your experiments.

Most membrane manufacturers recommend CFVs of 5 to 35cm/s for their spiral wound membrane elements. The flat sheet membranes can be operated in this range to simulate use in such elements. Additionally, the flat sheet membranes may be operated outside of this range by users that want to examine the relationship between CVF and fouling and membrane performance in their applications.

Cross flow velocity (CFV) is the linear velocity of the feed flow tangential to the membrane surface and is typically reported in m/sec or ft/sec. It is calculated by dividing the volumetric flow rate through the feed channel by the cross-sectional area of the feed channel. Please refer to the CFV article at https://www.sterlitech.com/blog/post/tech-tips-cross-flow-velocity-cfv/ for more information.

The recommended TMP will depend on the type of flat sheet membrane and on the application. Typically, a reasonable TMP will be the same as that noted for the permeate flux specification of the membrane. The specifications for any of the flat sheet membranes can be easily found on our website by clicking the “application/specification” tab on the product web page and scrolling down as necessary. Please contact us at [email protected] if you need additional assistance.

For a feed solution of 2,000 ppm sodium chloride (NaCl) at 70 psi (0.5 MPa) and 77°F (25°C), the NF90 membrane has sodium chloride rejection of between 90-96% while the NF270 membrane has approximately 50% rejection.