Nanofiltration (NF) is a membrane separation process. NF membranes pass water and reject ionic and non-ionic species based on both size and charge. Nanofiltration is not simply “loose RO”, and does more than “pass more ions than an RO” (as the phrase implies). Nanofiltration membranes are tuned to perform the specific separation the customer needs. Each NF manufacturer chooses specific ranges of species rejection and water flux/pressure for their markets. SnowPure’s targets process separations with its specialty NF membranes.
NF membrane chemistry is an aliphatic-aromatic polyamide thin-film composite formed on top of a porous substrate, typically a polysulfone ultrafiltration membrane. It can be flat sheet (for spiral assembly) or hollow fiber in form. The NF chemistry and thin film formation process varies to control the ionic separation. Ionic separation is based on the negative (-) charge of the membrane at normal pH. NF rejects 100% of divalent anions like SO4-2, and to maintain neutrality then holds back first divalent cations like Ca+2 then singly-charged monovalent cations like Na+.
Because of the NF rejection mechanisms, the separation performance depends highly on the composition of the feedwater to be filtered.
SnowPure NF models predict ionic separations in different feedwaters.
NF separates TOC (organics) based on molecular weight; a typical cutoff is 100-300 Daltons. Sugars are rejected, for example, while NF passes small alcohols and monovalent salts.
Many NF membranes are “2-layer”, or thin-film composite on top of UF. These generic NF membranes are normally used for reducing some ions from relatively pure water.
Specialty NF membranes are “3-layer”, where an intermediate layer is placed on top of the NF for 2 reasons. The TFC separating membrane is coated on top of the second smooth, hydrophilic layer.
- this special layer makes the surface hydrophilic (polysulfone is hydrophobic)
- this special layer makes the surface smooth (polysulfone UF is microscopically very rough)
Specialty 3-layer nanofiltration membrane technology is normally used for process separations where durability, fouling resistance, and unique separations are required.
- Purifying NaCl brine
- Desalting and concentrating used antifreeze
- Removing sulfate SO4 from produced water before reinjection
- Lithium brine purification
- Treating Copper and other mining waste for discharge to surface water
- … and hundreds of others
SnowPure’s ExcellNano line of NF membranes include:
- SXN2 for pure water separations (like Filmtec NF)
- SXN3 for process separations (like DK and DL, originally Desal-made membranes)