Nanofiltration

Within water treatment, there are four filtration ranges. These are microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. It is the rejection rate that sets the four filtration ranges apart. The rejection rate defines the size of the particles that a membrane can reject. On one end of the scale, there is microfiltration, which separates larger particles. On the other end of the scale, there is reverse osmosis, which separates the smallest particles. NF separates low total dissolved solids such as inorganic salts and small organic molecules. Microfiltration and ultrafiltration are often used as a pretreatment before NF. These processes will remove larger objects, which will protect the more sensitive nanofiltration membranes. This will also lower the risk of fouling issues and the need for chemical cleaning.

NF is a pressure-driven membrane filtration technology. No chemicals are added to the filtration process. Thus, it is a clean and safe technology, enabling a greener future. The separation occurs as pressure forces liquid through the membrane. A feed stream enters the membrane, and a feed pump triggers pressure. This will separate the feed stream into two new streams, which are the permeate and the retentate. The permeate is filtered liquid, which can pass through the porous semi-permeable membrane barrier. The retentate is concentrated feedwater, consisting of the rejected particles held back by the membrane.

Nanofiltration membranes are typically made of organic materials such as polysulfone, cellulose acetate, polyimide, or polycarbonate. The advantage of organic membranes is the low capital expenditure. Yet, they possess a low degree of thermal, chemical, and mechanical strength. Thus, organic membranes cannot withstand the filtration of aggressive fluids.

It is the nanofilter membrane’s pore size that determines the size of the rejected particles. Nanofiltration membranes have a pore size in the range of 0.001-0.01 µm. The denser the pore size structure is, the smaller particles can be retained. Yet, a dense pore size structure also brings a few disadvantages. As nanofiltration membranes have a dense membrane pore size structure, they require relatively high pressure to force the liquid purification. Thus, NF generally has a higher energy consumption than microfiltration and ultrafiltration.

Moreover, the membranes are more sensitive to fouling issues. To lower the pressure, protect the membranes, and reduce the fouling rate, larger objects can be separated in a posttreatment process with microfiltration or ultrafiltration.

NF can be used within several industries, including food and beverage, dairy, wineries, alcohol distilleries, bakeries, cosmetics, and pharmaceuticals. NF separates most organic molecules, almost all viruses, some salts, and most organic matter. NF also separates divalent ions, making water hard. Therefore, NF is often employed to soften water and groundwater. Yet, NF is the filtration method, which is utilized the least. The NF pore sizes are limited to a range of few nanometers. Anything larger is performed by ultrafiltration, and anything smaller is performed by reverse osmosis.

NF is the filtration range before reverse osmosis. Although the technologies are highly alike, they differ in the membrane pore size structure. NF has a pore size in the range of 0.001-0.01, and RO has a pore size in the range of 0.0001-0.001 μm. As nanofiltration membranes have a larger pore size structure than RO membranes, they generally require a lower pressure, meaning lower energy. A larger pore size structure will typically also result in lower fouling issues. Yet, while NF only separates a range of salts, RO separates all salts. RO removes all organic molecules, viruses, monovalent ions, and minerals. RO is therefore also used to desalinate water to deliver drinking water. Still, NF can be used as a pretreatment before RO to lower pressure, fouling issues and protect the membranes.