Nanofiltration (NF) Membranes: Everything You Should Know
Nanofiltration (NF) membranes are finding new applications in the membrane separations industry, even as a 35-year-old technology. Their unique ability to selectively remove ions has made separation processes more efficient, which can provide a significant benefit to some applications. Additionally, some plants even use NF as pretreatment to reverse osmosis (RO) to get higher recoveries when used as a softening step.
NF membranes can remove hardness, color, precipitated and dissolved metals, bacteria, pathogens, viruses and other contaminants from the feed water. NF membranes do an excellent job at removing divalent ions like calcium, magnesium and sulfate. This has opened a wide range of applications for NF membranes.
Moreover, they require lower operating pressure than RO, which makes them ideal for processes that don’t require rigorous separation. Although their divalent ions rejection is above 90%, their ability to remove dissolved salts is lower than that of RO.
This article explores the unique technical benefits behind nanofiltration membranes and their applications. We also discuss their limitations, comparison with traditional membrane separation technologies, and how we can help you optimize your separation processes. Read on to know more.
An Overview of Reverse Osmosis
To understand the benefits of nanofiltration, it is helpful to recap the technical capability of traditional reverse osmosis. Reverse osmosis membranes work on the principle of diffusion and offer the highest degree of separation. They can remove most dissolved organic molecules, dissolved and precipitated metals, colloidal particles, bacteria, pathogens, viruses, total dissolved solids (TDS), and salts with molecular weights greater than approximately 100 gm/mol. They require an operating pressure between 75-1200 psi (5-84 bar).
Polyamide reverse osmosis membranes, also known as thin film composite (TFC) membranes, can remove monovalent ions as well as divalent ions, whereas nanofiltration membranes only effectively remove divalent ions. NF membranes let most monovalent ions pass and they’re used to separate some gases in addition to liquids. Except for divalent ions, RO membranes can remove a wide variety of monovalent ions, from 95% to greater than 99% efficacy.
While polyamide reverse osmosis membranes can remove most ions, gases, and solvents can pass through them. Typical reverse osmosis membrane applications include desalination, ion exchange systems, food, potable water, and pharmaceutical industries to mention a few.
The Unique Advantages of Nanofiltration
Much like an RO membrane, NF membranes are also pressure-driven membranes and require a working pressure gradient to drive the permeate flux. Additionally, the operating pressure is around 50 to 225 psi (3.5-16 bar). They’re used to remove suspended solids, colloids, bacteria, viruses, pathogens, color, hardness, and total organic carbon (TOC). The main area of focus with NF membranes is separating organic matter in water treatment, food, and process industry. They can also reduce TDS content from the feed solution with 50-90% efficacy.
Unlike reverse osmosis, nanofiltration membranes offer selective ion transport by allowing monovalent ions like sodium and chloride to pass while retaining divalent ions like sulfate, calcium, and magnesium with >90% efficacy. They can reject divalent ions with molecular weights greater than 200-400 gm/mol. An ideal NF membrane will have maximum divalent ion rejection while maximizing flux. Their salt rejection is lower (50-90%) than RO membranes. NF membranes reject particles in the range of approximately 1 to 10 nanometers, hence the name. Their chemistry, extent of dissolved salt removal, and divalent ions rejection are what differentiate them from RO.
NF Membranes: Taking Advantage of Their Capability
The addition of different additives to the membrane layer, like piperazine, lets the manufacturers modify the selectivity of NF membranes by allowing or rejecting various divalent and monovalent ions and salts. Offering specific salt selectivity that can fit unique process requirements is the cutting-edge technology that makes them uniquely suited for various applications.
Additionally, NF membranes have low operating pressures and can be used in conjunction with RO. Although RO membranes have a thin-film composite that can remove almost all dissolved salts and organic compounds from water, it does so at a slightly higher energy cost. By using NF upstream of an RO, the feedwater can first be softened using the larger, more selective membrane layer. This will in turn allow for a higher overall system recovery that is achieved when the NF permeate is supplied as the feed to the RO.
NF Membranes: Other Applications
The NF process is widely used across various verticals of the process industry. Below you’ll find some commercial nanofiltration membrane applications.
Water and Wastewater Treatment
This is one of the main areas of focus regarding NF membranes. In the water treatment industry, NF membranes are used for water softening as they can effectively remove bicarbonate, magnesium, and calcium ions from hard water. Hard water is undesirable since these ions cause scaling in pipes and subsequent equipment.
Moreover, NF membranes are widely used in wastewater treatment to remove heavy metals, multivalent salts, dissolved organic carbon (DOC), sulfates, and nitrates. They can also reduce the total organic carbon by up to 95% from effluent streams.
Food and Beverage
NF membranes provide several uses in the food and beverage industry, from purifying dairy products and juices to concentrating syrups. They also decolonize and demineralize sugar solutions and colored brine separation.
Oil and Gas Industry
NF membranes have the ability to remove carbon dioxide from gases in the oil and gas industry. This is important because carbon dioxide converts into carbon monoxide at high temperatures and damages the catalyst. It’s also used in wastewater treatment of aqueous effluent solution containing significant amounts of oil and other toxic chemicals.
Pharmaceutical and Biotech
NF membranes sterilize and concentrate antibiotics and separate clotting compounds from blood and plasma.
Textile, Dyes, and Leather
The textile industry uses dyes of all kinds, and NF membranes play an essential role in dye desalting and concentration. NF membranes fabricated with polyether sulfone and polyethylene glycol are used for better dye rejection and membrane performance. In the leather industry, NF membranes reject tannins.
Additionally, NF is being tested for desalination pretreatment applications where a large part of the feed solution is treated before it reaches subsequent stages, like RO membranes. NF membranes offer high natural organic matter (NOM) removal from the seawater feed solution. Permeate solutions reduce the amount of pressure required in later stages and the rate of fouling in downstream equipment.
In some cases, nanoparticle-templated NF membranes may be incorporated into ultra-high-performance desalination in the future. In terms of the effect on salinity, NF membranes can purify brackish water into potable water. Moreover, these membranes can remove 50-70% of TDS and, in some cases, go as high as 85% but it depends on the feed solution and operating conditions.
Nanofiltration Troubleshooting and Cleaning
Once the design and commissioning stages have been completed, it’s critical to implement proper best practices with NF. Data normalization is done in a similar fashion to RO and is the first step to ensure the successful operation of an NF system. Data normalization takes into account changes in water quality such as pH, temperature, and feed conductivity. It’s the first line of defense when identifying problems before they become severe.
Data normalization sets a reference point for acceptable operating conditions. It can also detect sudden events like membrane scaling/fouling and systematic failures such as underfeeding antiscalants or seasonal water quality changes. Avista’s NormRO allows you understand your plant’s data trends based on three key parameters: normalized flow, rejection, and pressure drop. This enables you to proactively schedule cleanings, detect problems, and optimize the membrane operation.
Since NF membranes are uniquely suited for removing divalent ions such as calcium, magnesium, and sulfate, they can be susceptible to carbonate and sulfate scaling. Our proprietary software Avista™ Advisor™Ci is an excellent tool for determining the scaling potential of your NF system and selecting the right antiscalant and chemical dosing for your membrane system.
Changes in pressure, flow, or rejection can be indicators of fouling or scaling. When this occurs, it’s important to identify it early to take action. When fouling and/or scaling occur, performing a Clean in Place (CIP) may be necessary to get the system back up and running. CIP involves implementing various steps to ensure rigorous cleaning of the membrane. Since NF membranes generally require stricter limits over cleaning pH, choosing a cleaner compatible with the membrane is essential. Avista has a large selection of membrane– compatible cleaners for all foulants and scales with varying pHs.
Benefits and Limitations of NF Membranes
- Lower energy consumption than RO (lower operating pressure) and greater flux for divalent ions removal. This translates into lower environmental emissions and reduced operating costs than RO.
- Readily used to soften water by removing calcium, magnesium, and sulfate divalent ions, which allows RO membranes to yield higher recoveries.
- In addition to removing suspended solids and large colloids, NF membranes can effectively remove bacteria, proteins, pathogens, and viruses.
- They can remove dissolved organic and metals.
- They can remove organics with molecular weight above 200-400 gm/mol.
- They can’t completely remove TDS compared to RO.
- They require stricter control over cleaning pH than polyamide RO membranes.
- They don’t tolerate exposure to chlorine, and repeated chlorine exposure can lead to lower salt rejection.
Take a look at some of the most commonly asked questions about NF membranes.
What is nanofiltration in water treatment?
Nanofiltration is part of the membrane technology that separates dissolved contaminants, including multivalent ions and small molecules, from water. It can be used as pre-treatment to RO process, where it softens the water by removing calcium and magnesium divalent ions, so the RO can reach higher recovery. Moreover, it’s used as a standalone treatment to soften water where a lower salt rejection may be acceptable.
NF membranes have been applied to purify drinking water and to soften and remove contaminants from groundwater and brackish water.
What’s the difference between nanofiltration and traditional reverse osmosis?
It depends on the type of membrane and plant requirements. RO membranes require more pressure and are generally used where greater salt rejection is required. They can remove contaminants up to 0.1 nanometers or even smaller. This includes all inorganic and organic molecules and salts like sodium chloride.
The reverse osmosis process can require a remineralization stage where some minerals are added to the feed solution to make it safe for drinking and to prevent it from being corrosive. This may not be needed with NF membranes, but it is dependent on the application.
NF membranes can selectively remove larger multivalent ions such as calcium sulfate and calcium chloride while allowing monovalent ions like sodium chloride to pass through. They can reject molecules in the range of 1 nanometer, working with relatively lower operating pressure than RO.
Can nanofiltration remove bacteria?
Yes, NF membranes can remove bacteria, viruses, pathogens, and most organic compounds.