Reverse Osmosis and Nanofiltration Membrane Filtration Systems: Common Problems and How to Fix Them

Reverse osmosis (RO) and nanofiltration (NF) are membrane filtration technologies that are designed to remove solute ions and molecules from a pretreated liquid stream. Well-designed RO/NF units are compact and demand relatively little maintenance, making them an attractive alternative to conventional treatment trains.

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Even despite the merits of today’s efficient RO/NF technologies, it pays to understand common pitfalls associated with them. If your facility currently uses RO/NF or is considering purchasing an RO or NF system, the following article will help you to understand the most common problems impacting reverse osmosis and nanofiltration, and some possible solutions.

RO/NF membrane fouling

Fouling occurs when contaminants collect on the surface of a filtration membrane and restrict the flow of water through the membrane’s pores. With the smallest pore sizes of any membrane filtration technology, RO/NF are particularly prone to premature membrane fouling. Without adequate pretreatment and process monitoring steps in place, RO/NF membrane fouling can reduce unit service life, compromise permeate quality, and increase operational costs.

Preventative steps and/or remediation strategies for membrane fouling depend on the types of contaminants present in the process or waste stream. Common types of fouling include:

Particulate and colloidal fouling

Particulate fouling occurs when solid materials build up on a filtration membrane surface, forming a cake layer that blocks water from flowing through the membrane’s pores. In many cases increased pressure differential measurements provide early indication of particulate fouling in RO/NF membranes. Common particulate contaminants include bacteria, viruses, sediment, macromolecules, iron oxides, salts, and colloidal silica. In many cases, particulate fouling of RO/NF units can be prevented by applying appropriate upstream filtration, which can include media filtration, microfiltration (MF), and/or ultrafiltration (UF), depending on the sizes and geometric shapes of particles present.

For streams with colloidal particles, it is sometimes necessary to apply an inorganic coagulant to separate out suspended solids. Commonly-used coagulants include aluminum sulfate, aluminum chloride, sodium aluminate, and ferric chloride, and the use of each will vary depending upon the contaminants present in the feed stream. The use of coagulant chemicals must be monitored closely, however, as coagulants can also lead to RO/NF membrane fouling if allowed to proceed downstream, where they can react with antiscalants or other substances and collect on the membrane.

Biofouling

Biofouling is a process where microorganisms, plants, algae or other biological contaminants grow on RO/NF membrane elements, forming a layer known as biofilm. As biofilm accumulates on the membrane surface, greater pressure is needed to force water through, resulting in higher energy costs, and eventual damage to the RO/NF membrane element. Key symptoms of biofouling include increased differential pressure from feed to concentrate, and decreased membrane flux.

RO/NF membrane elements tend to provide the warm, low-flow environments suitable to biological growth, making them particularly susceptible to biofouling. Some common solutions for control and prevention of biofouling include:

• Biofiltration to remove nutrients from the feed stream;
• Chlorination to chemically destroy biological contaminants; and
• Fouling-resistant membranes that prevent microbials from clinging to the RO/NF element.

Scaling or precipitation fouling

Scaling or precipitation fouling occurs when membrane pores are blocked by crystallized salts, oxides, or hydroxides that have precipitated from solution. Scaling is among the most common forms of fouling in RO/NF elements, especially by divalent calcium (Ca2+) and Magnesium (Mg2+) ions. Like other forms of fouling, scaling and precipitation fouling can compromise the efficiency of an RO/NF unit, and, over time, can irreversibly damage membrane elements.

Control and prevention strategies for scale and precipitation fouling focus on inhibiting crystal growth, resulting in particles that are small enough to be carried away in the reject stream. Control methods include:

• Acid injection to control calcium carbonate scale;
• Water softening, or the addition of lime to feed water as a means of reducing hardness, alkalinity, and silica to prevent scale crystal formation; and
• Scale inhibition, or the injection of a specialized chemical substance into the feed stream to inhibit the growth of salt crystals.

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Membrane material compatibility

RO/NF membranes are fabricated from a variety of materials, including cellulose acetate, polyamide, and polysulfone, among others. In order to prevent premature failure, a number of important process factors—including industrial application, pH, substances present, temperature, feed pressure, and biological load—should be considered in the selection of a membrane material. An example of this is the use of polyamide membranes. While widely used in RO/NF units, polyamide membranes are easily damaged by chlorine, making them a poor option in applications where chlorine is needed for disinfection purposes. In order to prevent chemical attack and oxidation, all process characteristics must be carefully considered when selecting a membrane material.

Reject water discharge

While they are extremely effective water purification technologies, RO/NF produce large volumes of wastewater—frequently up to 20-50% of the volume of feed water that they process. Disposal of the concentrated waste streams produced by RO can be challenging, especially if your facility is subject to zero liquid discharge (ZLD) regulations, or if you face high costs for sewer or surface water discharge. A popular solution to mitigate disposal costs is the reduction of RO waste stream volume through evaporation.

Overall impact of proper design on preventing common problems

Many of the common problems impacting RO/NF can be avoided through careful design that takes process conditions into account. While we’ve already discussed how pretreatment is essential for minimizing operational and maintenance issues, care must also be given to other system design elements, including:

• Flux: Flux is the volume of water to pass through a membrane in a given amount of time, often expressed as the number of gallons of water per square foot of membrane per day (GSFD). Flux is used to determine the number of membrane elements needed for an application, and is affected by feed water quality, temperature, and salt concentration.
• Flow rate: Generally measured in gallons per minute (GPM), feed and permeate flow rates are critical measures for efficient RO/NF operation. Good RO/NF system design takes water source into account, for example, when processing surface waters with high colloids, an optimal flow rate may be 10 – 14 GPM/ft2 of membrane.
• Array: An array is the physical arrangement of pressure vessels in an RO/NF system. An array can entail multiple stages, with multiple pressure vessels in each stage. Generally speaking, the higher a recovery rate demanded by a facility, the greater the number of stages in its array.

A trusted engineer can help you to weigh these and other factors to achieve optimal RO/NF performance, maintenance, and energy costs, both immediately and in the long term.

How can SAMCO help?

SAMCO has over 40 years’ experience custom-designing and manufacturing RO/NF systems for a range of industries and solutions, so please feel free to reach out to us with your questions.

For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your RO/NF treatment system needs.

Head on over to our blog to learn more about industrial filtration and process separation technology. Some articles that might be of specific interest to you include:

• Does Your Facility Need a Reverse Osmosis or Nanofiltration System?
• How Do You Choose the Best Reverse Osmosis and Nanofiltration Membrane System for an Industrial Facility?
• What Are the Best Companies for Reverse Osmosis and Nanofiltration Membrane Systems?
• Reverse Osmosis vs Nanofiltration Membrane Process: What Is the Difference?
• What Are Reverse Osmosis and Nanofiltration and How Do They Work?

Water filtration and purification are two crucial steps to ensure that the water you’re drinking is safe for your body. If you’re considering a water treatment system, here is a comparison between nanofiltration and reverse osmosis which are two of the most popular and common techniques employed for purifying and filtering drinking water.

Comparing Nanofiltration And Reverse Osmosis Filtration Systems

The Process

First and foremost, you need to understand how each process works. This is what will distinguish the two and it will make it easier to understand what process does what.

Nanofiltration

So, as far as nanofiltration is concerned, it is a process of removing solute from solvent with the help of a membrane. This process is quite easy to explain. First of all, water, which is in large quantities, is known as a solvent and the impurities present in trace, but unacceptable quantities, are known as solute.

So, the principle of nanofiltration works based on removing the solute or impurities from the solvent or water with the help of a membrane. This is a very special membrane of a specific micron size.

Reverse Osmosis

Reverse osmosis, on the other hand, is the process where pressure is applied on the semi-permeable membrane and water moves from the region of high concentration (with more impurities) to a lower concentration (with no or fewer impurities).

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The membrane that allows the transfer keeps the impurities on the other side of everything so that you get almost 99.9% pure water at the end of it all. It is a very effective process for removing impurities from water.

Benefits

To gauge a process, the benefits or pros are what matter the most. They tell you whether a process is even worth it or not.

Nanofiltration

So, when it comes to nanofiltration, there are a lot of things that can make you invest in this water filtration system.

First of all, thanks to the very small pore size, nanofiltration allows everything to be removed from the water, as long as it’s bigger than the pore size of the nanofilter. Secondly, it can also remove bacteria and it also helps in desalination of water, which is something not a lot of filtration processes can do.

Reverse Osmosis

A reverse osmosis system MD is also capable of removing a lot of impurities, ranging from hazardous metals, compounds, PFAS, chlorine, and even arsenic. It also gives 99.9% purity and it’s something that not a lot of water filtration or purification plants can offer. The main advantage of RO is the low maintenance costs.

Even though the initial investment is quite hefty, it will pay you off in the future when you’re not wasting money on plastic bottles or other experimental water purification techniques, when you’ve got a one-stop shop for everything related to water and its purification.

Disadvantages

To be realistic and make a better decision in the end, you also want to know about the disadvantages of the processes. This makes for an all-rounded comparison and nothing is hidden from you.

Nanofiltration

One of the most painstaking things about nanofiltration is the maintenance. Since the pore size is super small, it can lead to contamination and fouling, for which cleaning the membrane and maintaining it is super important.

This can cause a lot of problems because you can end up tearing the membrane if you’re not too careful. Also, improper use of the system can lead to nanofiltration membrane fouling which can cause secondary pollution.

Reverse Osmosis

There are also a couple of things that just don’t sit right with people about RO systems. First is the price. Reverse osmosis plants and systems are very expensive and even though it is a sound investment, a lot of people just can’t justify the price. In line with high prices, RO systems can also lead to higher electricity bills which is never good.

On top of it all, the maintenance is also heavy. If you don’t maintain the RO system, the water recovery rate will drop significantly and that is not ideal.

Pore Size

Even though both reverse osmosis and nanofiltration employ the use of a membrane to purify and treat water, the pore size is where everything is different.

Nanofiltration

Nanofiltration systems have a membrane with a pore size of anywhere between 1-10 nanometers, hence the name. This membrane is very fine and it lets all of the impurities stay back.

Reverse Osmosis

For reverse osmosis, the membrane is more like a semi-permeable membrane, almost as large as 0. micron. It allows larger molecules of water to pass through it but it will stop all of the impurities.

Operating Pressure

Besides the membrane, the pressure at which the water is pushed towards the membrane for purification is also important to consider. This is what sets apart different processes and the pressure can ultimately make or break the purification process of water.

Nanofiltration

Nanofiltration has much less operating pressure. The value lies anywhere between 0.5 to 2.5 MPa and it is the force exerted in the water when it comes in contact with the membrane. This allows the water to be pushed out successfully over to the other side, but the impurities stay behind.

Reverse Osmosis

The operating pressure in reverse osmosis is slightly higher than the former, and that is because reverse osmosis has a higher degree of filtration and purification. The pressure exerted on the water to pass through the membrane is anywhere between 3 to 4 MPa and it provides very precise filtration of the water.

Add to that the very fine, semi-permeable membrane and the water you’ll get will be almost 100% pure without any impurities in it. The operating pressure is ultimately what ensures better purification and here reverse osmosis takes the lead.

Which One Is Better?

This is a question whose answer is not as straightforward. There are a lot of things that you need to consider when it comes to choosing a certain process for water filtration and purification.

Nanofiltration

Nanofiltration an excellent filtration process, but it is not as precise as reverse osmosis filtration. It is second in line when it comes to effective purification and, sometimes, it’s preferred.

So, if your water has a lot of chemicals and bacteria in it that you want removed, and there is no other impurity in it that is hard to remove, then you can go for a nanofiltration system. It will get the job done, you will have pure water and there won’t be anything to worry about in the long run.

Reverse Osmosis

The first thing that you need to look out for is the composition of the water and the presence of impurities in it. If there are a lot of divalent and monovalent ions in the water, then you need to get an RO system for the best results.

It will remove almost everything from the water, leading to better-tasting water, which is also good for your health.

Conclusion