How can RO membranes be kept from fouling and scaling?
Fouling of membranes can be a significant problem for water treatment operators, when employing a membrane-based water treatment system. All membrane-based water treatment methods, including reverse osmosis (RO), microfiltration (MF), nanofiltration (NF), and ultrafiltration (UF), are subject to fouling.
Membrane fouling can result in less productivity, more of the energy usage, and even more expensive equipment replacements. But fouling can be avoided, and being aware of some of the warning signals, will improve your water treatment operations.
In this article, we will discuss about the reason of fouling and scaling in the RO membranes, their cause, and its remedy so that the membrane won’t foul and scale.
Why does fouling and scaling occur in RO membranes?
When particles gather on a filtration membrane's surface or in its pores, fouling occurs. Foulants prevent water from passing through the membrane, which has a number of effects including increased hydraulic resistance, increased energy use, and even damage to the membrane and other system parts.
It is common practise to categorise membrane fouling into different categories based on the foulants, which are present in the feed stream. These foulants include the particulate or colloidal fouling, biological or microbial fouling, scaling or precipitation fouling, as well as organic fouling. Each of these types of fouling can either be irreversible, such as when foulants chemically bond with the membrane material, or reversible, such as when a straightforward cleaning is able to remove the foulants, and restore a membrane to its full function.
What are the various membrane fouling types?
The following are some common membrane foulants:
· Colloidal and particulate fouling
Particulate fouling happens when colloidal or suspended materials clog a membrane's pores, or stick to its surface. The membrane develops what is described as a "cake" layer when particles build up on it, preventing water from passing through the pores and causing symptoms, including increased pressure differential measurements and increased energy usage.
What is the reason behind particulate fouling in RO membranes?
When non-biological and inorganic particles such as silt or clay are present in feed water, especially when the stream is received from a body of surface water, it results in particulate/colloid fouling. The Silt Density Index (SDI) of a feed stream is commonly measured by water treatment professionals, to assess the relative risk of particulate/colloidal fouling.
Since, RO systems have the tiniest pores of any membrane filtration systems, and are thus significantly more susceptible to particulate fouling, SDI measurements are particularly crucial when building RO systems. Usually, upstream coagulation and filtering techniques that are appropriate can prevent particle fouling.
· Biological and microbiological fouling
The process of biofouling is the growth of microbes, plants, algae, or other biological pollutants, on or inside the pores and surfaces of RO membranes. Warm, low-flow settings are ideal for biological and microbiological foulants to flourish, because they may connect to membranes there and grow while generating an extracellular polymeric substance or EPS, which serves as a protective layer.
The EPS and bacteria work together to create a biofilm, a slimy gel coating. Because, of its chemical make-up, the biofilm is resistant to cleaning techniques like the backwashing, or use of biocides.
Biofouling membranes can be difficult to remove, and in some situations they might need to be replaced. A bio-fouled membrane will eventually restrict water flow from one side to the other. This state is noticeable as a rise in the differential pressure from feed to concentrate, a decrease in membrane flux, an increase in pressure demand, and an increase in energy expenses.
· Precipitation or scale fouling
Scaling is brought on by the presence of crystallised salts, oxides, and hydroxides in the feed solution, also referred to as inorganic or precipitation fouling. Membrane scaling happens when components which are dissolved in a solution separate, and gather on the membrane's surface or gets stuck in its pores.
What is the reason behind precipitation fouling in RO membranes?
Precipitation fouling happens when a solution builds up against the feed side of the membrane, to a degree where it exceeds the saturation point, causing ionic components to precipitate out of solution, and adhere to the membrane surface. High conversion rate RO systems are particularly susceptible to scaling, especially when the feed stream contains high calcium or magnesium concentrations.
By using treatment methods that stop the formation of crystals, such as acid injection, softening, and the use of other chemical scale inhibitors, inorganic fouling can be avoided. While, each of these methods works, it's important to avoid using chemical treatments that clash with the selected membrane material.
· Fouling caused by the accumulation of organic waste
The accumulation of carbon-based material on a filter membrane is known as organic fouling. Natural organic matter is made up of carbon-based substances that are frequently found in soil, groundwater, and surface water, as a result of the breakdown of plant and animal materials.
The risks that OM poses as a foulant will depend on number of elements, including its affinity for membrane material. Organic matter is frequently extremely reactive. By performing some kind of raw water treatment, or choosing a membrane material that prevents organic material from adhering to the membrane, facilities can reduce problems with organic fouling.
How do you know fouling has occurred in the RO Membranes?
· Poor water quality is a sign of membrane fouling
Typically, a drop in water quality indicates membrane fouling. Check the composition of the foulant to learn more about what may be the root of the problem, if your membrane-based water treatment system starts to produce bad quality permeate.
If the foulant is colloidal, you might need to inspect your pretreatment system or, if you don't have one already, install a new pretreatment system. Your membranes may be scaling if they simultaneously exhibit poor salt rejection, and an increase in transmembrane pressure.
· Strong, strange, or mouldy odours
Odd and intense odours may indicate biological growth on your membranes. Check the filters for a slime-like substance or mould growth, if your membrane-based water treatment system, such as RO Plants has pretreatment such as carbon filters. A temperature change can typically prevent this, though it can also be necessary to add a biocide.
· Build-up on Membrane Edges
This is a blatant indication that your membranes are fouling. It's typically foulants accumulating into a solid mass, if you notice a real build-up of solids on your membranes. It's crucial to carefully look into the makeup of any build-up on membranes.
Sometimes it can be harmless, but understanding its composition might help you decide how to respond if there is a problem. If a clean-in-place (CIP) is required, your chemical provider may be able to solve the problem by adding some chemicals to the water.
How can membrane fouling be prevented? What is the remedy that can stop or slow-down membrane fouling?
Membrane fouling can sometimes, but not always, be reversed. It is therefore advisable to take proper actions to avoid, or reduce membrane fouling before it occurs. We've listed a few typical preventative strategies for preventing membrane fouling below:
· Planned cleaning
A regular cleaning schedule can aid in preventing the accumulation of foulants on the membrane. For the maximum advantage, cleaning cycles should be planned once a month or at other regular intervals.
The architecture of the membrane filtration system and the types of contaminants present, can affect maintenance procedures, which may use one or more cleaning techniques like:
1: Mechanical cleaning: Using physical force to remove pollutants from the membrane and from the system requires mechanical cleaning. Typical methods include vibration and backward or forward flushing, which involves running water or a cleaning solution through the device more quickly, or under higher pressure than during a typical service cycle.
This creates turbulence that flushes contaminants from the membrane. To further boost turbulence, air is added to the backwash/forward flush solution in a related procedure called air scouring.
2: Chemical cleaning: It comprises of applying detergents, acids, caustics, dispersants, or antiscalants to the membrane surface, in order to loosen as well as remove foulants from it.
3: Pretreatment: Due to their smaller pores than MF and UF membranes, RO membranes are more likely to need some sort of pretreatment, in order to prevent membrane fouling or other problems.
To reduce the danger of membrane fouling, streams with high pollutant concentrations could additionally require pretreatment, before membrane filtering units. If colloidal particles are present, pretreatment options can also include coagulation, as well as gravity settling (sedimentation), flocculation, and medium filtering, to get rid of bigger or coagulated particles.
Ion exchange and chemical pH adjustment are two other pretreatment techniques that can be used, to stop foulants from adhering to or depositing on the membrane.
4: Proper system design: The greatest way to prevent membrane fouling is through thoughtful planning and design. When changing a membrane or installing a new system, there are several factors that should be taken into account, because they all affect how well the membrane filtration system operates. These consist of:
Membrane composition: A wide range of synthetic polymers, ceramics, and metallic materials can be used to create filtration membranes. The membrane's material properties, such as its surface ionic charge, hydrophobicity, and pH tolerance range, dictate how well it will tolerate process conditions, and the required maintenance schedule, as well as whether it will be resistant to certain types of fouling.
Pore size in a membrane: It is crucial for a membrane filtering system to effectively remove the intended pollutants. Additionally, by optimising permeate flux in consideration of other aspects including feed water quality, temperature, and salt concentration, choosing the right membrane pore size can help to prevent fouling.
Operating circumstances: Certain ranges of temperature, pH, transmembrane pressure, and flow rate can make membrane fouling worse. These factors should be balanced in a well-thought-out manner, to prevent foulants from accumulating on the membrane surface.
When to clean fouling RO membranes?
If your membranes are fouled, you might need to replace them if the issue has persisted for a while. However, you might be able to quickly restart water production by just cleaning the membranes. The greatest ways to assist prevent a shutdown, or the need to replace membranes earlier than necessary, is to take a proactive approach.
An excellent place to begin is with a membrane cleaning plan. Make an appointment for a meeting with a chemical supplier, if you don't already have one to see how they can help.
Firstly, it is suggested that the RO membrane system operator investigates the indicators of the fouling, before selecting a cleaning chemical as well as a cleaning strategy. Cleaning should typically start once the normalised flux has dropped by 10% to 15%, and the permeates normalised salt content has risen by 10%.
Additionally, certain membrane-based systems work to prevent fouling and scaling. Closed-Circuit based RO (CCRO) avoids fouling, since it offers a membrane environment that is continually changing. The plug flow sequence used in the CCRO method also aids in the several hourly purges of foulants from the membranes.
To thoroughly assess process needs and conditions due to the intricacy of these aspects, it is frequently best to contact a water treatment specialist. To find the best separation method that minimises membrane fouling, a specialist from Netsol Water Solutions can conduct treatability studies and pilot testing.
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