Membrane filtration systems are used for a variety of applications across many industries and verticals. If you’re considering a membrane filtration system for your own facility, you may be asking “What is membrane fouling and how can it be avoided?”
While membrane filter units tend to require relatively little maintenance compared to other treatment and separation technologies, they are often subject to fouling. Fortunately, there are many strategies to prevent membrane fouling and keep your filtration system running efficiently.
What causes membrane fouling?
Membrane filtration systems, including microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF), all share in that they use semi-permeable membranes to capture particles from liquids. Membrane fouling occurs when contaminants are deposited on the surface of a filtration membrane, restricting the flow of liquids through the membrane’s pores. There can be one or more factors that contribute to fouling, such as the presence of excess biological, colloidal, and/or organic particles in the source water; an inappropriate choice of membrane material; and/or unsuitable process conditions such as flow rate, temperature, and pressure.
How do we know if our facility has a problem with membrane fouling?
Problems with membrane fouling can emerge gradually or suddenly. Early on, facilities will often see symptoms such as rising energy costs and decreased membrane flux. As foulants continue to accumulate on the membrane surface, even greater pressure is needed to force water through, which can ultimately lead to irreparable damage to the membrane and other system components.
How can we avoid membrane fouling?
Membrane fouling is sometimes reversible—but not always. That’s why it’s best to implement preventative measures to avoid or minimize membrane fouling in the first place. Below, we’ve outlined some common preventative measures to avoid membrane fouling.
A systematic cleaning regimen can help to prevent foulants from building up on the membrane. Cleaning cycles should be scheduled monthly or at other regular intervals to provide the greatest benefit. Maintenance strategies can vary depending upon the membrane filtration system design and the types of contaminants involved, and can employ one or more cleaning methods, such as:
- Mechanical cleaning involves the use of physical force to loosen contaminants from the membrane and flush them out of the system. Typical approaches include vibration, as well as backward or forward flushing, where water or a cleaning solution is run through the unit at a faster speed or higher pressure than in a normal service cycle, resulting in turbulence that removes foulants from the membrane. In a related process known as air scouring, air is added to the backwash/forward flush solution to further increase turbulence.
- Chemical cleaning involves the application of detergents, caustics, acids, antiscalants, or dispersants to loosen and remove foulants from the membrane surface. Cleaning chemicals are selected based on the type of contaminants present, with consideration also given to the membrane material to ensure that the chemicals used do not damage it.
RO/NF membranes have smaller pores than MF/UF membranes, therefore, they are more likely to require some form of pretreatment to avoid membrane fouling or other issues. Streams with high concentrations of contaminants may also demand pretreatment ahead of membrane filtration units in order to minimize the risk of membrane fouling. Pretreatment options can include coagulation if colloidal particles are present, as well as gravity settling (sedimentation), flocculation, and media filtration for removal of larger or coagulated particles. Other types of pretreatment can include chemical pH adjustment and ion exchange to prevent adsorption or deposition of foulants on the membrane.
Preventing membrane fouling is best accomplished by good planning and design. There are many variables that play a role in proper system function for a membrane filtration system, each of which should be considered when replacing a membrane or installing a new system. These include:
- Membrane material: Filtration membranes may be fabricated from a wide variety of synthetic polymers, ceramic, and metallic materials. Properties of the membrane material, such as its surface ionic charge, hydrophobicity, and pH tolerance range, determine whether the membrane will be resistant to certain types of fouling, and how well it will withstand process conditions and the necessary maintenance regimen.
- Membrane pore size: Pore size is the key factor to ensuring efficient removal of targeted contaminants by a membrane filtration unit. Additionally, selection of the proper membrane pore size can help to avoid fouling by optimizing permeate flux in light of other factors, such as feed water quality, temperature, and salt concentration.
- Operating conditions: Membrane fouling can be exacerbated by certain ranges of temperature, pH, transmembrane pressure, and flow rate. A well-designed system will balance these variables to ensure that foulants do not collect on the membrane surface.
Due to the complexity of these factors, it is often best to contact a water treatment specialist to fully evaluate process needs and conditions. A professional can run treatability studies, membrane autopsy, and pilot testing to determine an ideal separation solution that minimizes membrane fouling.
Can SAMCO help?
SAMCO has over 40 years’ experience custom-designing and manufacturing membrane filtration systems for a range of industries and applications, 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 MF, UF, NF, or RO treatment system needs.
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