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 it is true that membrane filter systems tend to require relatively little maintenance compared to other separation technologies, these are by their nature subject to clogging (or, “fouling”).
Fortunately, there are many strategies to prevent this membrane state and keep the filtration system running in the long term.
Membrane filtration systems, including microfiltration, ultrafiltration, reverse osmosis and nanofiltration, all share the use of semi-permeable membranes to capture particles from liquids. Membrane fouling occurs when contaminants settle on the surface of a filtration membrane, restricting the flow of liquids through the membrane pores. There can be many reasons contributing to fouling, and such clogging problems can emerge gradually or suddenly. Some symptoms could be the increasing energy costs and decreasing filtrate flow. As contaminants continue to accumulate on the membrane surface, increasing pressure is required to force the product through, which can ultimately lead to irreparable damage to the membrane.
How to avoid such problems then?
Scheduled cleaning
A systematic cleaning process 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 remove contaminants from the membrane and flush them out of the system. Typical approaches include vibration as well as backwashing.
- Chemical cleaning involves the application of chemicals, caustics, acids, antiscalants or dispersants to dissolve and remove contaminants from the membrane surface. Cleaning chemicals are selected based on the type of contaminants present, also taking into account the membrane material to ensure that the chemicals used do not damage it.
Pretreatment
In many cases, before implementing a microfiltration process, and even more so if it is ultrafiltration or reverse osmosis, a pre-treatment is likely to be necessary to avoid clogging or other problems. Pre-treatment options may include coagulation if colloidal particles are present, as well as gravity settling (decantation), flocculation, or pre-filtration to remove larger or coagulated particles. Other types may include chemical pH regulation and ion exchange to prevent adsorption or deposition of contaminants on the membrane.
System design
Preventing membrane fouling is best accomplished by good system 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 can be manufactured with a wide variety of synthetic polymers, ceramic and metallic materials. The material properties of the membrane, such as its surface ionic charge, hydrophobicity and pH tolerance range, determine whether the membrane will be resistant to certain types of clogging and how long it will withstand the process conditions.
- Membrane pore size: pore size is the key factor in ensuring efficient removal of targeted contaminants by a filtration unit. Additionally, selecting the correct membrane pore size can help prevent scale build-up by optimizing permeate flow.
- Operating conditions: membrane fouling can be aggravated by certain ranges of temperature, pH, transmembrane pressure and flow rate. A well-designed system will balance these variables to ensure that contaminants do not accumulate on the membrane surface.