Among the most common problems impacting microfiltration (MF) and ultrafiltration (UF) or nanofiltration (NF) and reverse osmosis (RO) is the issue of membrane fouling. Addressing difficulties with fouling starts with addressing the question “what are the different types of membrane fouling and what causes them?”
Understanding how and why membrane fouling occurs is key to getting the longest possible service life out of a membrane filtration unit. In this article, we’ll break down common types of membrane fouling and explore the major contributing factors associated with each.
Types of membrane fouling
Fouling occurs when contaminants collect on the surface or in the pores of a filtration membrane. Foulants restrict water flow through the membrane, resulting in several consequences such as higher hydraulic resistance, greater energy consumption, and even damage to the membrane and other system components.
Types of membrane fouling are often defined according to the type of foulants present in the feed stream, which can include particulate/colloidal fouling, biological/microbial fouling, scaling/precipitation fouling, and organic fouling, as detailed below. Each of these types of fouling can be either reversible, as when a simple cleaning is able to dislodge the foulants and return a membrane to its full function, or irreversible, such as when foulants chemically bind with the membrane material, and permanently compromise its performance. Major types of membrane foulants include:
Particulate and colloidal fouling
Particulate fouling occurs when suspended solids and/or colloidal material clog the holes of a membrane or adhere to its surface. As particles accumulate on the membrane, they form what is known as a “cake” layer that obstructs water from flowing through the membrane’s pores, resulting in symptoms such as increased pressure differential measurements and increased energy consumption.
Particulate/colloid fouling is caused by the presence of non-biological and inorganic particles (e.g. silt or clay) in feed water, especially when the stream is sourced from a body of surface water. To measure the relative risk of particulate/colloidal fouling, water treatment specialists typically measure the Silt Density Index (SDI) of a feed stream. SDI measurements are especially important when installing RO systems, as these have the smallest pores of any membrane filtration systems and are consequently much more vulnerable to particulate fouling. Particulate fouling can typically be prevented by applying appropriate upstream coagulation and filtration.
Biological and microbial fouling
Biofouling is a process where microorganisms, plants, algae or other biological contaminants grow on or in filtration membrane surfaces and pores. Biological and microbial foulants tend to thrive in warm environments with low flow rates, where they are able attach to the membrane and multiply, while releasing a protective substance known as extracellular polymetric substance (EPS). Collectively, the microorganisms and EPS form a slimy gel layer known as biofilm. The chemical properties of biofilm make it resistant to normal cleaning strategies like backwashing or applications of biocides, such as chlorine. Membrane oxidation, although not a foulant, is most often caused by free chlorine attack. It’s permanent and cannot be reversed.
Biofouled membranes can be challenging to correct, and in some cases may need replacement.
Over time, a biofouled membrane will restrict water flow from one side of the membrane to the other, a condition which is perceptible in the form of greater differential pressure from feed to concentrate, as well as decreased membrane flux, greater pressure demand, and higher energy costs.
Scaling or precipitation fouling
Scaling, also known as inorganic or precipitation fouling, is caused by the presence of crystallized salts, oxides, and hydroxides in the feed solution. Membrane scaling occurs when dissolved constituents precipitate out of solution and collect on the membrane surface or lodge in its pores. Precipitation fouling occurs when a solution grows more and more concentrated against the feed side of the membrane, and eventually surpasses the solution saturation point, causing ionic constituents to fall out of solution and crystallize and/or bind to the membrane surface. RO/NF systems with high conversion rates are at particular risk for scaling, especially when the feed stream has significant calcium or magnesium concentrations.
Inorganic fouling can be prevented by treatment strategies that inhibit crystal growth, either though acid injection, softening, and application of other chemical scale inhibitors. While each of these strategies is effective, care must be taken to avoid choosing chemical treatments that are incompatible with the chosen membrane material.
Organic fouling is defined as the collection of carbon-based material on a filtration membrane. Natural organic matter consists of carbon-based compounds commonly found in soil, ground and surface water, resulting from decomposition of plant and animal material. Organic matter is often quite reactive, and the risk that it poses as a foulant depends upon a number of factors, including its affinity for the membrane material. Facilities can minimize issues with organic fouling by implementing some form of raw water treatment, and/or selecting a membrane material that resists adsorption of organic material to the membrane.
How can SAMCO help?
SAMCO has over 40 years’ experience custom-designing and manufacturing membrane 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 MF, UF, NF, or RO treatment system needs.
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