Membrane fouling is the most common issue affecting pressure-driven membrane filtration systems, including microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF) units alike. While it is not always possible to prevent membrane fouling altogether, there are usually ways to minimize its impact. In this post we’ll answer the question “how can you reduce membrane fouling in your water treatment system?” by exploring various optimization strategies for membrane filtration systems.
This article is focused on membrane fouling reduction strategies, or measures that seek to minimize fouling by making minor modifications to feed stream and process conditions. These can be distinguished from fouling prevention measures, which are comparatively more involved, and can include the addition of various pretreatment technologies, redesign of treatment system components, and replacement of the existing membrane material.
Feed stream variables
Membrane filtration systems remove contaminant solids from the feed stream, resulting in a treated stream (filtrate), and a waste stream (retentate). Ideally, the retained contaminants can be flushed away with the retentate stream or can be removed from the membrane unit during a normal cleaning cycle. Under certain conditions, however, constituents in the bulk stream can precipitate out of solution, crystallize, adsorb to the membrane, or otherwise interfere with the flow of the permeate through the filter unit.
Depending upon the process at hand, it is possible to reduce membrane fouling by making adjustments to stabilize the feed stream. Variables such as pH, temperature, and/or concentration of dissolved or suspended solids in the feed stream have a significant effect on the stability and reactivity of constituents in the feed stream. Specifically, this can help to address:
- Precipitation and scaling: Optimizing the pH, temperature, and concentration of the feed stream can help to prevent precipitation of salts that can form scale deposits on membranes.
- Protein denaturation: For streams with significant protein content, especially those used in dairy production, adjustments to temperature and/or pH can help to prevent protein denaturation and associated fouling issues.
- Biofouling: Warm, low-flow environments tend to invite growth of microorganisms, plants, algae and other biological contaminants, which can form a biofilm on membrane elements. In some cases, adjusting the temperature, flow rate, or pH of the feed stream can slow the growth of such biological contaminants and reduce biofouling.
Process variables include all those characteristics that make up the environment within the membrane filtration unit, such as the pressure, velocity, and temperature that govern the movement of feed, filtrate, and retentate streams into and out of the unit. Flux, or the flow rate of a liquid through a membrane, is an important measure of a membrane filtration unit’s performance. Too low, and foulants can build up on the membrane; too high, and the membrane can fail due to excess pressure.
The pressure applied to a feed stream in a membrane filtration system usually has a direct relationship with flux: as pressure increases, so does flux. However, this is often complicated by a phenomenon known as concentration polarization, where slow- or non-permeating components of the feed stream collect along the surface of the membrane. As a result, liquids and permeable components are not able to reach the membrane, causing a drop in flux, and a heightened risk of membrane fouling.
When looking for ways to reduce membrane fouling, it is important to check that your system is operating within manufacturer guidelines for temperature, pressure, and other conditions. Beyond that, there are a few variables that can be adjusted to minimize membrane fouling. These include:
- Transmembrane pressure: Increased transmembrane pressure results in increased flux—but only up to a certain threshold. If system pressure is too high, it can exacerbate fouling by causing excess deposition of foulants on the membrane, or shearing that diminishes the membrane’s permeability.
- Concentration polarization: Drops in flux can be a symptom of concentration polarization. If this is the case, it can be beneficial to introduce turbulence at the membrane’s surface, either through addition of an air injection or physical vibration unit. Other options include increasing the frequency of cleaning or backwashing cycles to limit the formation of an interruptive concentration gradient.
- Cross-flow velocity: For cross-flow type filtration units, increasing the speed at which the feed stream circulates through the unit (known as the cross-flow velocity) can increase permeate flux, and agitate foulants enough to prevent them from depositing on the membrane surface.
How 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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