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Nowadays, the membrane filter is becoming increasingly popular in producing fresh potable drinking water from the ground, surface, and seawater sources. The membrane filters with different configurations work by using membranes to filter all the contaminants in water.
If you want to learn more about this filter system, you’ve come to the right place! Scroll down to know more about “what is a membrane filter and how does it work?“
What Is A Membrane Filter?
The membrane is a highly designed physical barrier that only allows specific molecules in the feed stream to pass through. This filter involves passing a single feed stream through a membrane system that divides it into two individual streams: the permeate and the retentate.
Moreover, the membrane filter has different configurations, including ultrafiltration, reverse osmosis, and nanofiltration. Each of them has a distinctive process.
Molecules normally move from locations of high concentration to lower concentration. By providing an external pressure, molecules can flow from areas of low concentration to higher concentrations.
Particularly, the pressure difference on both sides of the membrane will help the permeate penetrate the membrane at a steady state. For that reason, the final product – permeate or retentate, will have higher overall yields.
Membrane Modules
There are four main types of modules: plate-and-frame, tubular, spiral wound, and hollow fiber:
- The plate-and-frame module is the simplest design, consisting of two end plates, the flat sheet membrane, and spacers.
- In tubular modules, the membrane is normally inside of a tube, and the feed solution is pumped through the tube.
- Meanwhile, the most used module belongs to the spiral wound module. This module consists of a perforated permeate collection tube wrapped in a flat sheet membrane. The feed flows through one side of the membrane, permeate flows on the other side, and spirals toward the central collection tube.
- Hollow fiber modules used for seawater desalination contain bundles of hollow fiber in a pressure tank. They may have a shell-side feed configuration, in which the feed runs along the outside of the fibers and gets through the fiber ends.
Hollow fiber modules can also be useful in a bore-side feed configuration, where the fibers circulate the feed. Yet, these fibers employed for wastewater treatment and membrane bioreactors are not always used in pressure tanks. Bundles of fibers could be dangling in the feed solution, and one end of the fibers will collect the permeate.
How Does It Work?
As mentioned above, there are four main types of membrane filters: reverse osmosis (RO), microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF).
- Reverse osmosis (RO) uses membranes with pores so tiny that only small portions of salts can go through, along with the water that is the major component of the permeate. Specific organic compounds with low molecular weights can also permeate, but only in small quantities. Consequently, other dissolved components within the liquid flow are unable to pass through.
- The main function of microfiltration (MF) is to remove small-diameter dispersed solids without influencing the balance of the components dissolved within the stream. Microfiltration membranes are extremely cost-efficient to operate, owing to their low energy consumption. Also, they don’t require frequent replacement and disposal of other consumables used in standard dead-end filtration.
- Ultrafiltration (UF) involves using membranes in which the pores are smaller (with a cut-off range from 1,000 to 100,000 Molecular Weight), and the pressure is quite low. Salts, sugars, organic acids, and smaller peptides can pass through the membrane’s pores, while proteins, lipids, and polysaccharides can not. Besides, using suitable ultrafiltration membranes can also separate a feed stream into two individual streams. Each contains various components with different molecular weights.
- Nanofiltration (NF) membrane technology functions very similarly to reverse osmosis. Specifically, Nanofiltration allows small ions to move through while flushing bigger ions and most organic components (e.g., bacteria, spores, fats, proteins, gums, and sugars). Nanofiltration is not a fine separation process as it uses membranes that are slightly more open than the RO.
Membrane Materials
Most membranes are synthetic organic polymers. Microfiltration and ultrafiltration membranes often come from the same materials but with various pore sizes due to the different membrane formation conditions.
Membranes can also be inorganic materials like ceramics or metals. Ceramic membranes are microporous, thermally stable, and chemically resistant. However, drawbacks such as high cost and mechanical fragility have limited their widespread use.
Metallic membranes are often stainless steel and can be extremely porous. Their primary application is in gas separations, but they can also be useful for water filtration at high temperatures or membrane supports.
The prevailing tendency on membrane trend is to use nano functionalized membranes. Polymer membranes doped with silver nanoparticles to prevent biofouling are typical examples of such modern membranes.
Fouling And Scaling In A Membrane Filter
Membrane Scaling
As the membrane flushes the dissolved minerals away, some of those start getting out of solution and form scale. It is clearly visible when you boil water for a long time, as a white ring will appear around the pot.
You can imagine how that membrane surface looks when minerals have gone. If insufficient water draining and excessive water flow, scale and accumulation will occur quickly.
Membrane Fouling
Fouling is similar to scaling, but instead of inorganic minerals accumulating, living organisms begin to slime and close down the membrane just like the scales.
How Often Should You Replace A Membrane Filter?
Reverse osmosis (RO) membrane: Every two or three years, based on water quality. As the reverse osmosis membrane declines more minerals, some of those minerals begin to precipitate out of solution and clog the surface area of the membrane. If you provide the RO system with softened water, its longevity could increase up to five years, under the condition that you replace the filters regularly.
Ultrafiltration membrane: For a point-of-use (POU) application, replace the ultrafiltration membrane every year for the UF membranes to do their work effectively.
Membrane Cleaning Methods
Some common membrane cleaning techniques include forward flushing, backward flushing, and air flushing.
- Forward flushing is the process of pumping permeate water through the feed side at high cross-flow velocity to remove foulants from the membrane surface. As a result of the faster flow and turbulence, the membrane will liberate some particles and then exclude them.
The particles absorbed into membrane pores will stay in. We only can remove these particles by applying backward flushing techniques. Particularly, forward flushing techniques are especially effective at removing colloidal matter.
- Backward flushing is a reversal of the purification process. Under pressure, permeate is pumped into the feed water side of the system, delivering twice the flux utilized during filtration. If the flux cannot restore itself adequately after backward flushing, you may need to apply a chemical cleaning technique instead.
- We can soak the membranes in a liquid of chlorine bleach, muriatic acid, or hydrogen peroxide with chemical cleaning. Firstly, the liquid soaks into the membranes for a few minutes. Then we may apply a forward flushing technique or backward flushing method to clean and wipe the contaminants out.
- One of the latest cleaning methods is air flushing. This forward flush injects air into the supplier pipes; the used air will result in a much more turbulent purification system.
Advantages Of Membrane Filter
Lower Overall Production Costs
The standout advantage of membrane filtration is its low price compared to many other water filter systems. The installation costs are cheaper, as well as the energy consumption costs.
Membrane filtration requires fewer processing steps that lead to greater purity and better overall yields.
Furthermore, since membrane filtration does not result in a filter cake, you won’t have to pay any extra costs for removing or disposing of this residue!
High Flexibility
Membrane filtration can help feed products with various viscosities, even with products that are difficult to process.
A diverse range of membrane filtration products ensures the most effective solution is available for each application. This also cuts down on wasted energy bills.
High-End Product Quality
Membrane filtration is an eco-friendly technique without a doubt. The separation process occurs only on the basis of molecular size, which eliminates the need to use additives. This produces a high-quality end product that meets the various strict requirements.
Conclusion
We believe you have found out the answer to the question, “What is a membrane filter, and how does it work?”. Membrane filtration has set a solid position in biotechnological applications with its high separation efficiency, ease of operation, and no requirement of chemicals during the process.
Besides, this technology allows you to cut down on overall production costs and improve the quality of products. This advanced technology will surely thrive in the future, serving us a much better life!