How are membranes used?
The small pores of the membranes can serve as a physical barrier, preventing passage of certain materials such as salt, bacteria and viruses while allowing the free passage of water and air. The desalination of water using reverse osmosis is a well known use of membranes as a filter.
Recently, recovery of water from sewage and recovery of whey protein from waste streams during cheese making have been carried out with ultrafiltration and microfiltation membranes which require much less pressure than reverse osmosis. While pressure is be used to drive filtration, electrical current, osmotic pressure, and temperature can also be used to preferentially allow one component in a mixture to pass freely through the membrane while retaining the rest. The membrane structure and chemistry can also serve to carry out other separations.
Membranes provide a high surface area material where chemical reactions or diffusion can take place. For example, bundles of hollow fiber membranes (membranes in a thin tubular form) are used in dialysis to purify the blood by removing certain toxins. Membranes can also be used to carry out solvent extraction and catalysis while also serving to separate the reactants.
Hydrophobic membranes can be used to prevent passage of liquid water but allow vapor to pass (like Goretex). This property has been exploited in membrane distillation where brackish water is heated using solar power and the pure water vapor passes through the membrane and condensed to produce very high quality water. This uses less energy than boiling and utilizes bountiful but low value energy in remote areas.
Types of Reverse Osmosis Membranes
As the pore size gets smaller, more pressure will be needed to start the filtration process. Also surface properties play an important role. For a hydrophobic (water rejecting) surface more pressure will be needed than with a hydrophilic (water attracting) surface.
MF and UF are typical particle filters, whereas NF and RO change the chemical or ionic composition of the water, as for instance the removal of dissolved minerals. See the filtration spectrum above.
This is the kind of RO membrane that has been used during the early experiments on RO during the early years of the 1950s. These types of membranes are made up of thin surface layers that are dense.
They are also asymmetric, and they also have a porous structure that is thick. The main purpose of the dense layer is to increase the rejection rate of your membrane and thus the reverse osmosis treatment system while the porous substructure shall provide the strength that the membrane may need.
One of the greatest advantages of the cellulosic membranes compared to other varieties of reverse osmosis membranes is the fact that they are very cheap. They’re also very convenient to install. However, despite these benefits, there are also a number of limitations.
For one, this kind of membrane can easily be compacted, especially if there’s an increase in temperature or in pressure. It’s also very vulnerable to hydrolysis, which means you may only be able to use it at such limited range of pH level, usually between 3 and 8 pH level.
This will depend on the brand or make of your RO membranes. If the temperature goes as high as 35 degrees Celsius, the cellulosic membrane will degrade progressively.
Furthermore, the cellulosic membranes are very vulnerable to attacks of bacteria. This is because they tend to reject poorly the low molecular weight of the contaminants.
Thin Film Composites (TFC)
These kinds of membranes are made up of surface film that is dense and thin. It is commonly placed atop the porous substructure. You can customize the manufacturing process and the construction materials of these varieties so your reverse osmosis membranes will be able to function more effectively.
There are also several kinds of TFC today. These include the polyfurane cyanurate, aromatic polyamide, and alkyl-aryl poly urea.
TFCs are one of the most efficient reverse osmosis membranes available in the market. However, you should also be wary about them, more so when they are exposed to free chlorine. This is because they can actually degrade because of high oxidation levels.
You should also be very consistent when you’re going to maintain the thin composite films. One of the integral parts that need constant monitoring is your carbon prefilter.
It’s essential to your RO membranes as they do not only get rid of the bad smell, taste, and appearance of water, but it can also prevent sediments from moving on to your membranes. When they do, the RO membranes will get damaged easily.
Aromatic Polyamide Membrane
This kind of reverse osmosis membrane has been developed by Dupont. It basically looks like the cellulosic membrane since it’s also asymmetric in structure and is very thin.
However, it is much better than the latter because aromatic polyamide membrane has higher resistance to the occurrence of biological attacks and hydrolysis. It’s also able to sustain sudden rise in temperature, though constant exposure to such condition can damage it forever.
Membrane Technology in General
Membranes can be subdivided according to their filtration properties in 4 categories:
- Micro-Filtration (MF), range 0,1 – 1 micron (1 micron = 1/1000 mm);
- Ultra-Filtration (UF), range 0,001 – 0,1 micron;
- Nano-Filtration (NF), range 0,0001 – 0,001 micron (0,001 micron = 1 nanometer – nm);
- Reverse Osmosis(RO), bereik < 0,0001 micron.