Membrane Filtration: Reverse Osmosis (RO) & Ultra Filtration
Effective Water Treatment with Membrane Filtration
A membrane is essentially a thin layer of semi-permeable material that separates substances when force is applied across the membrane. Membrane filtration processes are increasingly used for water quality parameters of removal of bacteria, microorganisms, particulates, and natural organic contamination. These can impart color, tastes, and odors to water and react with disinfectants for purification of water.
As advancements are made in membrane filtration and water quality purification systems, capital and operating costs continue to decline. The most commonly used membranes are reverse osmosis membrane and ultra filtration membrane. As for the commonly used processes, microfiltration (MF), ultrafiltration (UF), and reverse osmosis membrane (RO) are some of the most popular membrane filtration processes for water treatment.
Microfiltration: Membrane Filtration for Water Treatment
Microfiltration is loosely defined as a membrane separation process using membranes with a pore size of approximately 0.03 to 10 microns (1 micr millimeter and a relatively low feed water operating pressure of approximately 100 to 400 kPa (15 to 60psi). Contaminants removed by microfiltration include sand, silt, clays, Giardia lamblia and Cryptosporidium cysts, algae, and some bacterial species. Microfiltration is not an absolute barrier to viruses. When used in combination with disinfection, it appears to control these microorganisms in water.
There is a growing emphasis on limiting the concentrations and number of chemicals that are applied during water treatment. By physically removing the pathogens, membrane filtration can significantly reduce chemical addition, such as chlorine.
Another application for microfiltration is for removal of natural synthetic organic contaminants to reduce fouling potential. In its normal operation, the membrane removes little or no organic contaminants. However, when pretreatment is applied, increased removal of organic contaminants can occur. Microfiltration is usually used as a pretreatment to reverse osmosis or nano filtration to reduce fouling potential. Reverse osmosis has been traditionally employed to desalt or remove hardness from groundwater.
Ultrafiltration is similar to microfiltration in terms of the process but differs in membrane thickness and water pressure. Ultrafiltration membrane has a pore size of approximately 0.002 to 0.1 microns, and an operating pressure of approximately 200 to 700 kPa (30 to 100 psi). Ultrafiltration will remove all microbiological species removed by microfiltration (partial removal of bacteria), as well as some viruses (but not an absolute barrier to viruses) and humic materials. Disinfection can provide a second barrier to contamination and is recommended.
The primary advantages of low-pressure membrane filtration processes are compared with conventional clarification and disinfection (post-chlorination) processes are:
• No need for chemicals (coagulants, flocculants, disinfectants, pH adjustment)
• Size-exclusion filtration as opposed to media depth filtration
• Constant water quality treatment in terms of particle and microbial removal
• Process and plant compactness
• Simple automation
Due to these multiple benefits, and water quality with post filtration water treatment, ultrafiltration comes second only to reverse osmosis in membrane filtration techniques.
Cellulose Acetate Membranes
Cellulose acetate has a higher flux and a smaller area of membrane is therefore required. It is also resistant to small concentrations of free chlorine.
Reverse Osmosis or RO Membrane
Reverse osmosis can effectively remove inorganic contaminants from water. RO can also effectively remove radium, natural organic contaminants, substances, pesticides, cysts, bacteria and viruses. RO is particularly effective when used in series with multiple units. Disinfection is also recommended to ensure the safety of water. Some of the advantages of RO are:
• Removes nearly all contaminant ions and most dissolved non-ions,
• Relatively insensitive to flow and total dissolved solids (TDS level and suitable for small systems with a high degree of seasonal fluctuation in water demand
• RO operates immediately, without any minimum break-in period
• Low effluent concentration possible
• Bacteria and particles are also removed
• Operational simplicity and automation allow for less operator attention and make RO suitable for small system applications.
Reverse osmosis membranes do not have definable pores in the way that the film used in ultrafiltration do. There’re only spaces between the fibers making up the film, which can take up water because of the acetyl or similar groupings forming the surface.
Two materials make up the bulk of commercial RO membranes, cellulose acetate and an aromatic polyamide. Each of the two materials has advantages and disadvantages:
The polyamide membrane can be used at a higher temperature than cellulose acetate. Polyamide membrane filtration does not only filters the water but also gets rid of the bacteria. However, the polyamide membrane cannot tolerate chlorine and water treatment should be done with chlorine post filtration.
The choice of membrane depends upon the nature of the input of water and it is essential to be able to use the most suitable one in any particular set of circumstances.
Since the reverse osmosis membrane is a plastic material rather than a sponge, there is a continuous, irreversible compression of the material under pressure, temperature and time. The salt passage through the membrane is not significantly affected by this compaction. You may however, need to replace the membrane in the long run.
Chemical changes can also take place in the membrane. For example, cellulose acetate can be hydrolyzed to cellulose. This process is accelerated at high pH and this is one reason for the limited use of cellulose acetate membranes in water treatment systems. Although these changes are inevitable, choosing the conditions of operation can reduce their effect but a finite life of 3 to 5 years could be expected.
Membrane Integrity Testing: Water Quality Parameters
One of the most critical parts of using membrane filtration techniques is ensuring that the membranes stay intact. There are different methods to monitor the membrane integrity like
- Particle count monitoring
When it comes to choosing from the market, DOW filters have been reliable and durable filters in market for long now. Their Filmtec Membranes for desalination and mineral removal are known for the wide product range. From Industrial and chemical usage to filtration for drinking water, Filmtec is one of the most trusted and well known membrane filters.
Connect with reliable vendors and suppliers on our IDS Water directory for all your membrane filtration needs. Find the type of pore and film your require in membranes for water quality and treatment. Look up various vendors and suppliers today at IDS Water!
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