Water Purifying Membrane Classifications
The three predominate membranes used in water filtration industry are Reverse Osmosis (RO), Ultrafiltration (UF), Microfiltration (MF). In certain specialized cases Nanofiltration (NF) is used.
The pore size on microfiltration membranes ranges from 0.1 – 5 um, and has the largest pore size of the four main membrane types. Its pores are large enough to filter out such things as bacteria, blood cells, flour, talc and many other kinds of fine dust in solution. Because its pores are relatively large compared to other membranes, it can be operated under low pressures and therefore low energy.
Ultrafiltration has a pore size range of 0.1um to 0.01um. UF membranes reject particles such as silica, viruses, endotoxins, proteins, plastics and smog/fumes such as ZnO. Due to the decrease in pore size, the osmotic pressure required is higher than that of MF.
Here is a couple of example of water purifiers that use UF filter to purify drinking water. One is for tap water and the other is for fresh water (stream, rivers, lakes)
UF Under Sink Water Purifier & UF Water Bottle
Nanofiltration has a pore size range of 0.001-0.01um. NF membranes can filter particles up to and including some salts, synthetic dies and sugars, however it is unable to remove most aqueous salts and metallic ions, as such, NF is generally confined to specialist uses.
Reverse Osmosis (RO)
Reverse Osmosis has a pore size range of 0.0001 – 0.001. It is by far the finest separation material available to industry. It is used on a large scale for the desalination and purification of water as it filters out everything but water molecules, with pore sizes approaching the radius of some atoms in many cases. This pore size means it is the only membrane that can reliably filter out salt and metallic ions from water. The small pore size of RO membranes means that a significant amount of osmotic pressure is required to force filtration.
More Information On Comprehensive Methods Of Filtering and Purifying Drinking Water.
The drinking water treatment technologies used in the majority of systems include one or more of the following methods or media types:
• Sediment Filters - Ranging from 1 to 20 microns are normally used as a pre-filter to protect and extend the life of other filters. These filters remove larger particles such as sand and other visible to the naked eye debris.
• Activated Carbon, including Granulated Activated (GAC), Carbon Block, and Catalytic Carbon – Standalone, or combined with other technologies to remove chlorine, chloramines, VOCs, MTBE, and a wide range of contaminants including lead and mercury.
• Water Softeners – Used to soften "hard" water using ion exchange technology that exchanges magnesium and calcium with sodium or potassium. Water softeners are not designed to purify water that is microbiologically unsafe.
These systems replace or suspend mineral ions with salt or potassium ions.
• BASF ATS Media – Home systems, usually combined with activated carbon.
• Steam Distillation – Systems for home, business and commercial use. Unfortunately make water taste flat as these systems remove most TDS (Total Dissolved Solids) from water.
• Reverse Osmosis (RO) - RO uses a semi-permeable membrane filter (Thin Film Composite or TFC), very pure water is produced but still retains traces of minerals. RO systems utilize a series of filters (sediment, granulated activated or block carbon), the TFC membrane. As chlorine can damage the TFC membrane, all RO systems have effective carbon pre-filters, and most use a carbon post-filter to improve the taste of drinking water.
Reverse osmosis, or RO, is a common form of water purification. RO uses a semi-permeable membrane that has pores large enough to allow water molecules to pass through but small enough to block other types of molecules at 97-99% such fluoride, chlorine, heavy metals, nitrates, nitrites, silver, aluminum, arsenic, cadmium, copper, iron, nickel, zinc, lead, chromium, barium, hydrogen sulfide, radon, selenium, trihalomethane, manganese, mercury, chloroform, trichloroethane, lindane, pesticides, radioactive isotopes, sugars, salts, pyrogens, fungicides, bad tastes and odors.. When pressure is applied to water on one side of the membrane, water molecules are forced through to the other side of the membrane, producing purified water. Contaminants are essentially filtered out by the process. RO removes .001 micron size contaminants. RO does drain some water in order to separate and remove dangerous contaminants from water completely.
• Deionization – Not recommended for drinking water as it's not practical. Often used by laboratories, deionization utilizes a series of ion exchange resins, but to produce pure water, deionization must be combined with other purification systems, as while this method removes dissolved solids and gases, many other impurities remain.
• Ozonation – Nature's natural sterilizer. Ozone is the strongest oxidant available for the disinfection of air and water. Ozone is used to purify a high percentage of the world's drinking water, including bottled and municipal water.
Ozone acts over 3000 times faster than chlorine, with the ability to kill 99% of all waterborne bacteria, germs, viruses and most pesticides by rupturing the cells of micro-organisms, or destroying odors and chemicals by oxidation. Ozone has a fairly short life of about 20 minutes, naturally changing back to O2.
While the use of ozone for most drinking water systems is unnecessary and impractical due to municipal chlorination
• KDF Media – Zinc-copper alloy with a wide range of contaminant removal. Usually combined with Granulated Activated Carbon. KDF does not remove fluoride and requires a special fluoride filter.
Using a process of electro-chemical oxidation known as “redox” (Oxidation-Reduction), KDF media is a zinc-copper alloy that removes 95-97% of free chlorine, heavy metals, nitrates, nitrites, silver, aluminum, arsenic, cadmium, copper, iron, nickel, zinc, lead, chromium, barium, hydrogen sulfide, radon, selenium, trihalomethane, manganese, mercury, chloroform, trichloroethane, lindane, pesticides, fungicides, bad tastes and odors. KDF media significantly extends the life of granulated activated carbon (GAC), controls and inhibits the growth of microorganisms and outperforms silver-impregnated carbon filters.
• Ceramic – Alternative to germicidal UV-C for filtration of bacteria. With a .5 to .9 micron rating, Ceramic filters are effective for filtering microbes such as E.coli, cysts and sediment. Most ceramic filters do not remove viruses effectively.
• Activated Alumina – Specifically designed to remove fluoride and arsenic by adsorption.
• Bone Char – Great at removing fluoride. Bone Char also removes chlorine, heavy metals and radioactive isotopes.
• Ultraviolet Germicidal Irradiation (UV-C) - To disinfect microbiologically unsafe water. UV can be added to all RO systems when used for well water. Great at removing viruses.
• Ultra-Filtration (UF) - Ultra Filtration uses membrane technology to reject contaminants from 0.01 to 0.2 microns. UF is effective in the removal of bacteria, pyrogens, high molecular weight dissolved solids and other water contaminants without removing healthful minerals. Does not remove radioactive isotopes, viruses or salts and sugars and usually is sold with a UV filter.