Let's Take A Look At Different Water Purifying Membrane Classifications
Before we talk about filtration and purification membranes it should be understood that there are a few more methods to purify water depending on the application, circumstances and desired purification level, some listed below:
- Chlorine Dioxide
This post is focused (for now) on the filtration and purification of water with the use of filters and membranes as that's our expertise and we believe the best way to purify water for drinking for residential or commercial application.
The three predominate membranes used in residential and commercial water filtration process are Reverse Osmosis (RO), Ultrafiltration (UF), Microfiltration (MF). In certain specialized cases Nanofiltration (NF) is used.
Each level of filtration removes certain sizes of molecules and reverse osmosis in addition can remove dissolved solids such as salts as well as dangerous particles such as radioactive particles occurring naturally in the drinking water or lead and other metals that seep into the water from aging plumbing or industrial plants or pesticides.
Not all particles in drinking water are bad for human consumption and some are even put back in which is called "remineralization process". Minerals found in water are actually good for our health and help with that refreshing taste vs. a "flat" taste, which is exactly what you get when you remove EVERYTHING.
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.
Examples of microfiltration applications are:
· Cold sterilisation of beverages and pharmaceuticals
· Clearing of fruit juice, wines and beer
· Separation of bacteria from water (biological wastewater treatment)
· Effluent treatment
· Separation of oil/ water emulsions
· Pre-treatment of water for nano filtration or Reverse Osmosis
· Solid-liquid separation for pharmacies or food industries
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
Examples of ultrafiltration applications are:
Removal of particulates and macromolecules from
raw water to produce potable water.
Filtration of effluent from paper pulp mill
- Cheese manufacture, see ultrafiltered milk
- Removal of some bacterias from milk
- Process and waste water treatment
- Enzyme recovery
- Fruit juice concentration and clarification
- Dialysis and other blood treatments
- Desalting and solvent-exchange of proteins (via diafiltration)
- Laboratory grade manufacturing
- Radiocarbon dating of bone collagen
- Removal of particulates and macromolecules from
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 water purifying or filtering 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 they do not "clean water" and are even slightly different than water conditioners.
• 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.