Mechanical filtration is the most common way of removing physical contaminants in water. In this article, we look at four types of mechanical filtration and their uses in residential, commercial and industrial water purification processes.
Types of Mechanical filtration:
- Microfiltration
- Ultrafiltration
- Nanofiltration
- Reverse Osmosis
Microfiltration
Microfiltration is suitable for contaminants that range from 0.1 to 5 microns in size. MF has the largest pore size among the four membrane types listed. It filters out protozoa, bacteria, blood cells, flour, talc, sand, silt, clays, cysts, algae, and suspended solids. However, microfiltration is not effective in removing viruses or chemicals. It is often used as a pre-treatment for RO and Nanofiltration.
Examples of MF usage:
- Wastewater treatment process
- Clarification of gelatin, wine and plant extract
- Cold sterilization of beverages and pharmaceuticals without sacrificing flavor
- Separation of certain oil/ water emulsions
- Solid-liquid separation for pharmacies or food industries
Ultrafiltration
Ultrafiltration has a pore size range of 0.1um to 0.01um. It can reject particles such as silica, endotoxins, proteins, plastics and smog/fumes. Basically, it removes everything microfiltration can as well as most viruses and some chemicals. Like MF, UF is also often used as a pre-treatment to an RO as it removes the majority of particulates that fouls RO.
Examples of UF usage:
- Processing and treating wastewater
- Dialysis and other blood treatments
- Removal of pathogens from milk
- Chemical process separation
- Cheese manufacture
- Disaster site, for providing drinkable water without electricity
Nanofiltration
Nanofiltration typically filters particles from 0.001 to 0.01 microns such as multivalent ions, synthetic dies, protozoa, cysts, bacteria, viruses, sugars and salts with divalent anions like magnesium sulfate. Nanofiltration has attained the name of the ‘softening membrane’ as it is also used to soften water. However, the hard water from NF needs pre-treatment to avoid precipitation of hardness ions on the membrane. In terms of design and operation, nanofiltration is quite similar to the RO filtration process.
Examples of NF usage:
- Textiles
- Pharmaceuticals
- Bakeries
- Water treatment
- Dairy
- Pre-treatment for RO
Reverse Osmosis
Reverse Osmosis is the most effective water filtration process in removing many types of dissolved and suspended particles found in the water. Its semi-permeable membrane has a pore size of approximately 0.0001 micron. It allows water molecules or atoms to pass through a series of filters and leaves behind the majority of dissolved salts, organics, bacteria and viruses in the reject stream (also known as the concentrate or brine stream). RO provides many benefits such as preventing the risk of health issues, water with better odor and taste, and lower system cost.
Examples of RO usage:
- Pharmaceutical
- Power & Energy
- Agricultural
- Food & Beverage
- Printing press and other industrial process
- Dialysis
Conclusion
The four purification processes filter different contaminants through their membranes. However, the coarser filters act to prolong the life of the finer filters. MF, UF and NF can also be used as pre-treatment for RO.