Author: Site Editor Publish Time: 2022-07-31 Origin: Site
Currently in water filtration, reverse osmosis membranes and nanofiltration membranes are the most commonly used membrane elements, but many users have a vague understanding of reverse osmosis membranes and nanofiltration membranes. Today Jun Xin will introduce to you what the differences between RO and NF membranes are, as well as the commonalities and differences.
What do RO and NF membranes have in common?
RO and NF are membrane filtration technologies in which pressure is applied to a stream of liquid, driving it through a semi-permeable membrane to remove dissolved solids. In this way, RO and NF membranes are very similar to other forms of membrane filtration, including microfiltration (MF) and ultrafiltration (UF), but are able to remove smaller molecules such as dissolved organics, pesticides and agrochemicals.
Both RO and NF membranes benefit from pretreatment
Without the removal of larger particles by upstream filtration techniques such as media filtration or microfiltration and ultrafiltration, all filtration membranes have minimal pore space and RO and NF membranes can rapidly scale. Pretreating the process stream to remove these problematic particles also reduces the amount of energy required to maintain adequate pressure in the RO/NF system.
When the process stream is adequately pretreated, RO and NF membranes can have a long service life and require relatively little maintenance. This is because in so-called cross-flow filtration, the feed water continuously flows over the surface of the filter membrane, which results in waste material being swept away with the waste stream, rather than clumping onto the membrane.
Both reverse osmosis and nanofiltration membranes require pressure to function
While reverse osmosis and nanofiltration are quite effective purification technologies, they both require energy to move water across the membrane. This is because their fine pores produce high concentrations of salts and other compounds on the stagnant side of the membrane, so sufficient pressure must be applied to enable the water to overcome the osmotic pressure that causes it to resist flow through the membrane.
Both are used in place of conventional treatment technologies
Conventional water treatment series usually consist of several unit processes including: coagulation, flocculation, sedimentation, clarification, ion exchange and filtration. While conventional treatment systems are effective at removing dissolved solids, they do so through a complex series of steps that often require a large footprint and investment in a variety of specialised equipment and chemicals. As reverse osmosis and nanofiltration membrane technologies have become more efficient and cost-effective in recent years, they are increasingly being adopted as a more compact, efficient and environmentally friendly alternative to traditional treatment series. When using NF instead of conventional lime softening for example, multiple treatment steps are combined into one and no concentrated brine by-products are produced.
Although reverse osmosis and nanofiltration membranes are very similar, they can be distinguished by the size of particles each is capable of removing. In contrast, reverse osmosis and nanofiltration membranes are capable of removing finer contaminants than ultrafiltration and microfiltration, and their applications include the removal of hardness, nitrate, sulphate, total dissolved solids (TDS), heavy metals, radionuclides and organic macromolecules from process and waste streams.
RO membranes are the best of all membrane filtration systems, with extremely small pores capable of removing particles as small as 0.1nm.RO has been around since the 1950s and is used primarily for desalination, as in seawater or brackish sources of drinking water. Other applications for RO include the filtration of process water for industrial applications, such as in the printing industry, in order to maintain optimum equipment performance.RO membranes very effective in removing all ions, regardless of size.
NF provides a lighter filtration than RO, capable of removing particles as small as 0.002 to 0.005 μm in diameter. NF is a relatively new technology used primarily for drinking water generation. NF removes harmful contaminants such as pesticide compounds and organic macromolecules, while retaining minerals that may be removed by RO. Nanofiltration membranes are capable of removing larger divalent ions such as calcium sulphate while allowing smaller monovalent ions such as sodium chloride to pass through.
Nanofiltration (NF) membranes: The filtration accuracy is between ultrafiltration and reverse osmosis, with a lower desalination rate than reverse osmosis. It is also a membrane separation technology that requires charging and pressurisation. The water recovery rate is very low. This means that nearly 30% of the tap water will be wasted in the process of making water with a nanofiltration membrane. This is unacceptable for the average household. It is commonly used in industrial pure water manufacturing.
Reverse Osmosis (RO) membrane: The filtration accuracy is approximately 0.0001 microns. In the early 1960s, an ultra-high-precision membrane separation technology using differential pressure was developed in the USA. It can filter almost all impurities (both harmful and beneficial) from water. Water molecules can only pass through. This means that nearly 50% of the tap water will be wasted in the process of making water with a reverse osmosis membrane. This is unacceptable for the average household. It is commonly used for purified water, industrial ultrapure water and pharmaceutical ultrapure water. Reverse osmosis technology requires pressurisation, energisation, low flow rates and low water utilisation, making it unsuitable for the purification of large quantities of drinking water.
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