Reverse osmosis and nanofiltration are physical separation technologies used to remove contaminants from liquids. In recent years, RO and NF technologies have become more efficient and affordable for a variety of industrial, commercial and residential applications, including drinking water production and treatment of brackish water, seawater, wastewater and drinking water.
Reverse osmosis (RO) and nanofiltration (Nano) are two very similar technologies. Visually, they are almost identical and both use essentially the same technology to remove impurities from water or other liquids. In both systems, membrane elements (or membranes or elements) are used to separate the liquid from the contaminants. Membranes are basically filters with very small "openings" that prevent contaminants from passing through as the liquid being purified is "pushed" through the membrane.
Because all filtration membranes have the smallest pores, RO and NF membranes can quickly scale if larger particles are not removed by upstream filtration technologies such as media filtration or MF/UF. 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 have a long service life and require relatively little maintenance. This is because the influent water continuously flows over the surface of the filtration membrane in what is called cross-flow filtration, which results in rejected material being flushed away with the effluent stream rather than clumping onto the membrane.
While RO and NF are quite effective purification technologies, both require energy to move water through their membranes. This is because their fine pores result in high concentrations of salts and other compounds on the retention side of the membrane, so sufficient pressure must be applied to enable the water to overcome the osmotic pressure that causes the water to resist flow through the membrane.
The traditional water treatment series (in the case of industrial applications) usually consists of several unit processes including: coagulation, flocculation, sedimentation, clarification, ion exchange and filtration. For residential treatment, these steps are sedimentation, clarification, ion exchange, filtration, purification and polishing.
While conventional treatment systems can effectively remove dissolved solids, they do so through a complex series of steps that typically require a large industrial footprint and investment in a variety of specialized equipment and chemicals. This problem does not exist in residential or commercial systems, where the system can be installed under a sink or in the customer's garage or basement.
As RO/NF membrane technologies have become more efficient, affordable, and smaller for residential and commercial applications in recent years, they are increasingly being used as more compact, efficient, and environmentally friendly alternatives to traditional treatment systems. For example, when using NF instead of traditional lime softening, multiple treatment steps are combined into one and no concentrated brine byproducts are produced.
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