Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water by pushing it under pressure through a semi-permeable Reverse Osmosis Membrane.
Reverse Osmosis works by using a high pressure pump to increase the pressure on the polluted water side of the RO and force the water across the semi-permeable RO membrane, leaving almost all of dissolved salts behind in the reject stream (Drain). The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.
The desalinated water that is demineralized or deionized, is called permeate (or product) water. The drain stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream.
As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream (also known as the concentrate), which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it.
Reverse Osmosis is capable of removing up to 99%+ of the dissolved salts (ions), particles, colloids, organics, bacteria and pyrogens from the feed water (although an RO system should not be relied upon to remove 100% of bacteria and viruses). An RO membrane rejects contaminants based on their size and charge. Any contaminant that has a molecular weight greater than 200 is likely rejected by a properly running RO system (for comparison a water molecule has a MW of 18). Likewise, the greater the ionic charge of the contaminant, the more likely it will be unable to pass through the RO membrane. For example, a sodium ion has only one charge (monovalent) and is not rejected by the RO membrane as well as calcium for example, which has two charges. Likewise, this is why an RO system does not remove gases such as CO2 very well because they are not highly ionized (charged) while in solution and have a very low molecular weight. Because an RO system does not remove gases, the permeate water can have a slightly lower than normal pH level depending on CO2 levels in the feed water as the CO2 is converted to carbonic acid.
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The figure above shows different impurity sizes and the filtering capabilities of each membrane type. A reverse osmosis system (with a TFC membrane) will eliminate almost every impurity from the intake water.
Implementations of the reverse osmosis principle vary depending on the following set of parameters:
Feed water can be tap water from municipal distribution net, groundwater, brackish water o even sea water. The more salty the feed water the higher the pressure the system will require to work:
Feed water will determine the pre-treatment stage. Sediment filters are always required, but if the source water contains a high density of sediment particles two or more sediment stages may apply. These pre-filters will keep the bigger particles away from the RO membrane extending its life. An Activated Carbon Filter (AC) is necessary when the feed water contains Chlorine since the majority of the RO membranes are affected by Chlorine reducing its life tremendously. If the feed water is not chlorinated, AC filters should not be used for pre-filtration because they can encourage microbial growth on the membrane surface.
The quality of the permeate water will define the type of membrane needed and the post-treatment unit. Activated Carbon post-filters can remove certain pesticides and organic solvents that the RO membrane does not remove. The activated carbon post-treatment process is also improved since the RO membrane removes compounds that may hinder adsorption by the carbon.
This parameter will determine the high pressure pump characteristics and therefore the electrical power consumption (and design requirements) and the quantity and size of the membranes. There are, depending on the application, RO systems with permeate flows from a few gallons per day to millions of gallons per day.
Recovery ratio on the most common membranes is in the range of 15~20%. Depending on the configuration used in the machine design the unit can reach recovery ratios higher than 50~60%. To accomplish this, the concentrate of the first membrane goes into the feed of the second stage and the second stage reject on the third. Serial set ups like this example will rise the recovery ratio but the permeate flow per membrane will be less than a parallel set up and the higher stage membranes lifespan will be shorter.