Reverse osmosis (RO)
Reverse Osmosis (RO) is a modern process technology to purify water for a wide range of applications, including food processing, biotechnology, pharmaceuticals, power generation, seawater desalting, and municipal drinking water.
RO is currently considered one of the most economic and effective process for water desalination and it is often the appropriate technique to treat solutions having salt concentrations from 100 to over 50,000 mg/ liter. Solutions with salinity from surface water to sea water, and even brines. Filtration process is performed without the use of chemicals and instead by exploiting an exclusively physical mechanism.
A large number of natural processes occur through osmosis, such as the way plants obtain nutritional substances though the root structure. Reverse osmosis (R.O.) is a mechanism derived from the phenomenon of osmosis, which occurs in nature, whereby a fluid is concentrated by flowing through a semi-permeable membrane. Semi-permeable refers to a membrane that selectively allows certain species to pass through it while retaining others. In actuality, many species will pass through the membrane, but at significantly different rates. In RO, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts).
The flow of water crosses the membrane in a tangential manner, separating into concentrate and permeate. The expression “permeate” refers to water whose salinity is eliminated (also called “osmotic water”) so that it can be transferred to the user services, while the expression “concentrate” refers to the water whose salinity has been increased after the process and is therefore to be discarded.
Reverse osmosis systems are essentially composed of one or more semi-permeable membranes accommodated in specific vessels designed to support the operating pressure gradient in the system. This pressure value is normally obtained with a multi-impeller electric pump configured in a single stage or in series (for high pressure values), able to boost the pressure to 16-20 bar in fresh water systems, from 20 to 40 bar in brine systems, and from 40 to 60 bar in sea water systems.
The membranes used in R.O. plants are composed of several sheets of film pressed together (TFC thin film) and arranged in a spiral configuration around a plastic pipe (fig. 4).
In a reverse osmosis plant only a given percentage of the inlet water is used, normally between 60 and 80% for plants with pre-treatment, and between 40 and 50% in the absence of pretreatment. The remaining water is discharged in the form of concentrate.
The reverse osmosis process has significant benefits:
• Applicability to water with any level of saline contents, from well water to sea water.
• No need to dispose of any depleted chemical products after the process.
• Relatively low running costs compared to resin bed systems in the presence of high effective salinity.
• Straightforward management, given that the process does not require periodic regeneration, unlike resin bed systems.
The R.O. system cannot always be fed with untreated water, because certain elements in the water (free chlorine, turbidity, hardness, iron) would impair the osmotic process, sometimes causing irreversible damage to the semi-permeable membrane.