Water of RO The process of osmosis

Water is the source of life. The
human demand for clean and available water is intense. With the rapid
development of the global economy, the shortage of water resources around the
world has become increasingly prominent. According to World Bank data, nearly
50 regions and countries will experience a severe shortage of water resources
by 2025, involving a population of 1.4 billion, mainly in underdeveloped areas.
By 2035, this size will continue to expand, it is estimated that nearly 3
billion people will not be able to protect the normal use of water1.
As we all know, most of the earth’s water is saline water, which can not be
used directly. Desalting saline water and purifying low-quality water are both
important for human.

Desalination is a technology converts saline water
into clean water, which offers important solutions to these problems2.
There are a lot of desalination technologies , which can produce freshwater
from seawater or brackish water using heat, an applied pressure or
Electrochemical techniques3, such as electrodialysis4,
thermal distillation processes5, capacitive deionization6,7,
battery electrode deionization8 and reverse osmosis9.
Among these technologies, reverse osmosis is the most widely used one10.

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Reverse osmosis is a water purification
technology using a semipermeable
membrane to
remove ions, molecules
and other larger particles from drinking water. In the reverse osmosis process,
applied pressure is used to overcome osmotic
pressure which is a colligative property driven by chemical potential differences of the
solvent. Reverse osmosis can remove many types of dissolved and suspended
species from water and is used in both industrial processes and the production
of drinking water. The result is that the solute is
retained on the pressurized side of the membrane and the pure solvent is
allowed to pass to the other side. To be “selective”, this membrane
should not allow large molecules or ions through the pores,
but smaller components like solvent molecules should be allowed to pass freely11.

2 History of RO

The process of osmosis through semipermeable membranes
was first observed  by Jean-Antoine
Nollet12in
1748. He inadvertently discovered that when an alcohol solution bottle was
covered with porcini, the water could gradually enter the bottle, opening the
door of osmosis. For the following 200 years, osmosis was only a phenomenon
observed in the laboratory.

In 1950, the University of California at Los
Angeles (UCLA) first investigated desalination of
seawater using semipermeable membranes. Researchers from UCLA and the University of Florida successfully
produced fresh water from seawater in the mid-1950s, but the flux was too low to
be commercially viable13 until the discovery at UCLA by Sidney Loeb14 and Srinivasa
Sourirajan at the National Research Council of Canada,
Ottawa, of techniques for making asymmetric membranes characterized by an
effectively thin “skin” layer supported atop a highly porous and much
thicker substrate region of the membrane. John Cadotte, of FilmTec Corporation, discovered that membranes
with particularly high flux and low salt passage could be made by interfacial polymerization of m-phenylene
diamine and trimesoyl chloride. Cadotte’s patent on this process15 was
the subject of litigation and has since expired. Almost all commercial reverse
osmosis membrane is now made by this method. By the end of 2001, about 15,200
desalination plants were in operation or in the planning stages, worldwide16.

3 Theory of RO

Osmosis is a natural process.
When two solutions with different concentrations of the solute are separated by
a semipermeable membrane, the solvent has a trend to move from low to high
solute concentrations to achieve chemical potential equilibration.

Formally, reverse osmosis is the process of forcing a
solvent from a region of high solute concentration through a semipermeable
membrane to a region of low solute concentration by applying a pressure in
excess of the osmotic pressure. The largest and most important application of
reverse osmosis is the separation of pure water from seawater and brackish waters;
seawater or brackish water is pressurized against one surface of the membrane,
causing transport of salt-depleted water across the membrane and emergence of
potable drinking water from the low-pressure side.This process requires that a
high pressure be exerted on the high concentration side of the membrane,
usually 0.2 – 1.7 MPa for fresh and brackish water, and 4 – 8.2 MPa for
seawater, which has around 2.7 MPa17 natural osmotic pressure
that must be overcome.

There are two major models of reverse osmosis.

3.1 Dissolution – diffusion model

Lonsdale et al.18-20 Proposed a
dissolution-diffusion model to explain the reverse osmosis phenomenon. He views
the reverse osmosis active surface cortex as a dense, nonporous membrane and
assumes both solute and solvent are soluble in the homogeneous non-porous
membrane surface layer, each diffusing through the membrane, driven by the
chemical potential due to concentration or pressure. Differences in solubility
and diffusivity of solutes and solvents in the membrane phase affect the amount
of energy they pass through the membrane. The specific process is divided into:
the first step, the solute and the solvent adsorbed and dissolved outside the
surface of the liquid side of the membrane; the second step, there is no
interaction between the solute and the solvent, they are in their respective
chemical potential difference to molecular diffusion Way through the reverse
osmosis membrane active layer; the third step, solute and solvent desorption on
the permeate side of the membrane.

In the above solute and solvent through the membrane
process, the general assumption that the first step and the third step is very
fast, then the rate of transmission depends on the second step, that solute and
solvent in the chemical potential difference under the promotion of molecules
Diffusion through the membrane. Due to the membrane selectivity, the gas
mixture or the liquid mixture is separated. The material’s ability to
penetrate, not only depends on the diffusion coefficient, but also on its
solubility in the membrane.

3.2 Preferential sorption – capillary flow mechanism

When
different kinds of substances are dissolved in the liquid, the surface tension
of the liquid will change differently. For example, dissolved in the water
alcohol, acid, aldehyde, grease and other organic substances, the surface
tension decreases, but with some inorganic salts, but the surface tension
increases slightly, this is because the solute dispersion is not uniform, i.e.
internal concentration and concentration of solute in the solution in the
surface layer, this is the solution of surface adsorption phenomenon. When the
water solution contacts with the polymer porous membrane, if the chemical
property of the membrane causes the membrane to adsorb the solute negatively,
and the water is the preferential positive adsorption, then a pure layer of
water with certain thickness will be formed on the interface between the
membrane and the solution. Under the external pressure, it will pass through
the fine pores on the surface of the membrane, so that the pure water can be
obtained21-23.

4 Developments and disadvantages

Since the 1970s, prefiltration of high-fouling waters
with another larger-pore membrane, with less hydraulic energy requirement, has
been evaluated and sometimes used. However, this means that the water passes
through two membranes and is often repressurized, which requires more energy to
be put into the system, and thus increases the cost.

Other recent developmental work has focused on
integrating reverse osmosis with electrodialysis24-26 to
improve recovery of valuable deionized products, or to minimize the volume of
concentrate requiring discharge or disposal27. In the production
of drinking water, the latest developments include nanoscale and graphene membranes28.

In recent years, energy consumption has dropped with
the development of more efficient energy recovery devices and improved membrane
materials. It has lower energy demands ranging from 0.5?2.5 kWh/m3 for
brackish water (<10 g/L) and 3?4 kWh/m3 for seawater (?35 g/L)29-31. The main disadvantage of reverse osmosis is that the flow of water through semipermeable membranes leads to rapid fouling, which decreases performance due to the need for frequent membrane cleaning32,33. 5 Applications of RO Reverse osmosis is best known for its use in desalination (removing the salt and other minerals from sea water to produce fresh water), but since the early 1970s, it has also been used to purify fresh water for medical, industrial, and domestic applications. First, it is widely used in potable water purification34. Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking. Portable reverse osmosis water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (saline water may need special membranes). Some travelers on long boating, fishing, or island camping trips, or in countries where the local water supply is polluted or substandard, use reverse osmosis water processors coupled with one or more ultraviolet sterilizers35. The reverse osmosis water purification unit is also used for military36,37.   In industry, reverse osmosis removes minerals from boiler water at power plants38. The water is distilled multiple times. It must be as pure as possible so it does not leave deposits on the machinery or cause corrosion. The deposits inside or outside the boiler tubes may result in underperformance of the boiler, bringing down its efficiency and resulting in poor steam production, hence poor power production at the turbine. It is also used to clean effluent and brackish groundwater39,40 as well as the production of deionized water. What' s more, In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids than conventional heat-treatment processes41. Research has been done on concentration of orange juice and tomato juice. Its advantages include a lower operating cost and the ability to avoid heat-treatment processes, which makes it suitable for heat-sensitive substances such as the protein and enzymes found in most food products. 6 Conclusion As the most widely used desalination technology, reverse osmosis will continue to play a very important role in our daily life, research area and industrial production. There are still many areas that require research scientists to overcome in the field of membrane materials and device structures. Reverse osmosis should be combined with other water desalination technology, to avoid weaknesses. Ref

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