A solar stilldistills water with substances dissolved in it by using the heat of the Sun
to evaporate water so that it may be cooled and collected, thereby
purifying it. They are used in areas where drinking water is
unavailable, so that clean water is obtained from dirty water or from
plants by exposing them to sunlight.
There are many types of solar still, including large scale concentrated solar stills and condensation traps (better known as moisture traps amongst survivalists).
In a solar still, impure water is contained outside the collector,
where it is evaporated by sunlight shining through clear plastic or
glass. The pure water vapour condenses on the cool inside surface and drips down, where it is collected and removed.
Distillation replicates the way nature makes rain. The sun's
energy heats water to the point of evaporation. As the water evaporates,
water vapour rises, condensing into water again as it cools and can
then be collected. This process leaves behind impurities, such as salts
and heavy metals, and eliminates microbiological organisms. The end
result is pure distilled drinking water.
History
Condensation traps have been in use since the pre-Incan peoples inhabited the Andes.
Today, a method for gathering water in moisture traps is still taught within the Argentinian Army
for use by specialist units expected to conduct extended patrols of
more than a week's duration in the arid border areas of the Andes.
Uses
Solar stills are used in cases where rain, piped, or well water is impractical, such as in remote homes or during power outages. In subtropical hurricane target areas that can lose power for days, solar distillation can provide an alternative source of clean water.
Solar Well
Methods
Several methods of trapping condensation exist:
First method
This
method was first used by the peoples of the Andes. A pit is dug into
the earth, at the bottom of which is placed the receptacle that will be
used to catch the condensed water. Small branches are placed with one of
their ends inside the receptacle and their other ends up over the edge
of the pit, forming a funnel to direct the condensed water into the
receptacle. A lid is then built over this funnel, using more small
branches, leaves, grasses, etc. The completed trap is left overnight,
and moisture can be collected from the receptacle in the morning.
This method relies on the formation of dew or frost
on the receptacle, funnel, and lid. Forming dew collects on and runs
down the outside of the funnel and into the receptacle. This water would
typically evaporate with the morning sun and thus vanish, but the lid traps the evaporating water and raises the humidity
within the trap, reducing the amount of water that is lost. The shade
produced by the lid also reduces the temperature within the trap, which
further reduces the rate of water loss to evaporation.
Modern method
Today, with the advent of plastic sheeting, the moisture trap has become more efficient.
The method is very similar to that described above, but a single
sheet of plastic is used instead of branches and leaves. The greater
efficiency of this type of trap arises from the waterproof nature of the
plastic, which doesn't let any water vapour pass through it (some water
vapour escapes through the leaves and branches of the first method).
This efficiency requires a certain amount of diligence of the part of
the user, in that the plastic sheet must be firmly attached to the
ground on all sides; this is often accomplished by using stones to
weight the sheet down and/or covering the edges of the plastic sheet
with earth (such as that dug out to make the hole in which the trap
sits). Weighting the centre of the plastic sheet down with a stone forms
the funnel via which the condensed water will run into the receptacle.
Transpiration method
Water can be obtained by placing clear plastic bags over the leafy branch of a non-poisonous tree and tightly closing the bag's open end around the branch. Any holes in the bag must be sealed to prevent the loss of water vapour.
During photosynthesis plants lose water through a process called transpiration. A clear plastic bag sealed around a branch allows photosynthesis to continue, but traps the evaporating water causing the vapour pressure
of water to rise to a point where it begins to condense on the surface
of the plastic bag. Gravity then causes the water to run to the lowest
part of the bag. Water is collected by tapping
the bag and then resealing it. The leaves will continue to produce
water as the roots draw it from the ground and photosynthesis occurs.
The vapour pressure of water in the sealed bag can rise so high
that the leaves can no longer transpire, consequently when using this
method, the water should be drained off every two hours and stored.
Tests indicate that if this is not done the leaves stop producing water.
If there are no large trees in the area, clumps of grass or small
bushes can be placed inside the bag. If this is done the foliage will
have to be replaced at regular intervals when water production is
reduced, particularly if the foliage must be uprooted to place it in the
bag.
Efficiency is greatest when the bag receives maximum sunshine at
all times. Exposed roots are tested for water content. Soft, pulpy roots
will yield the greatest amount of liquid for the least amount of
effort.
Condensation trap efficiency
Condensation
traps are not in themselves a sustainable source of water; they are
sources for extending or supplementing existing water sources or
supplies, and should not be relied on to provide a person's daily
requirement for water, since a trap measuring 400 mm (16 in) in diameter
by 300 mm (12 in) deep will only yield around 100 to 150 mL (3.4 to
5.1 US fl oz) per day.
One method to increase the water output is to urinate
into the pit before placing the receptacle in. This increases the
moisture content of the earth, reducing the amount of water vapour that
the earth can subsequently absorb.
Materials
A simple basin-type solar still can be constructed with 2–4 stones, plastic film or transparent glass,
a central weight to make a point and a container for the condensate. A
cubic hole in moist ground is created of about 300 mm (12 in) on each
side. Into the centre of this hole, a collection container is placed.
Then a sheet of plastic film is stretched over the hole. Stills can also
be made from water bottles or plastic bags.
Variations
Transpiration bag
An alternative method of the solar still is called the transpiration bag.
The bag is a simple plastic bag and it folds over a stemmed plant with a
corner pointing down to allow the condensate to pool. From there a
person can remove the water by taking the bag off and pouring the water
out or one can make a tiny incision into the corner to drip water into a
cup. Its advantage over the basin type solar still mentioned before is
that it only requires a bag like one can get at the grocery store. It
doesn't need to be completely transparent. A disadvantage of the
transpiration bag is the requirement for a plant in direct sunlight or
heat to take the condensate.
In a study performed in 2009,
variations to the angle of plastic and increasing the internal
temperature of the hole versus the outside temperature made for better
water production. Other methods used included using a brine to absorb
water from and adding dyes to the brine
to change the amount of solar radiation absorbed into the system.
During the adjusted tilt angle experiment, the different angles used by
the different researchers created different results and it was difficult
for any of them to get a definite answer. In the graph, a bell curve is
observed with the maximum water output being at 30 degrees angle
adjustment. Each brine depth created a different amount of water and it
is noted on the graph that about 25 millimetres (1 in) is optimal with a
decreasing trend if more is used.
Wick still
This image shows how a wick basin solar still works.
The “wick” type solar still is a glass-topped box constructed and held at angle to allow sunlight in.
Salt water poured in from the top is heated by sunlight, evaporating
the water. It condenses on the underside of the glass and drips to the
bottom. A pool of brine in the still is attached to the wicks which
separates the water into banks to increase surface area for heating. The
distilled water comes out of the bottom and, depending on the quality
of construction, most of the salt has been purged from the water. The
more wicks, the more heat can be transferred to the salt water and more
product can be made. A plastic net can also catch salt water before it
falls into the container and give it more time to heat up and separate
into brine and water. The wick type solar still is made vapour-tight, as
in the vapour does not escape to the atmosphere. To aid in absorbing
more heat, some wicks are blackened to take in more heat. Glass's
absorption of heat is negligible compared to plastic at higher
temperatures. A problem, depending on application, with glass is that it
is not flexible if the solar still is not a standard shape.
Practical considerations
The pit still may be inefficient as a survival still, requiring too much construction effort for the water produced.
In desert environments water needs can exceed 3.8 litres (1 US gal) per
day for a person at rest, while still production may average 240
millilitres (8 US fl oz) per day. Even with tools, digging a hole requires energy and makes a person lose water through perspiration; this means that even several days of water collection may not be equal to the water lost in its construction.
Seawater still
In
1952, the United States military developed a portable solar still for
pilots stranded on the ocean, which comprises an inflatable
610-millimetre (24 in) plastic ball that floats on the ocean, with a
flexible tube coming out the side. A separate plastic bag hangs from
attachment points on the outer bag. Seawater is poured into the inner
bag from an opening in the ball's neck. Fresh water is taken out by the
pilot using the side tube that leads to bottom of the inflatable ball.
It was stated in magazine articles that on a good day 2.4 litres
(2.5 US qt) of fresh water could be produced. On an overcast day, 1.4
litres (1.5 US qt) was produced. Similar sea water stills are included in some life raftsurvival kits, though manual reverse osmosis desalinators have mostly replaced them.
Distilling urine
Using a condensation trap to distill urine will remove the urea and salt, providing one with drinkable water as a result.
Solar desalination is a technique to produce water with a low salt concentration from sea-water or brine using solar energy. There are two common methods of solar desalination. Either using the direct heat from the sun or using electricity generated by solar cells to power a membrane process.
Methods
In
the direct method, a solar collector is coupled with a distilling
mechanism and the process is carried out in one simple cycle. Solar stills
of this type are described in survival guides, provided in marine
survival kits, and employed in many small desalination and distillation
plants. Water production by direct method solar distillation is
proportional to the area of the solar surface and incidence angle and
has an average estimated value of 3–4 litres per square metre
(0.074–0.098 US gal/sq ft).
Because of this proportionality and the relatively high cost of
property and material for construction direct method distillation tends
to favor plants with production capacities less than 200 m3/d (53,000 US gal/d).
Indirect solar desalination employs two separate systems; a solar collection array, consisting of photovoltaic and/or fluid based thermal collectors, and a separate conventional desalination plant.[2]
Production by indirect method is dependent on the efficiency of the
plant and the cost per unit produced is generally reduced by an increase
in scale. Many different plant arrangements have been theoretically
analyzed, experimentally tested and in some cases installed. They
include but are not limited to multiple-effect humidification (MEH), multi-stage flash distillation (MSF), multiple-effect distillation (MED), multiple-effect boiling (MEB), humidification–dehumidification (HDH), reverse osmosis (RO), and freeze-effect distillation.
Indirect solar desalination systems using photovoltaic (PV)
panels and reverse osmosis (RO) have been commercially available and in
use since 2009. Output by 2013 is up to 1,600 litres (420 US gal) per
hour per system, and 200 litres (53 US gal) per day per square metre of
PV panel.[5][6] Municipal-scale systems are planned.
Utirik Atoll in the Pacific Ocean has been supplied with fresh water this way since 2010.
Indirect solar desalination by a form of humidification/dehumidification is in use in the seawater greenhouse.
History
Methods
of solar distillation have been employed by humankind for thousands of
years. From early Greek mariners to Persian alchemists, this basic
technology has been utilized to produce both freshwater and medicinal
distillates. Solar stills were in fact the first method used on a large
scale to process contaminated water and convert it to a potable form.
In 1870 the first US patent was granted for a solar distillation device to Norman Wheeler and Walton Evans.
Two years later in Las Salinas, Chile, Charles Wilson, a Swedish
engineer, began building a direct method solar powered distillation
plant to supply freshwater to workers at a saltpeter and silver mine. It
operated continuously for 40 years and produced an average of 22.7 m3 of distilled water a day using the effluent from mining operations as its feed water.
Solar desalination of seawater and brackish groundwater in the
modern United States extends back to the early 1950s when Congress
passed the Conversion of Saline Water Act, which led to the
establishment of the Office of Saline Water (OSW) in 1955. The OSW's
main function was to administer funds for research and development of
desalination projects.
One of the five demonstration plants constructed was located in Daytona
Beach, Florida and devoted to exploring methods of solar distillation.
Many of the projects were aimed at solving water scarcity issues in remote desert and coastal communities.
In the 1960s and 1970s several modern solar distillations plants were
constructed on the Greek isles with capacities ranging from 2000 to 8500
m3/day.[3] In 1984 a MED plant was constructed in Abu-Dhabi with a capacity of 120 m3/day and is still in operation. In Italy, an open source design called "the Eliodomestico" by Gabriele Diamanti was developed for personal use at the building materials price of $50.
Of the estimated 22 million m3 of freshwater being produced a day through desalination processes worldwide, less than 1% is made using solar energy. The prevailing methods of desalination, MSF and RO, are energy intensive and rely heavily on fossil fuels.
Because of inexpensive methods of freshwater delivery and abundant low
cost energy resources, solar distillation has, up to this point, been
viewed as cost prohibitive and impractical.
It is estimated that desalination plants powered by conventional fuels
consume the equivalent of 203 million tons of fuel a year. With the approach (or passage) of peak oil
production, fossil fuel prices will continue to increase as those
resources decline; as a result solar energy will become a more
attractive alternative for achieving the world's desalination needs.
Types of solar desalination
There
are two primary means of achieving desalination using solar energy,
through a phase change by thermal input, or in a single phase through
mechanical separation.
Phase change (or multi-phase) can be accomplished by either direct or
indirect solar distillation. Single phase desalination is predominantly
accomplished in a solar-powered desalination unit,
which uses photovoltaic cells that produce electricity to drive pumps,
although there are experimental methods being researched using solar
thermal collection to provide this mechanical energy.
Multi-stage flash distillation (MSF)
Multi-stage flash distillation
is one of the predominant conventional phase-change methods of
achieving desalination. It accounts for roughly 45% of the total world
desalination capacity and 93% of all thermal methods.
Solar derivatives have been studied and in some cases implemented
in small and medium scale plants around the world. In Margarita de
Savoya, Italy there is a 50–60 m3/day MSF plant with a
salinity gradient solar pond providing its thermal energy and storage
capacity. In El Paso, Texas there is a similar project in operation that
produces 19 m3/day. In Kuwait a MSF facility has been built
using parabolic trough collectors to provide the necessary solar thermal
energy to produce 100 m3 of fresh water a day. And in Northern China there is an experimental, automatic, unmanned operation that uses 80 m2 of vacuum tube solar collectors coupled with a 1 kW wind turbine (to drive several small pumps) to produce 0.8 m3/day.
Production data shows that MSF solar distillation has an output capacity of 6-60 L/m2/day versus the 3-4 L/m2/day standard output of a solar still.
MSF experience very poor efficiency during start up or low energy
periods. In order to achieve the highest efficiency MSF requires
carefully controlled pressure drops across each stage and a steady
energy input. As a result, solar applications require some form of
thermal energy storage to deal with cloud interference, varying solar
patterns, night time operation, and seasonal changes in ambient air
temperature. As thermal energy storage capacity increases a more
continuous process can be achieved and production rates approach maximum
efficiency.
Problems with thermal systems
There
are two inherent design problems facing any thermal solar desalination
project. Firstly, the system's efficiency is governed by preferably
high heat and mass transfer rates during evaporation and condensation.
The surfaces have to be properly designed within the contradictory
objectives of heat transfer efficiency, economy, and reliability.
Secondly, the heat of condensation
is valuable because it takes large amounts of solar energy to evaporate
water and generate saturated, vapor-laden hot air. This energy is, by
definition, transferred to the condenser's surface during condensation.
With most forms of solar stills, this heat of condensation is ejected
from the system as waste heat. The challenge still existing in the
field today, is to achieve the optimum temperature difference between
the solar-generated vapor and the seawater-cooled condenser, maximal
reuse of the energy of condensation, and minimizing the asset
investment.
Solutions for thermal systems
Efficient
desalination systems use heat recovery to allow the same heat input to
provide several times the water than a simple evaporative process such
as solar stills.
One solution to the barrier presented by the high level of solar
energy required in solar desalination efforts is to reduce the pressure
within the reservoir. This can be accomplished using a vacuum pump, and
significantly decreases the temperature of heat energy required for
desalination. For example, water at a pressure of 0.1 atmospheres boils
at 50 °C (122 °F) rather than 100 °C (212 °F).
Solar humidification–dehumidification
The solar humidification–dehumidification (HDH) process (also called
the multiple-effect humidification–dehumidification process, solar multistage condensation evaporation cycle (SMCEC) or multiple-effect humidification (MEH) is a technique that mimics the natural water cycle on a shorter time frame by evaporating and condensing
water to separate it from other substances. The driving force in this
process is thermal solar energy to produce water vapor which is later
condensed in a separate chamber. In sophisticated systems, waste heat
is minimized by collecting the heat from the condensing water vapor and
pre-heating the incoming water source. This system is effective for
small- to mid- scale desalination systems in remote locations because of
the relative inexpensiveness of solar thermal collectors.
Single-phase solar desalination
In
indirect, or single phase, solar-powered desalination, two different
technological systems are combined: a solar energy collection system
(e.g. through the use of photovoltaic panels) and a proven desalination
system such as reverse osmosis, are combined. Single phase solar
desalination is predominantly accomplished by the use of photovoltaic
cells that produce electricity to drive pumps used for reverse osmosis
desalination. However, alternative experimental methods are being
researched, which use solar thermal collection to provide mechanical
energy to drive the reverse osmosis process.
Solar-powered reverse osmosis
In reverse osmosis
desalination systems, seawater pressure is raised above the natural
osmotic pressure, forcing pure water through membrane pores to the fresh
water side. Reverse osmosis (RO) is the most common desalination
process in terms of installed capacity due to its superior energy
efficiency compared to thermal desalination systems, despite requiring
extensive water pre-treatment. Furthermore, part of the consumed
mechanical energy can be reclaimed from the concentrated brine effluent
with an energy recovery device.
Solar-powered RO desalination is common in demonstration plants due to the modularity and scalability of both photovoltaic (PV) and RO systems. A detailed economic analysis and a thorough optimisation strategy
of PV powered RO desalination were carried out with favorable results
reported. Economic and reliability considerations are the main
challenges to improving PV powered RO desalination systems. However, the
quickly dropping PV panel costs are making solar-powered desalination
ever more feasible.
While the intermittent nature of sunlight and its variable
intensity throughout the day makes PV efficiency prediction difficult
and desalination during night time challenging, several solutions exist.
For example, batteries, which provide the energy required for
desalination in non-sunlight hours can be used to store solar energy in
daytime. Apart from the use of conventional batteries, alternative
methods for solar energy storage exist. For example, thermal energy storage
systems solve this storage problem and ensure constant performance even
during non-sunlight hours and cloudy days, improving overall
efficiency.
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex.
In most cases the term is used to denote the processes of purification,
separation, and decontamination of aqueous and other ion-containing
solutions with solid polymeric or mineralic "ion exchangers".
Typical ion exchangers are ion-exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soilhumus. Ion exchangers are either cation exchangers, which exchange positively charged ions (cations), or anion exchangers, which exchange negatively charged ions (anions). There are also amphoteric exchangers
that are able to exchange both cations and anions simultaneously.
However, the simultaneous exchange of cations and anions can be more
efficiently performed in mixed beds, which contain a mixture of
anion- and cation-exchange resins, or passing the treated solution
through several different ion-exchange materials.
Ion exchanges can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure.
This can be dependent on the size of the ions, their charge, or their
structure. Typical examples of ions that can bind to ion exchangers are:
Ion exchange is a reversible process, and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.
Applications
Ion
exchange is widely used in the food and beverage industry,
hydrometallurgy, metals finishing, chemical, petrochemical,
pharmaceutical technology, sugar and sweetener production, ground- and
potable-water treatment, nuclear, softening, industrial water treatment,
semiconductor, power, and many other industries.
Industrial and analytical ion-exchange chromatography is another area to be mentioned.
Ion-exchange chromatography is a chromatographical method that is widely used for chemical analysis and separation of ions. For example, in biochemistry it is widely used to separate charged molecules such as proteins. An important area of the application is extraction and purification of biologically produced substances such as proteins (amino acids) and DNA/RNA.
There are two series of rare-earth metals,
the lanthanides and the actinides, both of whose families all have very
similar chemical and physical properties. Using methods developed by Frank Spedding
in the 1940s, ion exchange processes were formerly the only practical
way to separate them in large quantities, until the development of the
"solvent extraction" techniques that can be scaled up enormously.
A very important case of ion-exchange is the PUREX process (Plutonium-URanium Extraction Process), which is used to separate the plutonium-239 and the uranium from americium, curium, neptunium, the radioactive fission products that come from nuclear reactors.
Thus the waste products can be separated out for disposal. Next, the
plutonium and uranium are available for making nuclear-energy materials,
such as new reactor fuel.
The ion-exchange process is also used to separate other sets of very similar chemical elements, such as zirconium and hafnium,
which is also very important for the nuclear industry. Physically,
zirconium is practically transparent to free neutrons, used in building
nuclear reactors, but hafnium is a very strong absorber of neutrons,
used in reactor control rods.
Thus, ion-exchange is used in nuclear reprocessing and the treatment of radioactive waste.
Large cation/anion ion exchangers used in water purification of boiler feedwater
Ion exchange can also be used to remove hardness from water by
exchanging calcium and magnesium ions for sodium ions in an ion-exchange
column. Liquid-phase (aqueous) ion-exchange desalination has been demonstrated. In this technique anions and cations in salt water are exchanged for carbonate anions and calcium cations respectively using electrophoresis. Calcium and carbonate ions then react to form calcium carbonate,
which then precipitates, leaving behind fresh water. The desalination
occurs at ambient temperature and pressure and requires no membranes or
solid ion exchangers. The theoretical energy efficiency of this method
is on par with electrodialysis and reverse osmosis.
Other applications
In soil science, cation-exchange capacity is the ion-exchange capacity of soil for positively charged ions. Soils can be considered as natural weak cation exchangers.
In pollution remediation and geotechnical engineering, ion-exchange capacity determines the swelling capacity of swelling or expansive clay such as montmorillonite, which can be used to "capture" pollutants and charged ions.
In planar waveguide manufacturing, ion exchange is used to create the guiding layer of higher index of refraction.
Most ion-exchange systems contain containers of ion-exchange resin that are operated on a cyclic basis.
During the filtration process, water flows through the resin
container until the resin is considered exhausted. That happened only
when water leaving the exchanger contains more than the desired maximal
concentration of the ions being removed. Resin is then regenerated by
sequentially backwashing the resin bed to remove accumulated solids,
flushing removed ions from the resin with a concentrated solution of
replacement ions, and rinsing the flushing solution from the resin.
Production of backwash, flushing, and rinsing wastewater during regeneration of ion-exchange media limits the usefulness of ion exchange for wastewater treatment.
Water softeners are usually regenerated with brine containing 10% sodium chloride.
Aside from the soluble chloride salts of divalent cations removed from
the softened water, softener regeneration wastewater contains the unused
50 – 70% of the sodium chloride regeneration flushing brine required to
reverse ion-exchange resin equilibria. Deionizing resin regeneration
with sulfuric acid and sodium hydroxide
is approximately 20–40% efficient. Neutralized deionizer regeneration
wastewater contains all of the removed ions plus 2.5–5 times their equivalent concentration as sodium sulfate.
Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water, and is used in both industrial processes and the production of potable 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 (holes), but should allow smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (high water potential),
through a membrane, to an area of high solute concentration (low water
potential). The driving force for the movement of the solvent is the
reduction in the free energy of the system when the difference in
solvent concentration on either side of a membrane is reduced,
generating osmotic pressure due to the solvent moving into the more
concentrated solution. Applying an external pressure to reverse the
natural flow of pure solvent, thus, is reverse osmosis. The process is
similar to other membrane technology applications.
Reverse osmosis differs from filtration in that the mechanism of
fluid flow is by osmosis across a membrane. The predominant removal
mechanism in membrane filtration is straining, or size exclusion, where
the pores are 0.01 micrometers or larger, so the process can
theoretically achieve perfect efficiency regardless of parameters such
as the solution's pressure and concentration. Reverse osmosis instead
involves solvent diffusion across a membrane that is either nonporous or
uses nanofiltration with pores 0.001 micrometers in size. The
predominant removal mechanism is from differences in solubility or
diffusivity, and the process is dependent on pressure, solute
concentration, and other conditions. Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules.
History
A process of osmosis through semipermeable membranes was first observed in 1748 by Jean-Antoine Nollet. For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1950, the University of California at Los Angeles first investigated desalination of seawater using semipermeable membranes. Researchers from both University of California at Los Angeles and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable until the discovery at University of California at Los Angeles by Sidney Loeb 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 process
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, worldwide.
Reverse osmosis production train, North Cape Coral Reverse Osmosis Plant
In 1977 Cape Coral,
Florida became the first municipality in the United States to use the
RO process on a large scale with an initial operating capacity of 11.35
million liters (3 million US gal) per day. By 1985, due to the rapid
growth in population of Cape Coral, the city had the largest
low-pressure reverse-osmosis plant in the world, capable of producing
56.8 million liters (15 million US gal) per day (MGD).
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.
The membranes used for reverse osmosis have a dense layer in the
polymer matrix—either the skin of an asymmetric membrane or an
interfacially polymerized layer within a thin-film-composite
membrane—where the separation occurs. In most cases, the membrane is
designed to allow only water to pass through this dense layer while
preventing the passage of solutes (such as salt ions). This process
requires that a high pressure be exerted on the high-concentration side
of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater, which has around 27 bar (390 psi) natural osmotic pressure that must be overcome. This process 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.
Fresh water applications
Drinking water purification
Around the world, household drinking waterpurification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.
Such systems typically include a number of steps:
a sediment filter to trap particles, including rust and calcium carbonate
optionally, a second sediment filter with smaller pores
a reverse osmosis filter, which is a thin film composite membrane
optionally, a second carbon filter to capture those chemicals not removed by the reverse osmosis membrane
optionally an ultraviolet lamp for sterilizing any microbes that may escape filtering by the reverse osmosis membrane
The latest developments in the sphere include nano materials and membranes.
In some systems, the carbon prefilter is omitted, and a cellulose triacetate
membrane is used. CTA (cellulose triacetate) is a paper by-product
membrane bonded to a synthetic layer and is made to allow contact with
chlorine in the water. These require a small amount of chlorine in the
water source to prevent bacteria from forming on it. The typical
rejection rate for CTA membranes is 85–95%.
The cellulose triacetate membrane is prone to rotting unless
protected by chlorinated water, while the thin film composite membrane
is prone to breaking down under the influence of chlorine. A thin film
composite (TFC) membrane is made of synthetic material, and requires
chlorine to be removed before the water enters the membrane. To protect
the TFC membrane elements from chlorine damage, carbon filters are used
as pre-treatment in all residential reverse osmosis systems. TFC
membranes have a higher rejection rate of 95–98% and a longer life than
CTA membranes.
Portable reverse osmosis water processors are sold for
personal water purification in various locations. To work effectively,
the water feeding to these units should be under some pressure (280 kPa
(40 psi) or greater is the norm).
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
sterilizers.
In the production of bottled mineral water, the water
passes through a reverse osmosis water processor to remove pollutants
and microorganisms. In European countries, though, such processing of
natural mineral water (as defined by a European directive)
is not allowed under European law. In practice, a fraction of the
living bacteria can and do pass through reverse osmosis membranes
through minor imperfections, or bypass the membrane entirely through
tiny leaks in surrounding seals. Thus, complete reverse osmosis systems
may include additional water treatment stages that use ultraviolet light
or ozone to prevent microbiological contamination.
Membrane pore sizes can vary from 0.1 to 5,000 nm depending on filter type. Particle filtration removes particles of 1 µm or larger. Microfiltration removes particles of 50 nm or larger. Ultrafiltration
removes particles of roughly 3 nm or larger. Nanofiltration removes
particles of 1 nm or larger. Reverse osmosis is in the final category of
membrane filtration, hyperfiltration, and removes particles larger than
0.1 nm.[11]
Decentralized use: solar-powered reverse osmosis
A solar-powered desalination unit produces potable water from saline water by using a photovoltaic
system that converts solar power into the required energy for reverse
osmosis. Due to the extensive availability of sunlight across different
geographies, solar-powered reverse osmosis lends itself well to drinking
water purification in remote settings lacking an electricity grid.
Moreover, Solar energy overcomes the usually high-energy operating costs as well as greenhouse emissions of
conventional reverse osmosis systems, making it a sustainable
freshwater solution compatible to developing contexts. For example, a
solar-powered desalination unit designed for remote communities has been
successfully tested in the Northern Territory of Australia.
While the intermittent nature of sunlight and its variable
intensity throughout the day makes PV efficiency prediction difficult
and desalination during night time challenging, several solutions exist.
For example, batteries, which provide the energy required for
desalination in non-sunlight hours can be used to store solar energy in
daytime. Apart from the use of conventional batteries, alternative
methods for solar energy storage exist. For example, thermal energy storage
systems solve this storage problem and ensure constant performance even
during non-sunlight hours and cloudy days, improving overall
efficiency.
Military use: the reverse osmosis water purification unit
A reverse osmosis water purification unit (ROWPU) is a portable, self-contained water treatment plant. Designed for military use, it can provide potable water from nearly any water source. There are many models in use by the United States armed forces and the Canadian Forces. Some models are containerized, some are trailers, and some are vehicles unto themselves.
Each branch of the United States armed forces has their own
series of reverse osmosis water purification unit models, but they are
all similar. The water is pumped from its raw source into the reverse
osmosis water purification unit module, where it is treated with a polymer to initiate coagulation.
Next, it is run through a multi-media filter where it undergoes primary
treatment by removing turbidity. It is then pumped through a cartridge
filter which is usually spiral-wound cotton. This process clarifies the
water of any particles larger than 5 µm and eliminates almost all turbidity.
The clarified water is then fed through a high-pressure piston
pump into a series of vessels where it is subject to reverse osmosis.
The product water is free of 90.00–99.98% of the raw water's total dissolved solids and by military standards, should have no more than 1000–1500 parts per million by measure of electrical conductivity. It is then disinfected with chlorine and stored for later use.
Within the United States Marine Corps,
the reverse osmosis water purification unit has been replaced by both
the Lightweight Water Purification System and Tactical Water
Purification Systems. The Lightweight Water Purification Systems can be transported by Humvee and filter 470 litres (120 US gal) per hour. The Tactical Water Purification Systems can be carried on a Medium Tactical Vehicle Replacement truck, and can filter 4,500 to 5,700 litres (1,200 to 1,500 US gal) per hour.
Water and wastewater purification
Rain
water collected from storm drains is purified with reverse osmosis
water processors and used for landscape irrigation and industrial
cooling in Los Angeles and other cities, as a solution to the problem of
water shortages.
In industry, reverse osmosis removes minerals from boiler water at power plants. 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 under-performance of the boiler,
reducing its efficiency and resulting in poor steam production, hence
poor power production at the turbine.
It is also used to clean effluent and brackish groundwater. The effluent in larger volumes (more than 500 m3/day)
should be treated in an effluent treatment plant first, and then the
clear effluent is subjected to reverse osmosis system. Treatment cost is
reduced significantly and membrane life of the reverse osmosis system
is increased.
The process of reverse osmosis can be used for the production of deionized water.
Reverse osmosis process for water purification does not require
thermal energy. Flow-through reverse osmosis systems can be regulated by
high-pressure pumps. The recovery of purified water depends upon
various factors, including membrane sizes, membrane pore size,
temperature, operating pressure, and membrane surface area.
In 2002, Singapore announced that a process named NEWater
would be a significant part of its future water plans. It involves
using reverse osmosis to treat domestic wastewater before discharging
the NEWater back into the reservoirs.
Food industry
In
addition to desalination, reverse osmosis is a more economical
operation for concentrating food liquids (such as fruit juices) than
conventional heat-treatment processes. 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.
Reverse osmosis is extensively used in the dairy industry for the
production of whey protein powders and for the concentration of milk to
reduce shipping costs. In whey applications, the whey (liquid remaining
after cheese manufacture) is concentrated with reverse osmosis from 6%
total solids to 10–20% total solids before ultrafiltration processing. The ultrafiltration retentate can then be used to make various whey powders, including whey protein isolate. Additionally, the ultrafiltration permeate, which contains lactose,
is concentrated by reverse osmosis from 5% total solids to 18–22% total
solids to reduce crystallization and drying costs of the lactose
powder.
Although use of the process was once avoided in the wine
industry, it is now widely understood and used. An estimated 60 reverse
osmosis machines were in use in Bordeaux, France, in 2002. Known users include many of the elite-classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.
Maple syrup production
In 1946, some maple syrup producers started using reverse osmosis to remove water from sap before the sap is boiled down to syrup.
The use of reverse osmosis allows about 75–90% of the water to be
removed from the sap, reducing energy consumption and exposure of the
syrup to high temperatures. Microbial contamination and degradation of
the membranes must be monitored.
Hydrogen production
For small-scale hydrogen production, reverse osmosis is sometimes used to prevent formation of mineral deposits on the surface of electrodes.
Aquariums
Many reef aquarium
keepers use reverse osmosis systems for their artificial mixture of
seawater. Ordinary tap water can contain excessive chlorine,
chloramines, copper, nitrates, nitrites, phosphates, silicates, or many
other chemicals detrimental to the sensitive organisms in a reef
environment. Contaminants such as nitrogen compounds and phosphates can
lead to excessive and unwanted algae growth. An effective combination of
both reverse osmosis and deionization
is the most popular among reef aquarium keepers, and is preferred above
other water purification processes due to the low cost of ownership and
minimal operating costs. Where chlorine and chloramines
are found in the water, carbon filtration is needed before the
membrane, as the common residential membrane used by reef keepers does
not cope with these compounds.
Freshwater aquarists also use reverse osmosis systems to
duplicate the very soft waters found in many tropical water bodies.
Whilst many tropical fish can survive in suitably treated tap water,
breeding can be impossible. Many aquatic shops sell containers of
reverse osmosis water for this purpose.
Window cleaning
An
increasingly popular method of cleaning windows is the so-called
"water-fed pole" system. Instead of washing the windows with detergent
in the conventional way, they are scrubbed with highly purified water,
typically containing less than 10 ppm dissolved solids, using a brush on
the end of a long pole which is wielded from ground level. Reverse
osmosis is commonly used to purify the water.
Landfill leachate purification
Treatment with reverse osmosis is limited, resulting in low recoveries on high concentration (measured with electrical conductivity)
and fouling of the RO membranes. Reverse osmosis applicability is
limited by conductivity, organics, and scaling inorganic elements such
as CaSO4, Si, Fe and Ba. Low organic scaling can use two different
technologies, one is using spiral wound membrane type of module, and for
high organic scaling, high conductivity and higher pressure (up to 90
bars) disc tube modules with reverse-osmosis membranes can be used. Disc
tube modules were redesigned for landfill leachate purification, that
is usually contaminated with high levels of organic material. Due to the
cross-flow with high velocity it is given a flow booster pump, that is
recirculating the flow over the same membrane surface between 1.5 and 3
times before it is released as a concentrate. High velocity is also good
against membrane scaling and allows successful membrane cleaning.
Power consumption for a disc tube module system
Disc tube module with RO membrane cushion and Spiral wound module with RO membrane
energy consumption per m3 leachate
name of module
1-stage up to 75 bar
2-stage up to 75 bar
3-stage up to 120 bar
disc tube module
6.1 – 8.1 kWh/m3
8.1 – 9.8 kWh/m3
11.2 – 14.3 kWh/m3
Desalination
Areas that have either no or limited surface water or groundwater may choose to desalinate. Reverse osmosis is an increasingly common method of desalination, because of its relatively low energy consumption.
In recent years, energy consumption has dropped to around 3 kWh/m3, with the development of more efficient energy recovery
devices and improved membrane materials. According to the International
Desalination Association, for 2011, reverse osmosis was used in 66% of
installed desalination capacity (0.0445 of 0.0674 km³/day), and nearly
all new plants. Other plants mainly use thermal distillation methods: multiple-effect distillation and multi-stage flash.
Sea-water reverse-osmosis (SWRO) desalination, a membrane
process, has been commercially used since the early 1970s. Its first
practical use was demonstrated by Sidney Loeb from University of California at Los Angeles in Coalinga, California,
and Srinivasa Sourirajan of National Research Council, Canada. Because
no heating or phase changes are needed, energy requirements are low,
around 3 kWh/m3, in comparison to other processes of
desalination, but are still much higher than those required for other
forms of water supply, including reverse osmosis treatment of
wastewater, at 0.1 to 1 kWh/m3. Up to 50% of the seawater
input can be recovered as fresh water, though lower recoveries may
reduce membrane fouling and energy consumption.
Brackish water reverse osmosis refers to desalination of water
with a lower salt content than sea water, usually from river estuaries
or saline wells. The process is substantially the same as sea water
reverse osmosis, but requires lower pressures and therefore less energy. Up to 80% of the feed water input can be recovered as fresh water, depending on feed salinity.
The Ashkelon sea water reverse osmosis desalination plant in Israel is the largest in the world. The project was developed as a build-operate-transfer by a consortium of three international companies: Veolia water, IDE Technologies, and Elran.
The typical single-pass sea water reverse osmosis system consists of:
Intake
Pretreatment
High-pressure pump (if not combined with energy recovery)
Pretreatment
is important when working with reverse osmosis and nanofiltration
membranes due to the nature of their spiral-wound design. The material
is engineered in such a fashion as to allow only one-way flow through
the system. As such, the spiral-wound design does not allow for
backpulsing with water or air agitation to scour its surface and remove
solids. Since accumulated material cannot be removed from the membrane
surface systems, they are highly susceptible to fouling (loss of
production capacity). Therefore, pretreatment is a necessity for any
reverse osmosis or nanofiltration system. Pretreatment in sea water
reverse osmosis systems has four major components:
Screening of solids: Solids within the water must be removed and
the water treated to prevent fouling of the membranes by fine-particle
or biological growth, and reduce the risk of damage to high-pressure
pump components.
Cartridge filtration: Generally, string-wound polypropylene filters are used to remove particles of 1–5 µm diameter.
Dosing: Oxidizing biocides, such as chlorine, are added to kill
bacteria, followed by bisulfite dosing to deactivate the chlorine, which
can destroy a thin-film composite membrane. There are also biofouling inhibitors, which do not kill bacteria, but simply prevent them from growing slime on the membrane surface and plant walls.
Prefiltration pH adjustment: If the pH, hardness and the alkalinity
in the feedwater result in a scaling tendency when they are concentrated
in the reject stream, acid is dosed to maintain carbonates in their
soluble carbonic acid form.
CO32− + H3O+ = HCO3− + H2O
HCO3− + H3O+ = H2CO3 + H2O
Carbonic acid cannot combine with calcium to form calcium carbonate
scale. Calcium carbonate scaling tendency is estimated using the
Langelier saturation index. Adding too much sulfuric acid to control
carbonate scales may result in calcium sulfate, barium sulfate, or
strontium sulfate scale formation on the reverse osmosis membrane.
Prefiltration antiscalants: Scale inhibitors (also known as
antiscalants) prevent formation of all scales compared to acid, which
can only prevent formation of calcium carbonate and calcium phosphate
scales. In addition to inhibiting carbonate and phosphate scales,
antiscalants inhibit sulfate and fluoride scales and disperse colloids
and metal oxides. Despite claims that antiscalants can inhibit silica
formation, no concrete evidence proves that silica polymerization can be
inhibited by antiscalants. Antiscalants can control acid-soluble scales
at a fraction of the dosage required to control the same scale using
sulfuric acid.
Some small-scale desalination units use 'beach wells'; they are
usually drilled on the seashore in close vicinity to the ocean. These
intake facilities are relatively simple to build and the seawater they
collect is pretreated via slow filtration through the subsurface
sand/seabed formations in the area of source water extraction. Raw
seawater collected using beach wells is often of better quality in terms
of solids, silt, oil and grease, natural organic contamination and
aquatic microorganisms, compared to open seawater intakes. Sometimes,
beach intakes may also yield source water of lower salinity.
High pressure pump
The high pressure pump
supplies the pressure needed to push water through the membrane, even
as the membrane rejects the passage of salt through it. Typical
pressures for brackish water
range from 1.6 to 2.6 MPa (225 to 376 psi). In the case of seawater,
they range from 5.5 to 8 MPa (800 to 1,180 psi). This requires a large
amount of energy. Where energy recovery is used, part of the high
pressure pump's work is done by the energy recovery device, reducing the
system energy inputs.
Membrane assembly
The layers of a membrane
The membrane assembly consists of a pressure vessel with a membrane
that allows feedwater to be pressed against it. The membrane must be
strong enough to withstand whatever pressure is applied against it.
Reverse-osmosis membranes are made in a variety of configurations, with
the two most common configurations being spiral-wound and hollow-fiber.
Only a part of the saline feed water pumped into the membrane
assembly passes through the membrane with the salt removed. The
remaining "concentrate" flow passes along the saline side of the
membrane to flush away the concentrated salt solution. The percentage of
desalinated water produced versus the saline water feed flow is known
as the "recovery ratio". This varies with the salinity of the feed water
and the system design parameters: typically 20% for small seawater
systems, 40% – 50% for larger seawater systems, and 80% – 85% for
brackish water. The concentrate flow is at typically only 3 bar / 50 psi
less than the feed pressure, and thus still carries much of the
high-pressure pump input energy.
The desalinated water purity is a function of the feed water
salinity, membrane selection and recovery ratio. To achieve higher
purity a second pass can be added which generally requires re-pumping.
Purity expressed as total dissolved solids
typically varies from 100 to 400 parts per million (ppm or mg/litre)on a
seawater feed. A level of 500 ppm is generally accepted as the upper
limit for drinking water, while the US Food and Drug Administration
classifies mineral water as water containing at least 250 ppm.
Energy recovery
Schematics of a reverse osmosis desalination system using a pressure exchanger. 1: Sea water inflow, 2: Fresh water flow (40%), 3: Concentrate flow (60%), 4: Sea water flow (60%), 5: Concentrate (drain), A: Pump flow (40%),B: Circulation pump,C: Osmosis unit with membrane,D: Pressure exchanger
Schematic of a reverse osmosis desalination system using an energy recovery pump. 1: Sea water inflow (100%, 1 bar), 2: Sea water flow (100%, 50 bar), 3: Concentrate flow (60%, 48 bar), 4: Fresh water flow (40%, 1 bar), 5: Concentrate to drain (60%,1 bar), A: Pressure recovery pump,B: Osmosis unit with membrane
Energy recovery can reduce energy consumption by 50% or more. Much of
the high pressure pump input energy can be recovered from the
concentrate flow, and the increasing efficiency of energy recovery
devices has greatly reduced the energy needs of reverse osmosis
desalination. Devices used, in order of invention, are:
Turbine or Pelton wheel:
a water turbine driven by the concentrate flow, connected to the high
pressure pump drive shaft to provide part of its input power. Positive
displacement axial piston motors have also been used in place of
turbines on smaller systems.
Turbocharger: a water turbine driven by the concentrate flow, directly connected to a centrifugal pump
which boosts the high pressure pump output pressure, reducing the
pressure needed from the high pressure pump and thereby its energy
input, similar in construction principle to car engine turbochargers.
Pressure exchanger:
using the pressurized concentrate flow, in direct contact or via a
piston, to pressurize part of the membrane feed flow to near concentrate
flow pressure. A boost pump then raises this pressure by typically
3 bar / 50 psi to the membrane feed pressure. This reduces flow needed
from the high-pressure pump by an amount equal to the concentrate flow,
typically 60%, and thereby its energy input. These are widely used on
larger low-energy systems. They are capable of 3 kWh/m3 or less energy consumption.
Energy-recovery pump: a reciprocating piston pump having the
pressurized concentrate flow applied to one side of each piston to help
drive the membrane feed flow from the opposite side. These are the
simplest energy recovery devices to apply, combining the high pressure
pump and energy recovery in a single self-regulating unit. These are
widely used on smaller low-energy systems. They are capable of 3 kWh/m3 or less energy consumption.
Batch operation: Reverse-osmosis systems run with a fixed volume of fluid (thermodynamically a closed system)
do not suffer from wasted energy in the brine stream, as the energy to
pressurize a virtually incompressible fluid (water) is negligible. Such
systems have the potential to reach second-law efficiencies of 60%.
Remineralisation and pH adjustment
The desalinated water is stabilized to protect downstream pipelines and storage, usually by adding lime or caustic soda
to prevent corrosion of concrete-lined surfaces. Liming material is
used to adjust pH between 6.8 and 8.1 to meet the potable water
specifications, primarily for effective disinfection and for corrosion
control. Remineralisation may be needed to replace minerals removed from
the water by desalination. Although this process has proved to be
costly and not very convenient if it is intended to meet mineral demand
by humans and plants. The very same mineral demand that freshwater
sources provided previously. For instance water from Israel's national
water carrier typically contains dissolved magnesium levels of 20 to
25 mg/liter, while water from the Ashkelon plant has no magnesium. After
farmers used this water, magnesium-deficiency symptoms appeared in
crops, including tomatoes, basil, and flowers, and had to be remedied by
fertilization. Current Israeli drinking water standards set a minimum
calcium level of 20 mg/liter. The postdesalination treatment in the
Ashkelon plant uses sulfuric acid to dissolve calcite (limestone),
resulting in calcium
concentration of 40 to 46 mg/liter. This is still lower than the 45 to
60 mg/liter found in typical Israeli fresh water.
Disinfection
Post-treatment
consists of preparing the water for distribution after filtration.
Reverse osmosis is an effective barrier to pathogens, but post-treatment
provides secondary protection against compromised membranes and
downstream problems. Disinfection by means of ultraviolet
(UV) lamps (sometimes called germicidal or bactericidal) may be
employed to sterilize pathogens which bypassed the reverse-osmosis
process. Chlorination or chloramination
(chlorine and ammonia) protects against pathogens which may have lodged
in the distribution system downstream, such as from new construction,
backwash, compromised pipes, etc.
Disadvantages
Household
reverse-osmosis units use a lot of water because they have low back
pressure. As a result, they recover only 5 to 15% of the water entering
the system. The remainder is discharged as waste water. Because waste
water carries with it the rejected contaminants, methods to recover this
water are not practical for household systems. Wastewater is typically
connected to the house drains and will add to the load on the household
septic system. A reverse-osmosis unit delivering 19 L of treated water
per day may discharge between 75–340 L of waste water daily. This has a disastrous consequence for mega cities like Delhi
where large-scale use of household R.O. devices has increased the total
water demand of the already water parched National Capital Territory of
India.
Large-scale industrial/municipal systems recover typically 75% to
80% of the feed water, or as high as 90%, because they can generate the
high pressure needed for higher recovery reverse osmosis filtration. On
the other hand, as recovery of wastewater increases in commercial
operations, effective contaminant removal rates tend to become reduced,
as evidenced by product water total dissolved solids levels.
Reverse osmosis per its construction removes both harmful
contaminants present in the water, as well as some desirable minerals.
Modern studies on this matter have been quite shallow, citing lack of
funding and interest in such study, as re-mineralization on the
treatment plants today is done to prevent pipeline corrosion without
going into human health aspect. They do, however link to older, more
thorough studies that at one hand show some relation between long-term
health effects and consumption of water low on calcium and magnesium, on
the other confess that none of these older studies comply to modern
standards of research.
Waste-stream considerations
Depending
upon the desired product, either the solvent or solute stream of
reverse osmosis will be waste. For food concentration applications, the
concentrated solute stream is the product and the solvent stream is
waste. For water treatment applications, the solvent stream is purified
water and the solute stream is concentrated waste. The solvent waste stream from food processing may be used as reclaimed water, but there may be fewer options for disposal of a concentrated waste solute stream. Ships may use marine dumping and coastal desalination plants typically use marine outfalls. Landlocked reverse osmosis plants may require evaporation ponds or injection wells to avoid polluting groundwater or surface runoff.
New developments
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 electrodialysis to improve recovery of valuable deionized products, or to minimize the volume of concentrate requiring discharge or disposal.
In the production of drinking water, the latest developments include nanoscale and graphene membranes.
The world's largest RO desalination plant was built in Sorek, Israel, in 2013. It has an output of 624,000 m3 a day. It is also the cheapest and will sell water to the authorities for US$0.58/m3.
