Reverse osmosis (RO) is a water purification technology that uses a
semipermeable membrane to remove ions, 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
Reverse osmosis can remove many types of dissolved and
suspended species from water, including bacteria, 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) 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. However, key
differences are found between reverse osmosis and filtration. The
predominant removal mechanism in membrane filtration is straining, or
size exclusion, so the process can theoretically achieve perfect
efficiency regardless of parameters such as the solution's pressure
Reverse osmosis also involves diffusion, making the
process dependent on pressure, flow rate, 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.
Fresh water applications
Drinking water purification
3.1.1 Military use: the reverse osmosis water purification unit
3.2 Water and wastewater purification
3.3 Food industry
Maple syrup production
3.5 Hydrogen production
3.6 Reef aquariums
3.7 Window cleaning
4 Landfill leachate purification
4.1 Power consumption for a disc tube module system
5.2 High pressure pump
5.3 Membrane assembly
5.4 Energy recovery
Remineralisation and pH adjustment
6.1 Waste stream considerations
7 New developments
8 See also
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
University of California at Los Angeles first investigated
desalination of seawater using semipermeable membranes. Researchers
University of California at Los Angeles
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
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
Reverse osmosis production train, North
Cape Coral Reverse Osmosis
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 3 million gallons (11350 m³) 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 15 million gallons per day (MGD) (56800 m³/d).
A semipermeable membrane coil used in desalination
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 water purification 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
optionally, a second sediment filter with smaller pores
an activated carbon filter to trap organic chemicals and chlorine,
which will attack and degrade thin film composite membrane reverse
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
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 (40 pounds per
square inch (280 kPa) 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
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 (4×10−9 to
2×10−4 in) depending on filter type. Particle filtration removes
particles of 1 μm (3.9×10−5 in) 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.
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
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
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 micrometres (0.00020 in) and eliminates almost all
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
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 125 US gallons (470 l) per hour. The Tactical Water
Purification Systems can be carried on a Medium Tactical Vehicle
Replacement truck, and can filter 1,200 to 1,500 US gallons (4,500 to
5,700 l) 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 underperformance of the boiler, bringing down its efficiency
and resulting in poor steam production, hence poor power production at
It is also used to clean effluent and brackish groundwater. The
effluent in larger volumes (more than 500 m3/d) 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
The process of reverse osmosis can be used for the production of
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.
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.
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
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.
For small-scale hydrogen production, reverse osmosis is sometimes used
to prevent formation of minerals on the surface of electrodes.
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.
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 be used two different
technology, one is using spiral wound membrane type of module, and for
high organic scaling, high conductivity and higher pressure (up to 90
bars) can be used disc tube module with reverse osmosis membranes.
Disc tube modules was redesigned for landfill leachate purification,
what usually is contaminated with high organics. Due to the cross-flow
with high velocity is given by a flow booster pump, what is
recirculating the flow over the same membrane surface between 1,5 and
3 times before is released as a concentrate. High velocity is also
good against membrane scaling and allows successful membrane
Power consumption for a disc tube module system
Disc tube module with RO membrane cushion and Spiral wound module with
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
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
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.
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
Veolia water, IDE Technologies, and Elran.
The typical single-pass sea water reverse osmosis system consists of:
High pressure pump (if not combined with energy recovery)
Energy recovery (if used)
Remineralisation and pH adjustment
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
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
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
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
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 225 to 376
psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they
range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). 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.
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
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 milligram/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 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.
Schematics of a reverse osmosis desalination system using a pressure
Sea water inflow,
Fresh water flow (40%),
3: Concentrate flow (60%),
Sea water flow (60%),
5: Concentrate (drain),
Pump flow (40%),B: Circulation pump,C:
Osmosis unit with
membrane,D: Pressure exchanger
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.
Schematic of a reverse osmosis desalination system using an energy
Sea water inflow (100%, 1 bar),
Sea water flow (100%, 50 bar),
3: Concentrate flow (60%, 48 bar),
Fresh water flow (40%, 1 bar),
5: Concentrate to drain (60%,1 bar),
A: Pressure recovery pump,B:
Osmosis unit with membrane
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.
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
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
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 freshwaters.
Post-treatment consists of preparing the water for distribution after
Reverse osmosis is an effective barrier to pathogens, but
post-treatment provides secondary protection against compromised
membranes and downstream problems. Disinfection by means of ultra
violet (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.
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 five gallons (19 L) of treated water per day may discharge
between 20 and 90 gallons (75–340 L) of waste water per day.
This is having disastrous consequence for mega cities like
large-scale use of household R.O. devices has increased the total
water demand of the already water parched National Capital Territory
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.
Due to its fine membrane construction, reverse osmosis not only
removes harmful contaminants present in the water, but it also may
remove many of the desirable minerals from the water. A number of
peer-reviewed studies have looked at the long-term health effects of
drinking demineralized water.
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.
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
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
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 (165 million U.S. gallons) a
day. It is also the cheapest and will sell water to the
authorities for USD $0.58/m³.
Reverse osmosis plant
Richard Stover, pioneered the development of an energy recovery device
currently in use in most seawater reverse osmosis desalination plants
Silt density index
^ a b c Warsinger, David M.; Tow, Emily W.; Nayar, Kishor G.;
Maswadeh, Laith A.; Lienhard V, John H. (2016). "Energy efficiency of
batch and semi-batch (CCRO) reverse osmosis desalination". Water
Research. pp. 272–282. doi:10.1016/j.watres.2016.09.029.
Missing or empty url= (help)
^ a b Crittenden, John; Trussell, Rhodes; Hand, David; Howe, Kerry and
Tchobanoglous, George (2005). Water Treatment Principles and Design,
2nd ed. John Wiley and Sons. New Jersey. ISBN 0-471-11018-3
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Wastewater Engineering. New York: McGraw-Hill
Dissolved air flotation
Solid phase extraction
API oil-water separator
Rapid sand filter
Rotary vacuum-drum filter
Vacuum ceramic filter
Aqueous two-phase system
Acid mine drainage
Adsorbable organic halides
Biochemical oxygen demand
Chemical oxygen demand
Total dissolved solids
Total suspended solids
Agricultural wastewater treatment
API oil-water separator
Decentralized wastewater system
Fecal sludge management
Industrial wastewater treatment
Rotating biological contactor
Sewage sludge treatment
Ultraviolet germicidal irradiation
Wastewater treatment plant
Septic drain field