ABSTRACT
The lack of clean drinking water is a
problem that plagues many areas of the world today. Approximately 884
million people suffer each day from insufficient quantities of clean, drinkable
water. Most of the current technologies available to combat this problem
are expensive and consume too much power to be effective in rural regions of
the planet. The solutions that do not consume an excess of power
generally require expensive and time consuming filter maintenance. The
use of chemical processing mechanisms of purification is an affordable
solution, but it has been known to be hazardous if used improperly. With
the idea of low cost and sustainability in mind, we plan to develop a water
filtration system that will take advantage of natural energy in order to power
a water purification system. Water will enter the system, where it will
flow through a sediment filter and then be processed by a UV purifier. By
using both standard sediment filtration and ultraviolet radiation purification
techniques, our goal is to produce water with a total concentration of less
than 0.01% Coliform bacteria. In order to save on energy and cut costs, water
will enter the system manually and use gravity to pull it through the system,
eliminating the need for a pump. We plan to use photovoltaic technology
to transform sun rays into electric potential that will be stored in a battery
backup system. This battery system will power the ultraviolet
purification process of the system.Solar-driven
reverse osmosis desalination can potentially break the dependence of
conventional desalination on fossil fuels, reduce operational costs, and
improve environmental sustainability. The experience with solar desalination is
investigated based on the analysis of 79 experimental and design systems
worldwide. Our results show that photovoltaic-powered reverse osmosis is
technically mature and — at unit costs as low as 2–3 US$ m–3 — economically
cost-competitive with other water supply sources for small-scale systems in
remote areas. Under favourable conditions, hybrid systems with additional
renewable or conventional power sources perform as good as or better than
photovoltaic-powered reverse osmosis. We suggest that in the short-term, solar
RO desalination will gain shares in the market of small-scale desalination in
remote areas. Concentrating solar power technologies have the highest potential
in the medium-term for breakthrough developments in large-scale solar
desalination.
INTRODUCTION
Many areas worldwide that suffer from severe
water scarcities are increasingly dependent on desalination as a highly
reliable, non-conventional source of freshwater. Desalination markets have
greatly expanded in recent decades and they are expected to continue expanding
in the coming years, particularly in the Mediterranean, Middle East and North
African (MENA) regions . Among desalination technologies, reverse osmosis (RO)
is rapidly overtaking thermal desalination in terms of market shares . A
pressure-driven process that relies on the properties of semi-permeable
membranes to separate water from a saline feed, the end result of reverse
osmosis comprises the separate flows of freshwater permeate and concentrated
brine. System flow rate is proportional to the difference between the applied
pressure and the osmotic pressure differential between brine and dilute
compartments. Commercially available RO membranes can retain about 98–99.5% of
the salt dissolved in the feed water and typical operating pressures range
between 10 and 15 bar for brackish water and between 55 and 65 bar for seawater
. The amount of freshwater that can be recovered from the feed is limited by
membrane fouling and scaling. Overall water recovery rates are typically 45–50%
for seawater RO systems, and they can be as high as 90% in brackish water
desalination systems . The coupling of reverse osmosis desalination with solar
energy is a promising field of development in the desalination sector, with the
potential to (i) improve its sustainability by minimizing or completely
eliminating the dependence on fossil fuels and (ii) significantly reduce the
operational costs of desalination plants. Despite a steady reduction in the
energy consumption of pressuredriven membrane processes in recent decades,
energy consumption is still a major cost component of RO desalination plants,
accounting for 40–45% of total costs . This paper provides an extensive
assessment of the experience gathered from solar-powered RO desalination. The
prospects for commercial penetration and for further development of the principal
technological solutions are identified and discussed.
REVERSE OSMOSIS
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 parameter.
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 and
concentration. Reverse osmosis also involves diffusion, making the process
dependent on pressure, flow rate, and other conditions.[1] 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
The 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[2] until
the discovery at University of California at Los Angeles by Sidney Loeb[3] 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 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).
PROCESS
Osmosis is a natural process. When two solutions
with different concentrations of a solute are separated by a semipermeable
membrane, the solvent has a tendency to move from low to high solute
concentrations for chemical potential equilibration.
Formally, reverse osmosis is the process of forcing
a solvent from a region of high solute concentration through a semipermeable
membrane to a region of low solute concentration by applying a pressure in
excess of the osmotic pressure. The largest and most important application of
reverse osmosis is the separation of pure water from seawater and brackish waters; seawater or brackish water is
pressurized against one surface of the membrane, causing transport of
salt-depleted water across the membrane and emergence of potable drinking water
from the low-pressure side.
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
get 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
The reverse osmosis water filter process
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 carbonate
·
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
osmosis membranes
·
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 (40 pounds per square inch (280 kPa) or greater is
the norm).[7] 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 (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
_Service_Support_Group_31,_prepares_a_chlorine_solution_for_water.jpg)
United States Marines from Combat Logistics
Battalion 31 operate reverse
osmosis water purification units for
relief efforts after the 2006 Southern
Leyte mudslide
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 micrometres
(0.00020 in) 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 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 the turbine.
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 increased.
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.[13] 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.
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 (44.5 of 67.4 Mm3/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)
·
Membrane assembly
·
Energy recovery
(if used)
·
Disinfection
·
Alarm/control
panel
PRETREATMENT
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.[20]
·
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
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 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.
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
freshwaters.
Photovoltaics
Photovoltaics (PV)
is a term which covers the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect,
a phenomenon studied in physics, photochemistry, and electrochemistry.
A typical photovoltaic system employs solar panels, each comprising a number of solar
cells, which generate electrical
power. PV installations may be ground-mounted, rooftop mounted or wall mounted. The mount may be
fixed, or use a solar
tracker to follow the sun across
the sky.
Solar PV has specific advantages as an energy
source: its operation generates no pollution and no greenhouse gas
emissions once installed, it shows simple scalability in respect of power needs
and silicon has large availability in the Earth’s crust
Photovoltaic-powered
RO desalination The design option that has been implemented most frequently
in solar-driven RO desalination systems is a combination of RO membranes and
arrays of photovoltaic (PV) modules. The wide use of the latter is probably
because photovoltaics were the first widely commercialized technology for
exploiting solar energy. Indeed, at the time of writing, PV panels still
dominate the solar technology market, and among the renewable energies, they
constitute the fastest growing market. In PV–RO desalination, the direct
current (DC) electrical energy generated in the solar cells by silicon or other
semi-conductors is used—directly or after regulation—to power the pumps that
generate the pressure required for the feed water to permeate the RO membranes.
Despite the many technological improvements of recent years, however, the
conversion efficiencies of PV modules remain low, rarely exceeding 15–16%. In
addition to such low efficiencies, the retail price for PV modules, which
currently stands at 4.83 US$ and 277.30 Rs per Watt peak (Wp) in the US and
Indian markets, respectively, make solar sub-unit cost a key factor in the
economic feasibility of PV–RO desalination. PV–RO technology was implemented for the desalination of
both brackish water and seawater (29 and 16 systems in Table 1 respectively).
The production flow of experimental units is small, ranging from less than 0.1
m3 d–1 to 75.7 m3 d–1 , the ratio
between the installed PV capacity and the production flow ranges between 0.1
and 5.5 m3 d– 1 kWp –1. Although vast experience has been accrued with PV–RO
system design since 1978 , a standard design approach has thus far not emerged.
(Design solutions that either include or omit battery storage, energy
inverters, and other features will be discussed in detail later in this
section.) Despite the lack of standardization, however, several components are
common to all design approaches, and they can be used to create a simplified,
general design scheme for PV–RO desalination systems (Fig. 2): • Solar sub-unit
(PV modules). Both mono-crystalline and multi-crystalline silicon modules were
used in experimental units. Whether module orientation was fixed or adjustable
was recognised as an important factor in determining the electrical power output
and thus the overall performance of the desalination plant. While modules with
fixed axes are tilted at a constant angle, modules with adjustable axes can be
manually repositioned based on seasonal changes, or, if a tracking system with
controller and drive motor is installed, the modules can automatically follow
the sun’s daily path in the sky. Alawaji et al.
estimated that utilizing seasonal tilt angle varia- estimated that
utilizing seasonal tilt angle variation increases the yearly average permeate flow
of a PV-RO desalination plant in Saudi Arabia from 15 to 17 m3 d–1. For a 0.1
m3 d–1 PV-RO testing rig in Jordan, Abdallah et al. measured gains in
electrical power output and permeate flow of 25% and 15%, respectively, when a
one-axis automatic tracking system was used rather than a fixed tilt plate.
Harrison et al determined that tracking solar arrays produced a 60% higher
permeate flow than a fixed array in a small desalinator with a capacity of 0.05
m3 d–1. The high initial investment costs required to install tracking systems,
however, have so far limited their use in PV-RO desalination. Maximum power
point tracker (MPPT) circuits or similar optimisers are generally installed to maintain system
operation at a voltage that achieves maximum power while ensuring efficiency
under conditions of low irradiance. • Water extraction unit. The pump(s) that
convey the feed water from the seawater intake or groundwater well to the RO
pretreatment may be powered either by the arrays of the PV modules of the RO unit,
or by other power sources such as wind turbines
or conventional grid electricity or a combination of the two. Solar
pumps — the solar sub-unit powers the feed pump(s) — were reported to be highly
reliable in remote locations and to require limited maintenance, and as such,
they have been used frequently.Pretreatment unit. Conventional RO pretreatment
is generally implemented. The main filter barrier typically has a pore size of
5 μm and is preceded by a coarser filter with pore sizes of 20–25 μm or larger.
Active carbon filtration follows for the removal of free chlorine, which can
damage the RO membranes. Where bacterial counts in the feed water are high,
disinfection by ozonation or
chlorination are used to protect the membranes from biofouling. The experience
with ultrafiltration (UF) as a pretreatment step was limited to several
experimental tests performed in Australia with different kinds of brackish
groundwater. UF pretreatment involves higher investment costs than conventional
pretreatment, but because it removes significant numbers of microorganisms and
generally delivers higher quality RO feed, which eliminates the need for
membrane disinfection, UF pretreatment may reduce RO membrane cleaning
Simplified
general design scheme of a PV–RO desalination plant. Dashed lines identify components and connections that may be
absent and replacement costs. Chemical pretreatment with antiscalants is
frequently implemented to reduce the risk of membrane surface scaling.
Alternatively, the plant may be operated at low recovery rates to prolong
membrane viability . .High-pressure pump and motor. As a rule, positive
displacement pumps are used because of their higher energy efficiencies — with
respect to centrifugal pumps — at low flows. Both rotary positive displacement
pumps (e.g., rotary vane and progressive cavity pumps) and reciprocating pumps
(e.g., piston and diaphragm pumps ) were
used. The Clark pump, a reciprocating pump that was specifically developed for
energy recovery in small desalination systems and that was used in several
PV–RO applications in combination with reciprocating plunger pumps and rotary vane pumps for seawater desalination, was shown to significantly
reduce energy consumption. For the desalination of brackish water, systems
using rotary pumps have the lowest energy consumption. Specific energy
consumptions (SEC) as low as 1.4 kWh m–3 were reported both for rotary vane
pumps (influent TDS = 3,480 mgL–1; Dankoff Solar Slow pump; ) and for
progressive cavity pumps (influent TDS = 5,300 mgL–1; custom-designed
MonoPumps; ). SEC values for systems using reciprocating pumps, however, were
only available for outdated units (SEC = 6.9 kWh m–3; influent TDS = 3,000
mgL–1;) and small prototypes (SEC = 29.1 kWh m–3; influent TDS = 2,137 mgL–1;
). Pump motors are powered with either direct current (DC) or alternating
current (AC). In the latter case, since both PV arrays and batteries produce
DC, a current inverter is required. • Reverse osmosis membranes. Spiral-wound,
thin film composite RO membranes are the standard choice for PV–RO desalination
systems. The most common RO configuration is single pass, in which the
membranes are organised in series within one or more pressure vessels.
Concentrate recirculation was used in some brackish water desalination
installations to increase the overall water recovery rate and reduce brine
disposal issues . PV–RO desalination systems are often designed with generous
membrane areas since, for a fixed recovery rate, they can operate at lower pressures
and thus at higher energy efficiencies. Large membrane areas, however,
introduce a trade-off with permeate quality, which decreases as operating
pressure increases. Nanofiltration membranes were suggested as a cost-effective
alternative to reverse osmosis in brackish water solar desalination due to their lower operating pressures and
energy requirements, but no study thus far has monitored the continuous
operation of a nanofiltration solar desalination plant. In addition to the
basic components of PV–RO desalination plants, a series of other elements may
also be present in such systems. These include: • AC/DC inverter. Desalination
plants that use AC induction motors for the high pressure pumps require
inverters to transform the DC current generated in the PV modules or stored in
the batteries. The use of DC motors eliminates the need for inverters but
generally involves a higher initial investment. Since DC motors do not
experience the energetic losses inherent in inverters, PV–RO desalination
plants with DC motors are expected to function at higher energy efficiencies .
In a study conducted on a 6 m3 d–1 brackish water PV–RO desalination system, however,
de Carvalho et al. experienced steadier operation and significantly lower
energy consumption (3 kWh m–3 vs. 4.7 kWh m–3) after replacing a DC motor with
an AC induction motor. Systems with DC motors are also more reliable compared
to systems with inverters, whose failures are frequently related to the
inverter overheating during plant operation or overloading when the motors in
systems with more than one pump in the RO unit and no soft-start features are
installed. Electrical storage. Batteries can be included in the system either
to balance the electrical output of the PV modules during day-time operation or
to provide extended operation during night-time and overcast days. Although
electrical storage enables steady plant operation and may increase overall
productivity, it entails a series of drawbacks: (i) Installation and
replacement add significantly to the investment cost of the plant. (ii)
Batteries imply additional losses of electricity and reduce system efficiency.
(iii) When all auxiliary components such as charge controller and wiring are
considered, the inclusion of batteries in the system results in a more complex
system. (iv) The absence of careful maintenance typical in remotely located
systems may dramatically reduce battery life, particularly for large storage
batteries �[ �]
. Batteryless PV–RO systems are based on the idea that water storage is often
more efficient and cost-effective than energy storage. These systems are
operated either at fixed or variable capacity. In the former, all radiation
below the threshold value for start-up of the high pressure pump is dropped and
the desalination plant works only during peak radiation hours (generally 5–8 h,
depending on the local meteorological conditions). Systems operating at
variable capacity achieve higher performance and flexibility by including speed
control systems on the pumps and electronic power converters. The technical
feasibility and shortterm operation of variable speed, batteryless PV–RO
systems was tested in a series of studies, but long-term performance has not
been monitored. RO membrane biofouling can prevent the long-term operation of
such systems. The hot climate typical of the regions where PV-RO systems are
implemented promotes biofouling when the plant is not operating. Automatic
shut-down devices and membrane cleaning systems are usually installed in such
systems so that during periods of low solar radiation, the pump is shut off,
thereby reducing the potential for membrane biofouling . The recirculation
process used for membrane cleaning can be gravity-driven, rely on the high
pressure pump, or on a dedicated flushing pump. Timing of membrane flushing is
crucial: recirculation should be activated while there is still enough
radiation to power the flushing pump, but limiting to a minimum the waste of
radiation that could be used for the desalination process. For this purpose,
ITN designed an electronic circuit that
initiates the shut-down cycle based on the current from a separate, small solar
cell. Adjustable delay may be built-in to avoid repeated shut-down cycles that
may be induced by passing clouds. • Energy recovery device. With the
development of suitable devices for implementation in small-scale units, the
use of energy recovery devices in seawater PV–RO desalination is rapidly
becoming standard practice. Pelton turbines were used in early systems. More
recently devices that are more efficient at low flows were developed, such as
Clark pumps, hydraulic motors, energy recovery pumps , and pressure exchangers.
Studies comparing different recovery mechanisms applied to the same PV-RO
system reached different conclusions, possibly indicating that the choice of
the most efficient energy recovery device is system-specific. Only a lim- Only
a limited number of studies investigated the use of energy recovery devices in
brackish water desalination since low concentrate pressure and high water
recovery rates make energy recovery less critical in such systems.
FUTURE PROSPECTS FOR SOLAR RO DESALINATION
In
this section, we briefly highlight the main technological advancements that are
envisaged in the solar power and RO units and in the integration of the two,
all of which may help improve the competitiveness of solar RO desalination in
the coming years.
SOLAR UNIT TECHNOLOGIES
Although the level of technological
development of PV–RO desalination plants has allowed their commercialization
[9,49], market penetration has thus far been small, mainly due to the high
investment costs for the PV modules. Research in the field of PV modules,
however, is developing rapidly, which seems to offer hope that significant cost
reductions can be expected in the short-medium term. Promising lines of
research are the exploration of the properties of both crystalline and
amorphous silicon and of other semi-conductors such as cadmium telluride and
copper indium gallium diselenide for application in thin-film cells [2], and
the development of concentrating PV systems [84]. The development of CSP
technologies, on the other hand, will be crucial in determining whether solar
desalination will become attractive for large-scale desalination systems. Previous
research has estimated that very large concentrating solar thermal desalination
units up to 100,000 m3 d–1 have the medium-term potential to achieve a cost of
water below 0.55 US$ m–3 (0.40 € m–3) and to become the lowest cost option for
desalted water in the MENA region [66].
RO UNIT DESIGN AND OPERATION
Advanced
membrane pretreatment by ultrafiltration or nanofiltration can potentially
reduce fouling and scaling of the RO membranes and thus decrease energy
consumption and overall costs [2]. Nanofiltration membranes may also be
applicable in the solar-powered desalination of brackish water due to their
lower operating pressures and energy requirements [31,59]. Finally, the
develop- [31,59]. Finally, the develop- . Finally, the development of mechanisms
to automatically control the water recovery rate in solar RO desalination
systems can help promote the development of energy efficient systems that will
require only minimal maintenance .
SOLAR
AND RO UNIT INTEGRATION
First,
to date, the full potential of dual-purpose designs for the co-generation of
power and water has not been sufficiently explored. Such designs have a
potential to operate at low water generating costs and levelized electricity
costs [66]. Second, an operation strategy with the potential to increase both
energy efficiency and 294 A. Ghermandi, R. Messalem / Desalination and Water
Treatment 7 (2009) 285–296 permeate water production comprises preheating the
RO feed by cooling the PV panels or absorbing rejected thermal energy in solar
thermal power systems [31,72]. Third, improvements in the durability of
batteries may eliminate the current maintenance and replacement issues and make
energy storage attractive for long daily operation.
Conclusions
This
paper investigated the main technological solutions and current developments in
the field of solarpowered RO desalination on the basis of the analysis of 79
experimental and design units worldwide. The study prompted the following
conclusions:
•
PV-powered RO desalination is mature for commercial implementation. Although no
standard design approach has been developed, the technical feasibility of
different design concepts was demonstrated in a relatively large number of case
studies. State-of-theart, batteryless systems that directly couple the PV
modules to variable speed DC pump motors seem to have the highest potential for
energy efficient and cost-effective small-scale PV–RO desalination. The
long-term performance and reliability of such systems was, however, not
sufficiently tested yet.
•
CSP–RO desalination is the most promising field of development for medium and
large-scale solar desalination. Preliminary design studies suggest that CSP–RO
systems may compete in the medium-term with conventional RO desalination and as
a result, gain large market shares. There is a need for testing such promising
potential in demonstration and fullscale plants.
•
The combination of solar power with additional power sources may be beneficial
both in small and large-scale desalination. In small-scale systems, PV panels may combine favourably with
wind turbines, achieving lower overall costs where the complementary aspects of
the two renewable sources can be exploited. In large-scale systems, CSP and
fuel co-firing of desalination plants help confer stability on desalination
unit operation during the night-time or periods of low irradiation. In both
small and large-scale systems, connection to the electrical grid for combined
power and water generation will promote stable operation of the system.
State-of-the-art
solar RO desalination is cost-competitive with other water supply sources only
in context of remote regions (e.g., islands and remote inland areas) where grid
electricity is not available and freshwater demand is met by water imports
or—taking into account surging oil prices—small-scale fuel-driven desalination
plants. In this market, the share of PV–RO and hybrid PV–RO desalination plants
is likely to increase in the near future. A penetration of solar RO
desalination in other markets seems unlikely in the short-term. The rapid
advancements of both CSP and PV solar technologies offer the best hope for the
wider implementation of these potentially sustainable water supply technologies
in the future
ACKNOWLEDGEMENTS
This study was conducted in the framework of
the CSPD–COMISJO project with the support of the German Federal Ministry for
the Environment, Nature Conservation, and Nuclear Safety.
ADVANTAGES OF RO WATER PURIFIER
·
RO
water purifier removes toxin such as lead, mercury, Fluoride, Arsenic, Chlorine
which case human body to be ill. Lead metal can cause brain damage and anemia.
·
RO
water filter is great for removing commonly found Cryptosporidium in lake,
river and public supply water.
·
For
example Pureit marvella
slim RO is
one of the best RO water purifier
·
Reverse osmosis
water filters remove many bacteria and pathogens from tap water. Disease
causing bacteria like Giardia and Cryptosporidium are effectively filtered out
from your water source, thus eliminating the risks of you developing
gastrointestinal or other types of illnesses associated with these types of
pathogens and bacteria;
·
Chlorine taste and
odours are also removed; your tap water will not only be safer, but also
tastier;
·
With an RO water
filter you will save money on expensive bottled water;
·
RO systems don’t
use up much space and have a low energy consumption;
·
Because RO water
filters produce highly purified water, foods cooked with RO filtered water will
have an improved taste;
·
Reverse osmosis
water filters are affordable and provide outstanding quality water.
DISADVANTAGES OF RO WATER PURIFIER
·
Removes
essential minerals: While RO water purifier removes
dissolved impurities it removes natural mineral such as iron, magnesium,
calcium and sodium which are essential to the human body and cause a mineral
deficiency in the body.
·
Not
kills bacteria, viruses: RO water purifier does not kill
waterborne disease-causing bacteria and viruses.There high probability that
microorganisms can pass through RO membrane( It is advisable to pass RO water
through the UV water purifier to treat microorganisms )
·
Water
taste altered: As natural minerals are removed water
gets de-mineralized as a result water taste affected, it becomes tasteless.
·
More
time to purify: RO water purifier takes too long to
the purification of water.
·
Water
wastage: Approximately much more water compared
to filtered out water flushed down as waste water.
·
Expensive: RO water purifier costlier compared to counterpart water
purifiers UV and RO water purifier consumes much more electricity.
·
RO
membrane breakage: No mechanism is there, to know when to
replace RO membrane. Chlorine can damage RO membrane. Chlorine makes small
pores of RO membrane clogged and makes drastic reduction in performance.On
breakage of membrane dissolved salts, bacteria, viruses can easily pass through
RO membrane. It is advisable to replace RO membrane once in a year.
PROPOSED DESGN
REFERENCES
[1] Global Water Intelligence. Desalination
markets 2005–2015: A global assessment and forecast, Oxford, UK, 2004. [2] C.
Fritzmann, J. Loewenberg, T. Wintgens and T. Melin, Stateof-the-art of reverse
osmosis desalination. Desalination, 216 (2007) 1–76. [3] M. Wilf, Fundamentals
of RO–NF technology, Proc. International Conference on Desalination Costing,
Middle East Desalination Research Center, Limassol, Cyprus, 2004. [4] M. Wilf
and K. Klinko, Optimization of seawater RO systems design. Desalination, 138
(2001) 299–306. [5] K. Betts, Desalination, desalination everywhere. Environ.
Sci. Technol., 38(13) (2004) 246A–247A. [6] G. Petersen, S. Fries, J. Mohn and
A. Müller, Wind and solar powered reverse osmosis desalination units:
Description of two demonstration projects, Ges. für Kernenergieverwertung in
Schiffbau u. Schiffahrt, 1979. [7] S. Alawaji, M.S. Smiai, S. Rafique and B.
Stafford, PV-powered water pumping and desalination plant for remote areas in
Saudi Arabia. Appl. Energy, 52 (1995) 283–289. [8] O. Headley, Renewable energy
technologies in the Caribbean. Solar Energy, 59 (1997) 1–9. [9] A. Kanzari, The
Solco PV–RO system — Maldives case study, Proc. International Seminar on
Desalination Units Powered by Renewable Energy Systems, Hammamet, Tunisia,
2005. [10] D.G. Harrison, G.E. Ho and K. Mathew, Desalination using renewable
energy in Australia. Renewable Energy, 8 (1996) 509–513. [11] Y. Kunczynski,
Development and optimization of 1000–5000 GPD solar power SWRO, IDA World
Congress on Desalination and Water Reuse, Bahamas, 2003. [12] F. Castellano and
P. Ramirez, PV–RO desalination unit in the village of Ksar Ghilène, Proc.
International Seminar on Desalination Units Powered by Renewable Energy
Systems, Hammamet, Tunisia, 2005. [13] A. Goetzberger, C. Hebling and H.W.
Schock, Photovoltaic materials, history, status and outlook. Materials Sci.
Eng., R 40(1) (2003) 1–46. [14] Solarbuzz. Price Survey, July 2008
[http://www.solarbuzz.com]. [15] A.M. Helal, S.A. Al-Malek and E.S.
Al-Katheeri, Economic feasibility of alternative designs of a PV–RO
desalination unit for remote areas in the United Arab Emirates. Desalination,
221 (2008) 1–16. [16] E.S. Mohamed, G. Papadakis, E. Mathioulakis and V.
Belessiotis, A direct coupled photovoltaic seawater reverse osmosis
desalination system toward battery based systems: A technical and economical
experimental comparative study. Desalination, 221 (2008) 17–22. [17] K.
Touryan, M. Kabariti, R. Semiat, F. Kawash and G. Bianchi, Solar Powered
Desalination and Pumping Unit for Brackish Water. USAID Project No. M20–076,
Final Report, 2006
Comments
Post a Comment