This review aims to understand Reverse Osmosis
as a method of water purification and holistically analyze and contrast it with
the other popularly used methods. It will also suggest the attempts made to
make the process more effective and throw light on some ways of reducing the
investment involved in the setup and better the environmental impacts caused
during running of RO units.
List of Abbreviations:
parts per million
Total Dissolved Solids
2) Process of RO
3) Membrane Types
4) Importance and Efficiency of process
5) Cons of the process
6) Improving RO systems
Reverse Osmosis (RO) is a widely followed, yet
expensive, water purification technology. It is a technology that evolved from
basic science of liquid flow across membranes. Osmosis is a process involving
the flow of liquid from lower concentration of salt to higher concentration
when separated by a semipermeable membrane. This is the process which plants
use to absorb soil nutrients through the roots in the cellular level. RO is the
opposite process. It involves pressurizing 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 to osmotic pressure (that
pressure which is required to just stop osmosis when applied in the region of
higher solute concentration). RO is very important in the process of
desalination of sea water. Pure water is thus separated from sea (brackish)
water. Brackish water is pressurized against one side of the semipermeable
membrane resulting in movement of salt less water across the membrane and producing
drinking water from the other side.
During the process molecules are pressurized
through a semi-permeable membrane to a higher solute concentration. The
membrane here behaves as a type of filter being extensively porous with tiny
pores that remove microscopic debris from the water hence making it pure and
potable. The semi-permeable membrane (0.0001 microns size) is called so because
only lets water molecules through while other contaminants are collected and
flushed away. Membranes used for RO have dense barrier layer in the polymer
base. Usually this process requires a pressure of 30 to 250 psi to be applied
on the other side of the membrane for normal tap water and 600-1000 psi for
brackish sea water.
Common membrane materials include polyamide
thin film composites (TFC), cellulose triacetate (CTA) and cellulose acetate
(CA) with the membrane material being spiral wound around a tube, or hollow
fibres held together as bundles. Hollow fibre membranes have a greater surface
area and hence capacity but are more easily blocked than spiral wound membranes
RO membranes are rated for their ability to reject compounds from contaminated
water. A rejection rate can be computed calculated for every specific ion type
or contaminant element as well as for the general reduction of total dissolved
Comparatively TFC membranes are stronger and
durable as well as possess higher rejection rates than CTA or CA membranes.
They are also more resistant against high pH values, microbe attack and high
TDS. CA/CTA’s have a better ability to tolerate chlorine.
There is another membrane type: Sulphonated polysulphone
membranes (SPS) which is chlorine tolerant and can tolerate higher pH’s. These
are best used where the water is soft and is of high pH or where high nitrates concentration
Effectiveness and Importance
RO can effectively remove suspended and
dissolved impurities: pathogens, dust and mineral matter apart from that is
used in various industrial processes. TDS is a measure of all soluble chemicals:
minerals (cations and anions) and microorganisms and biomolecules in water. TDS
is a very important parameter in the design of RO systems. It is often measured
and reported in units of ppm. The standard values are around 300 ppm for tap
water and 2,000 ppm for ground water. Contribution to TDS is mainly by sodium
chloride salt. RO can ensure a purity of 0 to 50 ppm of TDS values which is
considered safe to drink for a normal person. TDS values lesser than these are
not recommended to drink as they are short of dissolved minerals that is
necessary for humans to intake through water.
RO units are responsible for reducing barium,
chromium, radium, and other ions whose presence in drinking water can cause
cancer, kidney damage, and birth defects.
Household RO treatment unit (Source: Google images)
In industry, boiler water is demineralized
using RO before using it in the power plant. Further with few cycles of
distillation the water becomes as pure as possible and doesn’t leave mineral
deposits in machinery or corrode the turbines. Any such corrosion leads to poor
steam production as boiler efficiency is brought down and hence poorer power
production in the turbine. Thus a
natural conclusion would be to understand that their profitability is largely
based on the cost effectiveness and efficiency of water purification system.
In food industry, RO is extensively used for
concentrating fruit juices. RO is found to be more economical than the
conventional heating method to concentrate fruit juice. Moreover, heat
sensitive drinks such as enzyme or protein based drinks can be concentrated
through RO. A common example is milk and concentrated whey protein given to
children and body builders.
RO also finds its application in hydrogen
production plant as treated water prevents scale formation on the electrode
Cons of using RO
The household RO system has one main
disadvantage. Due to low back pressure (pressure from tap water) it generally
wastes two to three gallons of water for every gallon of pure water produced.
This water is used in cleaning the membrane first as the membrane is given
lateral flow of water. The wasted water is highly salty and diverting large
volumes of this into the sewage would cause acting against the smooth movement
sewer contents. The rejected water will not be tolerated by normal garden
plants due to very high salt concentration. It will also lead to corrosion as
it would form deposits on floor and sanitary ware.
Secondly, natural trace minerals present in
water can be removed by reverse osmosis. These minerals not only provide a good
taste to water, but they also serve a vital function in the body’s system and
can hence be unhealthy for the body if removed from water. Persons drinking
such water would have to take minerals necessary through supplementary tablets.
High power consumption is a major disadvantage
of RO especially in larger scale apart from leading to environmental imbalance
specifically in the application in desalination of brackish water. Most
industrial RO systems use large amounts of energy thus affecting carbon
footprint. Average annual RO based desalination consumes power equivalent to
power consumed by a refrigerator running in the house. Annual power required to
supply RO based seawater desalinated water in each household could be compared
to 10% increase in power consumption of that house.
Desalination of water can be harmful to the aquatic
ecosystem as its effects include sucking in fish eggs with intake water in the
system; using harsh chemicals to clean membranes which in-turn can get retained
in water and enter bodies of consuming organisms; and release of large volumes
of highly salty liquid brine back into the water body resulting change in environment
for water organisms.
Comparison between RO, UV and UF systems
The table below compares RO, UV and UF modes
of water purification which is with special reference to household/municipal
Improving the RO systems
RO systems are mainly inefficient due to discharge
of 1 part of water in every 4 parts purified (in most cases). In recent years,
one can find the development of novel RO filter systems called as
“zero-discharge”. This technology involves discharging the backwash water into
the water supply pipes. It further refers to the fact that the backwash water
is not discharged into the sewer system. This may require additional power
intake but the net effect is making it water resource friendly as the issues of
wasting water and simultaneously releasing high contaminant concentration water
into domestic sewers or in dump-lands is taken care of.
A recent research paper published that a
combination of RO and evaporation can suggest a very energy efficient and
simpler way to maximize recovery of water that is given as input to the RO system.
The paper proposed a very innovative
method of using high pressure nano-filtration and crystallizer/filtration. The
paper further suggested a water recovery efficiency greater than 95% from
RO is a very powerful way of water
purification. Although it is inspired from nature it is energy intensive. RO
must be resorted to only in cases where the salt content of water to be used
for drinking is extremely high. Even here reduction of the salt content to the
level of 10 or 20 ppm may not be very favorable. If the salt content of the
water is considerably high even for non-potable purposes, rainwater harvesting
can be resorted to.
In cases where the water contains coliform
bacteria, the source must be traced to and the contamination must be
eliminated. Although RO may be advised, elimination of the cause is the safer
and preferred method and ultimately the more cost effective option too.
Resorting to RO for water with high salt content
must be preceded by assessment of the volume likely to be subjected per day and
one must ask the supplier how long will the filters work effectively with that
volume, moreover the cost of replacement of RO membranes must also be considered
apart from the monthly running costs in addition to the capital cost.
Nevertheless for heavy purposes such as
desalination and massive water purification for food products industry, RO is
the best and most suitable method compared to most other methods such as UV and
UF especially in cases where virus free water and water free from any kind of
dissolved salt ions is desired.