Desalination of Sea Water using Membrane technology
The scarcity of fresh water resources and the need for additional
water supplies is already critical in many arid regions of the world
and will be increasingly important in the future. It is very likely
that the water issue will be considered, like fossil energy
resources, to be one of the determining factors of world stability.
Thus, it is of utmost importance to fabricate methods to use sea
water as drinking water so as to fulfill the rising demand of water
supply. This can be done by water desalination methods which
remove the salt content and other unwanted compounds from
water thus making it suitable for various applications.
It is a process that removes or separates salts from saline water to give fresh
water, at the expense of energy.
Depending upon the type or form of energy used, Desalination Processes
can be broadly classified into two groups:
1. Thermal Desalination
2. Membrane Desalination
Thermal desalination processes involves heating of saline
water to its boiling point to produce water vapor, this pure
vapor is condensed to produce fresh water.
The three types of thermal desalination units used
1. Multistage flash (MSF)
2. Multiple effect distillation (MED)
3. Low Temperature Evaporation (LTE)
Membrane processes use a semi permeable membrane to move water across the
membrane from the salt solution to produce fresh water on the other side of the
Membrane desalination is classified depending on the driving force.
Size of materials
5-100 nm molecules
- 4 bar)
0.5 - 5 nm molecules
- 15 bar)
< 1 nm molecules
- 60 bar)
Microfiltration is a process of separating material of colloidal size and larger
than true solutions.
The MF membranes are made from natural or synthetic polymers such as
cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides,
polysulfone, polycarbonate, polypropylene, PTFE etc. The inorganic
materials such as metal oxides (alumina), glass, zirconia coated carbon etc.
are also used for manufacturing the MF membranes.
Applications of MF are:
Food & beverages
Ultrafiltration is most commonly used to separate a solution that has a
mixture of some desirable components and some that are not desirable.
Rejected species include sugars, biomolecules, polymers and colloidal
Applications of MF are:
Oil emulsion waste treatment
Treatment of whey in dairy industries
Concentration of biological macromolecules
Electrocoat paint recovery
Concentration of textile sizing
Concentration of heat sensitive proteins for food additives
Concentration of gelatin
The separation mechanism of NF involves size exclusion as well as
electrostatic interaction. In NF, organic molecules with molecular wt.
greater than 200-400 are rejected.
Membranes used for NF are of cellulosic acetate and aromatic polyamide
Applications of NF are:
Concentration of sugars, divalent salts, bacteria, proteins, particles, dyes
and other constituents that have a molecular weight greater than 1000
Removal of color and total organic carbon (TOC) from surface water
Removal of hardness from well water
Overall reduction of total dissolved solids (TDS)
RO membranes give 96%-99% NaCl rejection. Greater than 95-99% of
inorganic salts and charged organics will also be rejected by the membrane
due to charge repulsion established at the membrane surface.
RO membranes are made of polymers, cellulosic acetate and aromatic
Potable water from sea or brackish water
Ultra pure water for food processing and electronic industries
Pharmaceutical grade water
Water for chemical, pulp & paper industry
Waste treatment etc.
Municipal and industrial waste treatment
18-20% (w/w) in N-Methyl pyrrolidone (NMP) solvent
Ultrafiltration membrane is basically used for pre-treatment of water so as to
reduce the process cost and it is also used as a support for nanofiltration and
reverse osmosis membranes.
1.Solution made of poly(ether)sulfone (PES)
2.Solution of polyamide (PA)
The nanofiltration membranes should possess good selectivity, good
rejection ability and good flux. The parameters involved in the development
of the membrane should be optimized so as to obtain a good quality
1. 2% amine solution (MPD)
2. Organic solution of acid chloride (TMC)
Comparison between Membranes:
Microfiltration, Ultrafiltration Nanofiltration, Reverse Osmosis
Energy required for the
operation is less.
Energy required for the operation is
They have a good selectivity.
They only separate suspended
They do not have a good selectivity.
They separate suspended as well as
Membrane preparation was done using the immersion precipitation technique.
Dry polysulfone beads were taken in air tight bottles and then a specific amount of DMF was
added to dissolve the polymer. The same method was applied the PVDF powder.
A piece of fabric was kept on the glass plate and the casting solution was spread on it evenly.
The entire assembly was immediately immersed in a room temperature gelling bath made by
using Ultra Filtered water.
The membranes were stored in laboratory refrigerator maintained at 5oC.
(% Solvent(DMF) (% w/w)
2% v/v DMF in
2% v/v DMF in
Renewable Energy based
Conventional sources of energy are depleting fast and hence there is an
urgent need to find renewable sources of energy. Desalination can also be
carried with the help of renewable sources of energy such as solar energy.
One of the methods is Solar Reverse Osmosis. This setup can also be done
in those areas where access to grid electricity is not possible. Also,
renewable energy based methods are pollution free and environmental
Solar Reverse Osmosis Unit:
REVERSE OSMOSIS MEMBRANE
PIPE PRESSURE DROP
Factors affecting pressure drop calculations:
1. Friction between the fluid and the wall of the pipe
2. Friction loss as the fluid passes through any pipe fittings, bends,
valves, or components
3. Pressure loss due to a change in elevation of the fluid (if the pipe is
4. Friction between adjacent layers of the fluid itself
Tube id is given by,
Tube id (D) = (tube od – (2*thickness))
The velocity (v) of fluid is estimated based on internal cross sectional area of pipe which
is shown as follow.
v = (mass flow rate/(ρ*cross sectional area))
Then, Reynold's number is:
Viscosity depends upon the temperature of the liquid and is calculated from the
correlation. The friction factor (f) is:
f = 1/ ( 16*(Log(((e/D)/3.7)+(5.74/(Re^0.9))^2))
From the friction factor, pressure drop (pd1) due to friction is calculated.
Pd1=dp = (2 * f * l * ρ * (v2)) / D
The pipe fitting is selected and its corresponding k value is selected.
Therefore, pressure head(hf) due to pipe fittings is,
Total pressure drop (Tpd) across the piping system is as follows:
Tpd = (pd1+pd2)
Pressure drop calculations in MS
A program has been developed in MS Excel for
estimation of pressure drop based on above
Designing of Low Temperature Evaporator ( LTE )for
Desalination of Sea Water
Calculation of Heat Transfer Area of the various stages of LTE and optimising
the design with the given flow rate of product water.