82
The Last Resource
charged particles or ions, the sodium ions being positive and the
chloride negative. If a current of electricity is passed through the
solution the negative ions move to the positive pole and positive
ions move to the negative pole. Electrodialysis exploits this funda—
mental behaviour and enhances its effect by placing special mem—
branes in the path of the migrating ions. Membranes which allow
only positive ions through them alternate with membranes permeable solely to negative ions and as a result alternate compartments
of concentrated brine and desalted water slowly build up (see
gure 8). Because the amount of energy required by the process
depends directly on the number of ions being pushed along and
because electrical energy is generally expensive, electrodialysis is
used mainly to desalt brackish, less salty, waters.
Reverse osmosis. In contrast to electrodialysis, in which the salt ions
are moved through the membranes, in reverse osmosis it is the
water which passes through them. If a sugar solution is separated
from water by a semi—permeable membrane—one that allows only
water through it—the water moves into the sugar solution. This
movement will continue indenitely although the rate at which the
water crosses the membrane will fall o as the sugar solution
becomes diluted. The only way to stop the migration of water
through the semi—permeable membrane is to apply a pressure equal
to the ‘attractive force’ or osmotic pressure of the sugar solution on
the sugar side of the membrane. An increase beyond this osmotic
pressure not only stops the flow of water into the sugar solution side
but actually reverses it, squeezing water back through the membrane.
This is the principle behind desalination by reverse osmosis (see
gure 8). Unfortunately it is difcult to make a membrane which
will let water through but hold back the sodium and chloride ions
which are only very slightly larger than the water molecules.
C. E. Reid rst suggested using reverse osmosis for desalination
in a research proposal submitted to the U.S. Department of the
Interior. Of the various membrane materials which he and his co—
worker E. ]. Breton tried, commercial acetate lms proved the most
impermeable to salt. In a typical experiment using a pressure of
100
atmospheres, the concentration of salt in the efuent was
approximately 5 per cent that of the feed solution. The most
serious drawback was the very slow rate at which the water moved
through the membranes—about a tenth of a gallon per square foot
The Last Resource
charged particles or ions, the sodium ions being positive and the
chloride negative. If a current of electricity is passed through the
solution the negative ions move to the positive pole and positive
ions move to the negative pole. Electrodialysis exploits this funda—
mental behaviour and enhances its effect by placing special mem—
branes in the path of the migrating ions. Membranes which allow
only positive ions through them alternate with membranes permeable solely to negative ions and as a result alternate compartments
of concentrated brine and desalted water slowly build up (see
gure 8). Because the amount of energy required by the process
depends directly on the number of ions being pushed along and
because electrical energy is generally expensive, electrodialysis is
used mainly to desalt brackish, less salty, waters.
Reverse osmosis. In contrast to electrodialysis, in which the salt ions
are moved through the membranes, in reverse osmosis it is the
water which passes through them. If a sugar solution is separated
from water by a semi—permeable membrane—one that allows only
water through it—the water moves into the sugar solution. This
movement will continue indenitely although the rate at which the
water crosses the membrane will fall o as the sugar solution
becomes diluted. The only way to stop the migration of water
through the semi—permeable membrane is to apply a pressure equal
to the ‘attractive force’ or osmotic pressure of the sugar solution on
the sugar side of the membrane. An increase beyond this osmotic
pressure not only stops the flow of water into the sugar solution side
but actually reverses it, squeezing water back through the membrane.
This is the principle behind desalination by reverse osmosis (see
gure 8). Unfortunately it is difcult to make a membrane which
will let water through but hold back the sodium and chloride ions
which are only very slightly larger than the water molecules.
C. E. Reid rst suggested using reverse osmosis for desalination
in a research proposal submitted to the U.S. Department of the
Interior. Of the various membrane materials which he and his co—
worker E. ]. Breton tried, commercial acetate lms proved the most
impermeable to salt. In a typical experiment using a pressure of
100
atmospheres, the concentration of salt in the efuent was
approximately 5 per cent that of the feed solution. The most
serious drawback was the very slow rate at which the water moved
through the membranes—about a tenth of a gallon per square foot