Knowledge Tip of
the Day: Heart of the Reverse Osmosis System: reverse osmosis membranes
Construction:
The semipermeable membrane used reverse osmosis systems are cast polymer
films of asymmetric density. That is, they have a dense barrier layer
which is very thin, perhaps 10 millionths of an inch, supported on a more
porous substrate a few thousandths of an inch thick.
Configurations
: Different configurations of membranes have been devised, each offering
certain advantages.The most popular membrane configuration is the spiral
wound. These are assembled by folding a sheet of membrane over a tube,
referred to as the product tube, and trapping a screen between the two
halves of the membrane. The membrane is bonded to the tube and glued together
along the three open edges. Another spacer screen is laid on the membrane
and the whole sandwich is rolled tightly around the product tube and then
bound with tape to hold it together. This method of packaging membrane
provides considerably more surface area per module than the tubular. However,
since the feed water must wind its way through the path created by the
spacer screen, dirt particles can be easily trapped, so 5 micron prefiltration
is generally recommended. The
general operation of all RO modules is the same. The feed stream is supplied
to the membrane and split into the permeate which has diffused through
the membrane, and the concentrate which passes over the membrane, carrying
away the minerals to waste.
Low
pressure RO operation generally refers to feed pressures of less than
100 psig. This includes most of the equipment capable of being installed
under the kitchen sink and those referred to as counter top modules. Although
for the counter top Reverse Osmosis modules and some permanently installed
units, the storage tanks are maintained at atmospheric pressure - the
majority of under-the-sink installations utilize accumulator storage vessels.
As water is added to the tank, the air charge is compressed and thus the
pressure in the tank rises. It is this elevated pressure that is used
to propel the drinking water to the faucet. The pressure in the tank also,
however, acts as a back pressure on the membrane, and as tank pressure
increases, the differential pressure across the membrane decreases.
Recalling
the expressions for water and salt transport across the membrane, as the
tank pressure rises, the water production rate drops and yet the salt
passage continues unaffected. Thus the quality of the water being delivered
drops significantly if the differential pressure is allowed to become
too low. Therefore, most equipment included some provision for limiting
the storage tank pressure to some value less than line pressure. A ratio
of two thirds is a commonly chosen limit, and may be done for a continuos
flowing system.
To
conserve water consumption in reverse osmosis devices another type of
control called "shutdown" is employed in the design using a shutoff
valves. At
the designed-in, present ration, the storage tank pressure will close
the valve and prevent further feed to the system. The valve will open
again when sufficient pressure reduction is sensed at the storage tank.
Whatever means is used to accomplish shut down, the end result is that
the differential pressure across the membrane is eliminated so that water
production ceases.
F.A.Q.
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