What is Reverse Osmosis?
 Osmosis
is a natural process, known for over 200 years, on which reverse osmosis
systems are based. The walls of living cells are natural membranes. A
membrane is selective, some materials can pass through it, some cannot.
Figure 1 illustrates osmosis and the
selectivity of the membrane. The semi-permeable nature of the membrane
allows the water to pass much more readily than the dissolved minerals.
Osmotic pressure works to make the concentration of the solution equal on
both sides of the membrane. Since the water in the less concentrated
solution seeks to dilute the more concentrated solution, the water
passage through the membrane generates a noticeable head difference
between the two solutions. This head difference is a measure of the
concentration difference of the two solutions and is referred to as the
osmotic pressure difference.  This
head pressure, converted to the familiar pressure units of pounds per
square inch (2.31 feet of water head equals 1 psi),
allows the observation of a valuable rule of thumb. That is, that each
100 mg/L total dissolved difference is equal to approximately 1 psi osmotic pressure difference.
When a pressure is applied to the
concentrated solution which is great that the osmotic pressure
difference, the direction of water passage through the membrane is
reversed and the process that we refer to as reverse osmosis is
established. That is, the membrane's ability to selectively pass water is
unchanged, only the direction of the water flow is changed. Thus, as
shown in Figure 2,
a water treatment technique in which the water is being separated from
the dissolved minerals is demonstrated.
Were the membrane to act as a perfect
separator, the permeate would contain 0-mg/L total dissolved solids, no
matter what the concentration on the feed side of the system. This is not
the case, however. And, in fact, let us consider, for the sake of
illustration, 90% rejection to be an average operating condition. By
considering the mechanism of salt and water passage through the membrane,
it will be clear why complete salt elimination is not possible and how
operating conditions can effect permeate quality and quantity.
The membrane's ability to hold back
salts while allowing water to pass is based on the fact that the salts
are in solution as ions, that is, charged particles. The dissolved salts
are in solution as cations, with a positive
charge, and as anions, with a negative charge. A descriptive analogy of
what is happening is to consider the membrane to be a mirror. As the
charged particles, ions, approach the membrane, they are repelled by a
reflection of their own charge. That is, similar charges repel, just as
similar magnetic poles repel each other. Therefore, the layer of water
immediately adjacent to the membrane is void of charged particles, and it
is this water which will subsequently diffuse through the pores and be
delivered as permeate. Since the anions and cations
are constantly moving around in solution, sometimes they are near enough
to each other to be attracted to one another, thus canceling their
individual charges. Without a net charge, these particles are free to
pass through the membrane.
 Although
Figure 2 was sufficient to illustrate the basic RO process, the feed and concentrate
ports added in Figure 3 are necessary to illustrate a continuously
operating RO system.
In order to keep the membrane from
fouling it is important to continually flush the brine side. As the water
is squeezed through the membrane, leaving most of the salts behind, the
brine side solution becomes increasingly concentrated. Without the reject
flow to drain, the brine side mineral concentration would eventually
exceed the solubility limits of the salts present and they would
precipitate, forming a scale on the membrane. To avoid excessive brine
side concentrations, the permeate volume recovered, in a low pressure
system, is usually kept in the range of 1- to 30 percent of the feed
stream volume. For example, if for each five gallons of water fed to the
membrane, one gallon of permeate is recovered, the membrane is operating
at 20% recovery.
REVERSE OSMOSIS MEMBRANES
Construction
The semi-permeable membrane used in RO
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. Figure 4 illustrates the different densities in the cross section
of the membrane.
Configurations
Different configurations of membranes
have been devised, each offering certain advantages.
The most popular membrane
configuration is the spiral wound, shown below in
Figure 6.
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 form. 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 pre-filtration is generally recommended.
Reverse Osmosis Operation
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 Systems
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, used for
aquariums and those referred to as counter top modules. Figures 8 and 9
define the elements commonly found in these systems and their arrangements.
Although for the counter top Reverse Osmosis
modules and for 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 continuous
flowing system as shown in Figure 8.
When the storage tank has been filled
to the point at which its pressure equals two thirds of line pressure, the
permeate is diverted to drain.
To conserve water consumption in
reverse osmosis devices another type of control called "shutdown"
is employed in the design using a shutoff valves and is illustrated in
Figure 9.
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. Unless provision
is made to eliminate the dissolved mineral concentration difference
across the membrane, salt passage will continue, creating a high TDS
water on the permeate side of the membrane. The phenomena is commonly
referred to as a TDS creep. A membrane flush kit will bypass the waste
water flow restrictor (which provides the pressure for the RO membrane to
work) and allow the full rate of feed water to flush across the membrane.
Feel free to call us with any additional questions you may have. We also
have a Technical
Data Sheet with all the specifications on the membranes we use.
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