.,
o
PRINCIPLES OF OPERATION
Reverse Osmosis is a process of the removal of dissolved ions from water in which pressure is used to
force the water through a semipermeable membrane element which will pass the water but reject most
other dissolved materials.
To understand reverse osmosis, the naturally occurring phenomenon of osmosis must be understood.
Osmosis can be defined as the spontaneous passage of a liquid solvent from a dilute solution across an
ideal semipermeable membrane element. The transfer of the solvent water- but not the solutes (dissolved
solids) - will continue until the concentrations of the solution on either side of a membrane element are
equal. The flow rate of the water is directly proportional to the concentrations of the two solutions.
This driving force, called the osmotic pressure, can be measured, and resulting flow can be halted by
applying a pressure equal to osmotic pressure on the more concentrated solution side. If this external
pressure is increased further, the flow of water will be reversed from its natural flowing direction and
towards the more dilute solution. The reversing of the flow is the process of reverse osmosis. For
example, if a variable pressure were applied on the more concentrated solution side of a semipermeable
membrane element, the following conditions could be realized:
(1)
(2)
P equals the osmotic pressure of the solution: The solvent flows at the same rate in both
directions; i.e., there is no net change in water volumes. This condition, as shown Fig. RO-l,
represents the phenomenon of osmosis.
P is greater than the osmotic pressure of the solution: Solvent flows from the more concentrated
solution to the "pure" solvent side of the membrane. This condition, as shown in Fig. RO-2,
represents the phenomenon of reverse osmosis.
There are many membrane elements which have good qualities of rejection (salt separation). The main
problem with the early membrane elements was that the water flow rate (flow per unit area) was very
low at any reasonable pressure drop across the membrane element.
The breakthrough that made RO feasible was the creation of an asymmetric cellulose acetate membrane
element; one surface of the element having a dense layer of skin (0.2 micron thick), and the remainder
being a relatively spongy porous mass. !fthis element was magnified many times its actual size, it would
be seen that the initial pore size in the dense skin is very small.
6010!OOA.3 2293
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o
Pure
Water
....
Semipermeable
Membrane
Figure RO-!
Osmosis Normal Flow From
Low Concentration Solution
to High Concentration Solution
Pressure
Pure
Water
....
Semipermeable
Membrane
Figure RO-2
Reversed by Application of
Pressure to High Concentration Solution
6010100A.4 2293
Pace 4
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Figure RO-3
5~ral Wound Aaaem~y
II" Preuure
v.....
Suppcn Pia II
J?igure RO-4
The Spiral-Wound Cartridge
Anli· TolescoPing
DevlC1l
FEE
D--~~~-==---"_1I11~~
FIb6rgl~
BRINE
~_..... PflOOUC T
OUlefWr~
FEED~~:J\S~
L
I
Food ChannII
__
SpaoeI'
1000(
Producl W.le(
eou.cuon
Chatv*
Product
Flow
o
Outsode
Dlameler
4.25 in.
\
~
:~- -~-I
1'211"1
).~
8:
\·t • .,
I
I-:·~_ ~ _l.I.~T.J
Concenlrale
FlOW
i
Feed
Flow
'·'12 in.
Extension
SPECIFICATIONS
Materials of Construction: 304 55, 316 SS, Polished 316 S5
Maximum Operating Pressure:
304 & 316 SS: 1000 psig (6890 kPa)
Polished 316 S5:1000 psig (6890 kPa)
D.D.: 4.25 in. (108 mm); J.D.: 4.0 in. (102 mm) manufactured
by Osmonics to ASTM specification A269 for industrial and
polished sanitary.
End Cap Assembly: 304 & 316 5S: Victaulic-type Style 77 or
HP70 to 1000 psig (6890 kPa);Victaulic-type Style 75 to 500
psig (3445 kPa); Polished 316 SS: high pressure dairy clamp to
1000 psig (6890 kPa).
Number of Elements:
Length (aPJ,J"QJ.){jncludjn~
1
2
3
CQIIDlin~s and ead caDS)
49
89
(124) (226)
52
lil.
92
(em) (132) (234)
4 in. Industrial: in.
(em)
4 in. Sanitary:
l29
(328)
132
(335)
Weight (....ithout clemelllsl:
Ibs. 20
(kgs) (9.1)
30
40
( 13.6) (18.l)
o
SYSTEM DESCRIPTION
The RO system is composed of two major parts; the high-pressure pump, and the pressure tubes
containing membrane elements. The high-pressure pump is water lubricated and must not be run in the
dry condition. The correct direction of rotation is shown on the pump housing. When it is connected
electrically, the direction of rotation should be checked. The pump should not be run for more than 60
seconds with reverse rotation as this may damage the internal parts.
IONICS ULTRAPURE WATER CORPORATION uses membrane elements which are placed in
pressure tubes rated at working pressures in excess of 400 psi. RO systems used by IONICS
ULTRAPURE WATER CORPORATION (IUWC) generally operate below 300 psi.
The supply water, which is pressurized by the pump, flows over these membrane elements. The system
is carefully designed to make certain that minimum flow rates over the elements are maintained. This
factor is critical to the efficient operation ofRO membrane elements. The reason for this is that as pure
water passes through the element under pressure, it leaves behind, at the element surface, a very high
percentage of the dissolved substances originally present in the supply water. For example, if the water
in contact with the element is 500 ppm, then the product water going through the element at that point
will be about 10 ppm (2%). A little further downstream, the water in contact with the element may be
concentrated to 1,000 ppm, (2%) and so on.
By maintaining the water flow velocity across the element smface above a critical valve, this
concentrated boundary layer is kept at a minimum, and product water quality is possible.
The other benefit of proper flow rates is that suspended matter tends to be carried out of the system more
effectively. For these reasons, the design flow rates should not be changed except in the "safe" direction.
The "safe" direction in general is increasing the concentrate flow from the system.
It should be noted that the less concentrated the supply water is in the RO system, the better the quality
of the product water. In other words, the lower the water recovery rate, the better the product water
quality. For some applications, the economic benefits of better quality product water far out weigh the
extra cost of rejected water. An example of this is where RO water is to be subsequently deionized.
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OPERATING SPECIFICATIONS
It appears that there are longer tenn benefits to be gained by operating at lower percent water recovery.
particularly in reducing maintenance to the system, and minimizing precipitation problems which may
arise do to chemical additive failures.
Itis important to realize that the product water from an RO system is delivered essentially at atmospheric
pressure. In general, it cannot be operated by opening and closing a valve in the product water line. The
reason for this is that the high pressure in the system drives the water across the membrane elements and
no flow of water would be possible if the pressure on both sides of the elements were the same. If, for
some reason, the product water line was closed while the system was operating, the pressure would build
up. Of course, if the product water side of the system were strong enough, which it is not, pressure would
end up the same as the pump pressure but at that point no water would flow across the membrane.
The membrane elements used by "IUWC" are capable of taking very high "forward" pressure; i.e., from
the direction of the supply water side to the product water side. However, they cannot tolerate much
"back" pressure; i.e., in the direction from the product water side to the supply water side. The maximum
back pressure should be 40 psi.
The quality of the product water produced by ROis aconstant percentage of the feedwater. Forexample,
if the feedwater is running 50 ppm, the product water may run about 2 to 5 ppm or less (90% or more
rejection of dissolved minerals). When the feed water is running 500 ppm, the product water would run
about 50 ppm or less.
601 01 OOA.l1 2293
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