WSP10809

• File Number: 8273.500
• Description: Colorado River Basin Salinity Control - Federal Agency Reports - EPA
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 4/1/1978
• Title: Integrating Desalination and Agricultural Salinity Control Alternatives
• Water Supply Pro - Doc Type: Report/Study

A, _ ['Og

(TDS ) - log (TDS.)
P ~
log (FSR)

+, ][

C
P
953.92

+ ,]-... (64)
m3/day.

N
00
w::..
w

K. =
~
C1l =
T =

feedwater concentration of potassium, mg/~l
feedwater concentration of chloride, mg/~; and
feedwater temperature, degrees Celsius.

In general, ED stacks (or units as shown in Figure 7) are ar-
ranged in stages to achieve the desired product quality and in
parallel rows to achieve the desired plant capacity. Thus, given
the desired product quality and the rating factor, the number of
stages (As) can be computed by first calculating the fraction of
salts remaining after each stage, FSR:

FSR = 0.53/RFo.5418,.........."....,.................. (63)

which assumes an individual stack capacity of 953.92

Reverse Osmosis

Another membrane process using hydraulic pressure rather
than electrical potential to separate water and salts via a semi-
permeable membrane is reverse osmosis. Much of the character-
istics and problems noted earlier for ED processes apply to RO
as well. Fpr example, energy consumption is proportional to
the quantity of salts to be removed. Consequently, the most
economic application of RO plants should be to soft, warm feed-
waters havi~g 1000-10,000 mg/~ TDS and producing water of 100-
,500 mg/~. The general RO plant flow network is illustrated in
Figure 8.

Two vessels of water having different salt concentrations
and separated by a semi-permeable membrane (permeable to water
but exclusive of salts) will produce a flow of relatively pure
water from the dilute solution to the more concentrated or until
either they are both the same concentration or a buildup of
pressure in the latter will stop the process. This phenomenon
is called osmosis. It should also be noted that the more sub-
stantial the initial concentration differential, the greater the
pressure (psmotic pressure) necessary to stop the flow. If a
pressure g~eater than the osmotic pressure is applied to the
solution of higher salinity, the flow of water can be reversed,
thus the concept of reverse osmosis as illustrated in Figure 9.

The brine to product ratio for RO plants is computed from
Eq, 61 but must exceed O.ll due not only to scaling or fouling,
but also because of existing membrane technology. It might
be emphasized that this limitation will improve as new membranes
are discovered and should therefore be evaluated periodically.
Total intake rate and brine volume are determined from Eqs. 48
and 60,

45