TM 5-813-8
6.5 gallons/minute of high chloride high total dissolved solids single-or first-stage reverse osmosis desalinated sea
water. 311.5 gallons/minute of high sulfate island well water
This blend will be most economical when:
1. The cost of island well water is between 14.25 times single-or first-stage product water and 1.161 times the
cost of this first-stage product water.
2. The removal of 311.5 gallons/minute of well water does not degrade the water quality in the wells.
b.
The availability of up to 312 gallons/minute of island well water and the quality of island well water should be
investigated extensively.
c.
If over roughly (500) + 85 = 600 gallons/minute of island well water can easily be withdrawn without degrading the
quality or quantity of island well water in the future, then the facility should be reviewed again for the installation of
electrodialysis-reversal treatment at 85% recovery and disposal of the resulting high sulfate brine.
d.
The use of a mere 6.5 gallons/minute of first-stage desalinated sea water will save at least (188.1 182)
gallons/minute = 6.1 gallons/minute of second-stage reverse osmosis water plus (6.1) + 0.85 6.5 gallons/minute =
0.68 gallons/minute first-stage reverse osmosis water. This savings is definitely worth the engineering,
construction, and operation costs.
e.
A table showing the final cost of blended waters consisting of the five possible minimum cost blends of $.10 well
water, .00 single-pass reverse osmosis water, and .75 double-pass reverse osmosis water has been
prepared and is shown in table A-3. Figure A-23 shows a plan view of the desalination system of a two pass
reverse osmosis system.
A-6.
Sample problem for simple blend. A base in the desert has a water with a total dissolved solids content of 907
mg TDS/liter. If reverse osmosis is to be used as a desalination technique in this application, then the final product water
will have far less than the required 500 mg TDS/liter. If 250,000 gallons/day of finished water will be required, how much
of the raw water can be reblended in meeting TB-MED 576 requirements?
Symbols:
x = Amount of water that must be treated
Given:
Daily flow is 250,000 gallons/day
Finished blend concentration is 500 mg TDS/liter.
Initial feed concentration is 907 mg TDS/liter.
Flow Concentrated = (Desired Concentration - Dilute Concentration) Flow Dilute
(High Concentration - Desired Concentration)
SAMPLE PROBLEM FOR SIMPLE BLEND
Assumptions: a. The total dissolved solids rejection of a low-pressure membrane is about 90%, leaving .1 x TDS in the
dilute water stream.
b. Total dissolved solids are the limiting factors in the reblending.
Therefore: The flow of raw undesalinated water used = 250,000 gallons/day - x
And:
(250,000 gallons/day - x) = [500 - .1(907)]x
(907 - 500)
250,000 gallons/day = x + (409.3/407)x
250,000 gallons/day = 2.00565x
'. x = 124,000 gallons/day
A-7.
Silt density index (SDI). This fouling index mentioned in Chapter 3, tables 4-2 and 4-3, and Chapter 5 is
measured on a 47-millimeter-diameter 0.45-micron pore size membrane filter. The data used by the manufacturers to
calculate the index are the time (T1) necessary initially to force 500 milliliters of solution through a clean 0.45-micron filter
with a 30-pound-per-square-inch pressure differential across the filter, followed by the time (T2) required to force an
additional 500
A-40
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