TM 5-813-8
CHAPTER 4
PROCESS SELECTION
100%
General process selection. In selecting a
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% recovery of product water
potable water production system, it is important to
estimate costs of various options. The conventional unit
x water demand = saline feed water flow
of comparison is cost in dollars per 1,000 gallons of
product water. Water quality and energy sources will be
In some cases, the limited quantity of an available
estimated from simple site reconnaissance.
For
saline water may require a decision to adopt a more
example, a sea coast site where the water source
expensive desalination process with a higher water
temperature exceeds 95 degrees Fahrenheit indicates a
recovery rate. However, it may require choosing a
high-salinity high-temperature combination favoring
different and more saline feed water with a greater
distillation/condensation processes. Reverse osmosis
availability.
requires a feed water temperature below 95 degrees
Blending a high
Fahrenheit.
If local well testing indicates salinity
c. Blending of waters.
concentration stream with a low concentration stream
between 500 and 3,000 milligrams per liter and
wastes the osmotic pressure energy between the two
electricity is inexpensive, electrodialysis reversal or
streams. Therefore, it is best to match the design of the
highflux reverse osmosis is indicated.
desalination system to the product quality desired.
a. Desalination requirements. The design of a
When a desalination process cannot be economically
desalination system requires a clear understanding of
matched to the desired product quality, then a process
the following: the quantity of product water desired; the
that yields water with a very low dissolved material
quality of the desired product; and the quality of the feed
content must be used.
To conserve capital and
water source. This manual addresses the production of
equipment costs and meet the desired water demand,
potable water containing less than 500 milligrams per
the high purity product water can be blended with the
liter of total dissolved solids. Laundries, boilers, mess
pretreated saline feed water to produce the required
halls, and hospitals may require water purer than 500
product quantity and quality. The following equation can
milligrams per liter of total dissolved solids. Potable
be used to calculate the concentration of a blended
water from the desalination system may be further
water stream:
treated to meet these requirements in accordance with
AR 420-46.
( Σii=c=1onncentration i x flow i)/total flow =
b. Saline feed water quantity. The production of
potable water from saline water usually requires a
significantly larger quantity of saline feed water than the
blended concentration
quantity of potable water produced. When desalination
When only two streams are blended, the equation can
is necessary to produce potable water, the process splits
be rearranged to show the flow of concentrated water
the feed water into two streams. One stream is the
that when blended with a dilute flow will result in the
product water; the other stream is the brine that contains
desired product concentration.
This rearranged
most of the salts originally in the feed water. In waters
equation is as follows:
that need very little desalination, high-rate reverse
(P - H)(D) =F
osmosis may only reject 5 percent of the feed stream as
C-P
brine. In reverse osmosis of sea water, more than 70
Where:
percent of the intake water may be rejected as brine.
P = Desired product water
Multiply the required product quantity by the reciprocal
concentration
of the product water recovery fraction to find the
H = High purity water concentration
quantity of saline water that must be processed to yield
D = Flow of the high purity water
the desired quantity of product water. In equation, form,
C = Concentration in the impure
it can be expressed as follows:
concentrated stream
F = Flow rate of the concentrated
stream
Reblending for remineralization is discussed in Chapter
9. The same blend equations will apply to blending
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