extremely complex. When the molybdenum reactive
for remineralization, which is a more common
silica concentration exceeds 30 milligrams per liter as
procedure. See Appendix A for sample problems.
SiO2 or the pH exceeds 8.3 in the brine stream, an
Process limitations. The various desalination
environmental chemist or engineer should be consulted.
processes presently available have limitations that must
Reverse osmosis is also limited to the treatment of
be considered prior to selecting a desalination process
waters with less than 1 milligram per liter of oil and
for a particular site. These limitations apply only to the
desalination processes themselves; pretreatment can be
(1) Cellulose acetate membranes. Cellulose
and is often used to bring a saline feed water within
acetate membranes are usually limited to pH levels
limits so that a desalination process can be used. The
between 4.0 and 7.5. Cellulose acetate membranes
raw feed water chemistry for all desalination systems
require some form of continuous disinfection with the
must be evaluated thoroughly for constituents that may
feed water to prevent microbial degradation of the
precipitate in the desalination system.
membranes and can tolerate up to 1 milligram per liter
a. High-temperature distillation. High-temperature
of free chlorine.
Therefore, cellulose acetate
distillation is limited by the saturation of alkaline earth
membranes are usually disinfected by maintaining 0.2 to
metal salts, such as CaSO4, BaSO4, SrSO4, CaCO3,
0.9 milligrams per liter of free chlorine in the feed water.
BaCO3, and SrCO3. Carbonate salt scaling can be
Cellulose acetate membranes cannot be used on waters
controlled by acid addition. The recovery of water from
where the temperature exceeds 88 degrees Fahrenheit.
a hightemperature distillation plant is usually limited by
Cellulose acetate membranes should not be used at
calcium sulfate solubility. When the concentration of
the sulfate and the limiting alkaline earth metal is one-
recommended pressure, since they are prone to
third of the saturated condition at ambient temperature,
membrane degradation by pressure compaction.
distillation design must include pretreatment to reduce
or inhibit the scaling ions. High-temperature distillation
Brackish water polyaromatic amide membranes are
is also limited to oil and grease levels below 1 milligram
generally limited to operation in feed waters between pH
per liter. All other limitations on the high-temperature
4 and pH 11. Polyaromatic amide membranes are less
distillation process are equipment specific and require
pH tolerant and should not be used outside of the range
pH 5 to pH 9. All polyaromatic amide membranes are
b. Low-temperature and mechanical distillation.
limited to use on feed streams that are free of residual
Low-temperature and mechanical distillation systems
chlorine. If chlorination is necessary or desirable as a
are limited to operation below saturation of alkaline
pretreatment option, complete dechlorination must be
earth sulfates and carbonates. The lower operating
effected. Polyaromatic amide membranes are tolerant
temperature permits economical operation on waters
of water temperatures up to 95 degrees Fahrenheit.
that are at or below half saturation at ambient
While polyaromatic amide membranes are not as
temperature. Oil and grease are limited to less than 1
quickly or completely compacted as are cellulose
milligram per liter. Any other limitations are equipment
acetate membranes, manufacturer's recommended
pressures must be followed to prevent mechanical
c. Reverse osmosis. The most severe limitation
damage to membrane modules.
on reverse osmosis is the maximum limit of 50,000
d. Electrodialysis reversal. While electrodialysis
milligrams per liter of total dissolved solids in the feed
reversal has been used to treat water as saline as sea
water. Another limitation is that there must be no iron in
water, 4,000 milligrams per liter of total dissolved solids
the feed water. This limitation is so rigid that only
is considered to be an upper limit for economical
stainless steel and nonferric materials will be used
Some electrodialysis membranes can
downstream of the iron removal. The solubility of
tolerate strong oxidants, like chlorine, but most cannot.
alkaline earth sulfates and carbonates limits reverse
The reversal of polarity used in electrodialysis reversal
osmosis treatment. Any water containing less than
for removal of scale allows operation on water that is
4,000 milligrams per liter of total dissolved solids that
saturated with alkaline earth carbonates. Saturation
would be saturated with an alkaline earth sulfate when
the concentration is multiplied by 1.5 should not be
considered for reverse osmosis desalination. Reverse
osmosis is limited to waters that do not have silica
saturation in the reject brine.
Silica chemistry is