systems are generally used. In some cases, mixed-
limits the efficiency of product water recovery during
media filtration may be necessary. Filters may use
either pressure or gravity flow methods.
magnesium hydroxide, and calcium sulfate can be
a. Single-media filtration. Single-media filtration
prevented through the addition of scale-inhibiting
chemicals. Two scale-inhibiting chemical types are
consists of one media. This media is often small-
commonly used: polyphosphates and polyelectrolytes.
grained silica sand; however, anthracite may be used
The most commonly used scale inhibitors are
after lime and lime-soda softening. Some desalination
polyphosphate chemicals. Polyphosphates lose their
pretreatment systems use an alternate media such as
effectiveness above 190 degrees Fahrenheit, which
greensand to remove iron compounds. Diatomaceous
limits upper operating temperatures. To prevent the
earth media is not recommended for primary filtration
formation of scale within desalination systems,
because of its characteristic high head loss and short
polyphosphate-based chemicals are the best general
The most commonly used
b. Dual-media filtration.
consists of two media with different specific gravities.
Polyelectrolytes or organic
The difference creates a two-layer separation effect.
polymers are relatively new scale inhibitors. The use of
Use silica sand or greensand for one layer; use
polyelectrolytes in potable water shall be in accordance
anthracite for the other layer. The use of dual media will
with paragraph 7 of AR 420-46.
allow larger quantities of material to be filtered and will
Degasification and deaeration. The removal
reduce head loss during operation. The use of two
media types will provide a good coarse-of-fine filtration
of dissolved gases from a feed water can be
process for desalination facilities.
accomplished in one of two ways. The dissolved gas
c. Mixed-media filtration. When three media are
may be stripped out with another gas, or the dissolved
gas may be stripped out with water vapor. A forced
used in filters, a better coarse-to-fine filtration pattern
draft degasification tower will remove nonatmospheric
can be constructed. High-density silica sand, garnet,
gases. This is done by blowing air up through a tower of
and anthracite are commonly used to provide the filter
packing. The water to be degasified is then sprinkled
bed. The different media do not stratify completely.
over the packing. When full deaeration is required to
Instead, there is a small amount of intermixing among
the different layers. This gradual change in media size
water vaporizaton or flashing, steam or water vapor
provides a gradient from coarse to fine and creates a
must be used as the stripping gas. On high-temperature
media flow pattern necessary to achieve a very low silt
distillation systems, a steam deaerator may be used.
On most distillation desalination systems, the only
economical method of deaeration the water is to lower
Secondary filtration. Secondary filtration is
the pressure with a vacuum pump or venturi eductor or
necessary to ensure that particulate material does not
ejector to produce enough water vapor to strip away all
pass from the pretreatment systems into a membrane
dissolved gases. There are two basic designs for water
Secondary filtration is used
distribution in deaerators. A design that sprays the
normally as a precautionary measure. Cartridge filters
water in the deaerator vessel will usually create the
are commonly used. Ultrafiltration systems can be used
most complete deaeration. The other commonly used
as a process and precautionary measure.
design involves cascading the water over trays. While
a. Cartridge units. The necessary protection is
the internal design of this deaerator is prone to short
most easily accomplished with cartridge filters. The
circuiting and incomplete deaeration, it is more compact
cartridges are replaced easily, and the filter mesh can
and requires no pressure to operate the sprayers.
be sized to remove from 20-micron to 0.05-micron
Although some cartridge filters can be
Filtration with granular media. Before the
backwashed, most are replaced when head loss reaches
filtration is designed, the possibility of use of
wells/infiltration galleries shall be evaluated. Filtration is
b. Ultrafiltration units. The newer process for
necessary when the desalination process requires a low
level of particulate material. Primary filtration with
granular media removes particulate materials left after
lime, coagulates, or other chemicals have been added
to the feed waters. Single-or dual-media filtration