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Inorganics:
Metals, Cyanide etc.
Inorganics in Ground
Water
Precipitation, filtration, and ion exchange
are widely used ex situ treatment technologies for inorganics
in ground water. In situ treatment technologies are used less
frequently. |
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Precipitation/Flocculation
and Sedimentation
The combination of precipitation/flocculation
and sedimentation is a well-established technology for
the removal of metals from ground water. This technology
pumps ground water through extraction wells and then treats
it to precipitate lead and other heavy metals.
Typical removal of metals employs precipitation
with hydroxides, carbonates, or sulphides. Hydroxide precipitation
with lime or sodium hydroxide is the most common choice.
Generally, the precipitating agent is added to water in
a rapid-mixing tank along with flocculating agents such
as alum, lime, and/or various iron salts. This mixture
then flows to a flocculation chamber that agglomerates
particles, which are then separated from the liquid phase
in a sedimentation chamber. Other physical processes,
such as filtration, may follow.
Filtration
Filtration isolates solid particles
by running a fluid stream through a porous medium. The
driving force is either gravity or a pressure differential
across the filtration medium.
Pressure differentiated filtration
techniques include separation by centrifugal force, vacuum,
or positive pressure. The chemicals are not destroyed;
they are merely concentrated, making reclamation possible.
Parallel installation of double filters is recommended
so ground water extraction or injection pumps do not have
to stop operating when filters are backwashed.
Ion Exchange
Ion exchange is a process whereby
the toxic ions are removed from the aqueous phase in an
exchange with relatively innocuous ions (e.g., NaCl) held
by the ion exchange material.
Modern ion exchange resins consist
of synthetic organic materials containing ionic functional
groups to which exchangeable ions are attached. These
synthetic resins are structurally stable and exhibit a
high exchange capacity. Other types of ion exchange materials
include clays, zeolites, and peat derivatives. They can
be tailored to show selectivity towards specific ions.
The exchange reaction is reversible and concentration-dependent;
the exchange resins are regenerable for reuse. The regeneration
step leads to a 2-to10% waste stream that must be treated
separately. All metallic elements present as soluble species,
either anionic or cationic, can be removed by ion exchange.
A practical influent upper concentration limit for ion
exchange is about 2,000 mg/L. A higher concentration results
in rapid exhaustion of the resin and inordinately high
regeneration costs.
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