Liquid-Phase
Separation
Description
Several types
of separation techniques are used
to separate either dissolved or undissolved particles in water. These
are
briefly described below:
´
Filtration.
Filtration is the physical process whereby particles suspended in water
are
separated by forcing the fluid through a porous medium (i.e., a
filter). The
suspended particles are trapped in the filter. Filtration relies on the
pore
size of the membrane, which can be varied to remove particles and
molecules of
various sizes. Microfiltration processes generally work best for
separating
very fine particles (0.1-0.001 microns) from the liquid. Sampling the
effluent
can monitor the effectiveness of these processes.
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Distillation. Simple
distillation is
a process in which heat is applied to a liquid, causing a portion of
the liquid
to vaporize. Contaminants with
different boiling
points from water are thereby separated. The vapors are condensed,
producing a
liquid product called distillate.
´
Freeze
Crystallization.
Freeze crystallization freezes contaminated water. When the solution is
slowly
frozen, water crystals form on the surface, from which they are
separated from
the remaining solution.
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Membrane
Pervaporation.
Membrane pervaporation uses several permeable membranes to
preferentially sorb volatile organic compounds (VOCs) from
contaminated
water. Contaminated water is heated, causing the VOCs to change into
the vapor
phase. With the assistance of vacuum pressure, VOCs pass through the
membrane
wall to a condenser where they return to the liquid phase. (Note that
Vapor-Phase Separation, described as a separate technology, is designed
to
separate vapors from off-gas. However, it
uses a
similar concept to membrane pervaporation.)
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Reverse
Osmosis.
Reverse osmosis uses a selectively permeable membrane that allows water
to pass
through it, but which traps heavy metals and radionuclide ions on the
other side
of the membrane. If used for removing radionuclides from water, the
size and
charge of the ion being treated affect reverse osmosis. Because radium
and
uranium ions are large and highly charged, reverse osmosis is
particularly
effective at removing these dissolved radionuclides from contaminated
solutions.
Limitations
and Concerns
Reverse
osmosis generates a concentrated waste
stream containing radionuclides that must be treated further or
disposed.
Removal efficiencies for microfiltration have been shown to be greater
than 99
percent for uranium, plutonium, and americium.
Depending on
what is fed into the system, the
microfiltration process generates three waste streams: a filter cake of
solid
material, a filtrate of treated effluent, and a liquid concentrate that
contains the dissolved contaminants. The filter cake and/or liquid
concentrate
require further treatment or disposal.
Membrane
pervaporation and freeze crystallization
are limited to aqueous waste streams.
The feed
stream should be dilute to make the
freeze crystallization process cost effective.
The presence
of oil and grease may interfere
with water separation processes by decreasing flow rate.
Space must be
adequate for distillation and
freeze crystallization treatment systems. Distillation units are
comparatively
tall.
Explosive
compounds should not be used in the
distillation process.
Applicability
These ex-situ
separation processes are for groundwater treatment. Most
liquid-phase
separation processes are used mainly as a pretreatment or
post-treatment
process to remove contaminants from wastewater. The target contaminant
groups
for ex-situ
separation processes are VOCs, semi-volatile organic compounds (SVOCs),
pesticides, metals, most
radionuclides, and
suspended particles. At Department of Energy (DOE) sites, these
processes are
used to separate tank wastes. However, tritium cannot be removed easily
because
of its chemical characteristics.
Technology
Development Status
Membrane
processes for removing contaminants
from liquid-phase are commercially available for most contaminants.
More
studies are needed to assess how effectively radionuclides can be
removed from
liquid media. Performance data on distillation and freeze
crystallization are
not available. Reverse osmosis is commercially available.
Web Links
http://www.frtr.gov/matrix2/section4/4-51.html
Other
Resources and Demonstrations
See
descriptions of Membrane Separation (designed for vapor-phase
separation) and Solid-Phase Separation.
Through
membrane processes, uranium
concentrations of 300 µg/L were reduced by 99 percent in Florida
ground water.
See U.S. DOE, ñCost and Performance Report, PerVapTM
Membrane
Separation Groundwater Treatment, Pinellas Northeast Site,î
October 1997.