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Mercury
Work Group
Phase II Reports >> Technology Identification Subgroup Report
Facilities Loadings
| Pretreatment Manual |
Tech. Identification
Hg Management Guidebook | Mercury Products
Database
For more
information, contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
V. CANDIDATE MERCURY REMOVAL PROCESSES
The
following summarizes the unit operations identified by the
Subgroup that could be used in mercury removal pretreatment
systems:
Simple
Filtration:
is designed to remove particulate matter from wastestreams.
Filtration is often applied to wastestreams where particulate
matter could disturb subsequent unit operations. Since mercury
tends to bind to particulate matter, simple filtration (through
a 1 to 25 micron or larger rated media) can often be used as a
roughing or coarse removal step. Sometimes, the simple
filtration step itself may be sufficient. Filtration equipment
in this category would include bag type, depth or fiber wound
cartridges, and sand or diatomaceous earth media. Nearly all
systems would operate under a nominal pressure (typically less
than 20 psig), requiring pumped flow as opposed to gravity flow.
Membrane
Micro (and Nano) Filtration:
is designed for the removal of smaller particulate matter from a
wastestream, typically down to 0.1 microns in size. These
systems commonly employ synthetic membranes that have pore sizes
in the particulate range but approaching the threshold
(typically taken as 0.45 microns) that could be defined as
dissolved. These systems operate at moderate pressures (60 psig)
and offer a variety of cleaning processes (using chemicals or
hydraulic back pulses) to restore a fouled membrane. Some
membranes are fairly rugged and reusable after cleaning.
Reverse
Osmosis:
is designed for sub-micron particulate removal and high
molecular weight compound removal. Most familiar to hospitals
are the reverse osmosis (RO) systems used for the desalinization
of incoming city water (often used with ion exchange processes).
Most RO membranes are easily fouled by oil and grease and
certain organic materials and can, therefore, be destroyed by
continuous exposure to chlorine or other oxidants and organic
solvents. Operating pressures are elevated (say 200 psig) and
membrane performance is expected to degrade typically over a
period of two to five years.
Ion
Exchange:
removes dissolved ionic (charged) molecules from water.
Depending upon the ion exchange resin and/or combinations of
resins used, it may be possible to achieve nearly total
deionization of a wastewater stream and, thereby, produce a pure
"megohm" quality product. Ion exchange resins are
expensive and are highly susceptible to degradation by oxidizers
or fouling by oil and grease and certain organic materials. Most
resins are organically derived synthetics that can be adversely
affected by biological activity.
Chemical
Precipitation/Redox Reactions:
are used to convert a pollutant from one form (typically
dissolved or soluble) to another form (typically particulate or
insoluble). Then, a particulate separation process (usually
sedimentation or filtration) is used to remove the resulting
particles from the wastestream. The type of chemistry selected
for these reactions is dependent upon many conditions (pH, flow,
residence time, interferences or synergistic effects of the
mixed waste) and success in pollutant removal varies with final
pollutant solubility.
Adsorption:
is a process involving a combination of concurrent reactions
including electrochemical bonding, micro- and macro-reticular
pore entrainment and, to a lesser extent, ion exchange
(depending upon the presence and form of surface-active
functional groups). Activated carbon is a surface-active
adsorption medium that is often used for removal of dilute
organic solvents from a wastestream. Activated carbon has also
been used for removing low levels of heavy metals, including
mercury, from both gaseous and aqueous streams. A disadvantage
to the use of activated carbon for certain hospital wastestreams,
however, may be that the carbon surfaces provide growth sites
for biological activity.
Disinfection:
is a method for limiting biological activity in the wastestream
to reduce possible antagonistic effects in subsequent unit
operations. Disinfection may be performed by using chemical
methods with hypochlorite, permanganate, ozone, or peroxides (i.e.,
oxidizers) or by using ultraviolet light (UV sterilization). The
choice of method could be dependent upon the forms of biological
activity present in the wastestream (e.g., UV might be
ineffective on cysts and spores) or subsequent unit operations (e.g.,
chlorine and hypochlorite may attack certain synthetic
membranes, as mentioned earlier). Thermal disinfection (which
would leave no chemical residuals in the wastestream) is also an
option but is usually impractical because of cost
considerations.
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