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Mercury
Work Group
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For more information,
contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
Technology
Evaluated
The Subcommittee
initially compiled a list of industrial wastewater pretreatment
technologies known to be used in the pollution control industry.
The thought was to focus more on each individual technology's
ability to remove mercury from the wastestream and less on the
ability of a packaged system's performance ability. The
technologies identified by the Subcommittee which were
subsequently reviewed for applicability via the presentations
made by equipment suppliers are as follows:
Simple Filtration:
is designed to removed particulate matter (typically in excess
of 5 microns in size) from the influent wastestream. For mercury
contained in biomass, simple filtration would be used as a
roughing or course removal step. For wastestreams heavily laden
with particulate matter which would prove deleterious to
subsequent unit operations processes, filtration might be
employed. Filtration systems in this category would include
reusable bag type, depth or fiber wound disposable cartridges,
sand and diatomaceous earth (DE) media. Nearly all systems would
operate under a nominal (typically less than 20 psig) pressure
(pumped discharge 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
micron in size. These systems employ synthetic membranes that
have pore sizes still in the particulate range but approaching
the threshold of material which could be defined as dissolved
(typically 0.45 micron). These systems operate at elevated
pressures (60 psig) and offer a variety of cleaning (mechanical
backpulse or chemical) processes to restore the sustained flux
of a fouled membrane. Membranes are fairly rugged and reusable
after cleaning.
Reverse Osmosis:
is designed for submicron particulate removal into the range of
molecular weights. Most familiar to Hospitals are RO systems
used for the desalinization of incoming City water in
conjunction with ion exchange components. Membranes used by RO
technology are easily fouled by organic material and can be
nearly destroyed by exposure to chlorine and other oxidants.
Operating pressures are further elevated (200 psig) and membrane
performance is typically expected to degrade after about three
years.
Ion Exchange: removes
dissolved ionic (charged) material from water. Depending upon
the resin and/or combinations of resins used, it may be possible
to achieve nearly total deionization of a wastewater and,
thereby, produce a megohm quality product. Resins are expensive
and are highly susceptible to degradation by oxidizers and
fouling by oil/grease and/or organic materials. Most resins are
organically derived synthetics which would also be adversely
affected by the presence of biological activity.
Chemical
Precipitation/Redox Reactions: is designed for use in
converting one form of a pollutant (typically dissolved) to
another (typically insoluble) then using a particulate
separation process to remove the formed solids from the
wastestream (sedimentation or filtration techniques). The type
of chemistry employed is highly dependent upon a myriad of
operating conditions (pH, flow, residence time, synergistic
affects of the mixed waste) and its predicted success is always
a function of pollutant solubility.
Adsorption: is a
process involving a combination of concurrent reactions
including electrochemical bonding, micro and macro reticular
pore entrainment and, though to a lesser extent, ion exchange.
Typically, activated carbon is used as an adsorbent in
situations where dilute solvents present in the wastestream must
be removed though carbon has also historically been used for
removing low levels of heavy metals, such as and including
mercury, from aqueous phase. A significant disadvantage to the
use of activated carbon in a hospital wastestream setting,
however, is its availability to biological activity as a food
source.
Disinfection: is
a method for neutralizing biological activity present in the
wastestream before it would enter subsequent unit operations.
Disinfection may be performed using chemical methods involving
hypochlorites, permanganates, ozones, peroxides, etc. or may
also be accomplished using ultraviolet light (UV sterilization).
The choice of method is highly dependent upon the form and mix
of biological activity present in the wastestream (UV would
likely be ineffective on cysts and spores) and subsequent unit
operations (hypochlorites attack certain synthetic membranes, as
mentioned earlier). Thermal disinfection, which imparts no
residual to the wastestream and is highly effective, is also
available but is impractical to continuous flow mode application
due to limitations in heat exchange.
Evaporation: is
a means of eliminating a wastewater discharge. This alternative
was only touched on by the Subcommittee since it does not
represent a true pretreatment technology but, rather, only
provides a means of transferring the mercury problem to other
media (air and hazardous waste).
Note that all of these
technologies would be considered "add-on" to an
end-of-pipe active neutralization system which is the minimum
system that should already be in place within an institution
holding a permit from the MWRA. This presumption is made with
the understanding that such a neutralization system is also
required pursuant to the Massachusetts Plumbing Code, under the
Special Waste section (248 CMR 2.13).
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