Technical
Paper
Optimizing Mercury Removal Processes
for Industrial Wastewaters
Authors: Gerald Walterick, Jr. and Larry Smith,
GE Water & Process Technologies, Trevose, PA, USA.
Introduction
Mercury (Hg) removal from both air emissions and
industrial wastewater, especially coal-fired power
plants in the U.S., has been a topic of extensive
study for the last decade. Much of the initial focus
has been on controlling air emissions of mercury
contaminants from coal-burning power plants
which were identified as the single largest source of
airborne mercury emissions and specifically targeted by the U.S. Environmental Protection Agency’s
(EPA’s) Clean Air Mercury Rule1. This problem was
addressed by the installation of wet flue gas desulfurization (FGD) systems at many coal-burning facilities. The FGD process has resulted in a significant
reduction in air emission of mercury, but has done
this by transferring the mercury contaminants to a
wastewater stream. Other industries such as petroleum refining, natural gas recovery and other light
and heavy industries also generate mercury contaminated wastewaters. Much of the recent research has been focused on removing mercury
from industrial wastewaters.
Mercury typically occurs at low parts per billion
(ppb) levels in industrial wastewaters. The severe
toxicity of some mercury compounds and the tendency of these compounds to bioaccumulate in
aquatic ecosystems have led to very stringent
wastewater discharge regulations to keep mercury
out of the environment.
The Ohio River Valley Sanitation Commission
(ORSANCO) standard of 12 parts per trillion (ppt) discharge limit on Hg for discharges into the Ohio River
and the Great Lakes Water Quality Initiative standard of 1.3 ppt Hg for discharges into bodies of water
in the Great Lakes Basin are examples of U.S. guidelines.
Meeting these limits presents a significant challenge to many industries. This paper will describe
the use of the MerCURxE* technology, which includes a patented chemical product developed by
GE Water & Process Technologies to aid in the removal of mercury to these low levels in industrial
wastewaters. Known as MetClear* MR2405, this
metals precipitant, when used in conjunction with
other unique coagulants and flocculants from the
GE portfolio, provides significant removal of both
soluble and insoluble mercury.
Mercury Occurrence and Speciation
The primary source of mercury contaminants in
coal-burning power plants is the coal. Mercury
concentrations vary with coal grade, chloride content and origin, but are typically in the range of
0.05-0.2 micrograms per gram (μg/g), equivalent to
0.0001-0.0004 lb. /ton.2 Although many organic
and inorganic mercury compounds exist, for the
purpose of this discussion, mercury contaminants
will be categorized into three different “species” that
are related to the chemical or physical process that
would remove them. These are: elemental mercury
(Hg0), ionic mercury (Hg+2) and particulate mercury
(Hgp).
Find a contact near you by visiting www.ge.com/water and clicking on “Contact Us”.
* Trademark of General Electric Company; may be registered in one or more countries.
©2012, General Electric Company. All rights reserved.
TP1198EN.doc Nov-12
Hg Speciation
Hg0 (g)
Stack
Hgp (s)
Hg0 (g)
Hgp (s)
Hg2+ (g)
Boiler
FGD
Hg0 (g)
ESP/FF
Hg2+ (g)
Hg0 (g)
Coal
Hgp (s)
Bottom Ash
Hgp (s)
Fly Ash
Hg2+
FGD Purge to
Wastewater treatment
Figure 1: Coal Combustion FGD System Mercury Speciation
Figure 1 illustrates the fate of various Hg species
through a typical coal combustion process. Prior to
combustion, mercury is primarily present in the raw
coal as naturally occurring mercuric sulfide, which
would be classified as particulate mercury (Hgp).
During combustion, most of the mercury associated
with the coal is volatilized to Hg0 and Hg2+. Some
small fines of Hgp may carryover with the fly ash
and a small amount of Hgp may remain with the
bottom ash. As the combustion products proceed
toward the exhaust stack, they cool down and Hg0
may be oxidized to Hg2+. The flue gases then pass
through an Electrostatic Precipitator (ESP) or Fabric
filter (FF) to remove "fly ash" particulates, including
Hgp. Many systems also include Flue Gas Desulfurization (FGD), which is a process originally intended
to remove sulfur dioxide, (SO2), from coal combustion processes, but is also an effective means of
removing mercury, particularly Hg2+ which is soluble
in the scrubber slurry. Oxidants and catalysts are
often incorporated into the process to promote oxidation of Hg0 to the more soluble Hg2+ species. This
enhances the removal of mercury by the FGD process.
Waste Treatment Processes
A significant portion of the airborne mercury contaminants removed from flue gas is transferred to
aqueous waste streams that must be treated prior
to discharge. The mercury species of most concern
in wastewaters are soluble mercury (Hg2+) and particulate mercury (Hgp). Treatment processes to
handle these wastewaters can be as simple as a
settling pond as shown in Figure 2, below.
Settling Ponds
Fly Ash
Precipitants
Bottom Ash
FGD Purge
Settling pond
Settling pond
Discharge
Leachate
Coagulant
Flocculant
Figure 2: Settling Pond Diagram
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Technical Paper
A more complex wastewater treatment system,
specifically designed to incorporate chemical precipitation reactions required for adequate treatment of highly contaminated FGD wastewater
streams is shown in Figure 3.
FGD Wastewater Treatment
System
Precipitant
lime
Flocculant
Coagulant
Flocculant
Filter Aid
pH Adj.
Desaturator
Clarifier
Clarifier
Reaction Tanks
Filter
Discharge
or
Pond
FGD Purge
Figure 3: FGD Wastewater Treatment System
This type of system typically includes several separate unit operations, including: desaturation (addition of lime to precipitate sulfate as gypsum),
equalization (to stabilize influent pH and water
chemistry), metals precipitation, coagulation, clarification and filtration. Biological waste treatment
processes may also be included to remove organics, nitrogen compounds and selenium.
Chemical Additives
Proper selection and application of chemical additives are critical to the success of a wastewater
treatment program for mercury removal. Additives
used to enhance the removal of contaminants in
conjunction with the unit operations described
above may include: lime, coagulants, flocculants
and heavy metal precipitants. Use of an appropriate precipitant is essential to ensure that soluble
mercury contaminants are reduced to low ppt concentrations. Several years of lab, pilot and full-scale
testing have determined that some of the most effective mercury precipitants are types like GE’s
metals precipitant, MetClear MR2405, which has a
very strong affinity for mercury and will also precipitate other heavy metals such as silver, cadmium,
copper, lead, zinc, cobalt and nickel.
Technical Paper
Experimental
Bench scale mercury removal studies (jar tests)
were conducted in the laboratory on samples of
mercury-contaminated wastewater from several
industrial sources. Due to the extremely low concentrations of mercury typically found in these
wastewaters, great care was taken to ensure that
the test apparatus and sample bottles were meticulously clean. The procedures used for the studies
reported here are summarized below:

Test apparatus - Mercury removal studies were
done in a dedicated clean laboratory using customized apparatus designed and operated to
minimize the potential for sample contamination.

Samples for low-level mercury analysis were
processed in a dedicated clean lab using EPA
recommended protocols.

Personal Protective Equipment (safety gloves,
protective eyewear and protective clothing)
were worn by lab personnel at all times.
The jar test procedures used for mercury removal
studies were customized for each application using
a proprietary computer program to design mixing
protocols that simulated the mixing conditions and
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reaction times of the full-scale wastewater treatment process. The use of this program improves
the accuracy of bench tests and facilitates scale-up
to full-scale processes.
Results
A variety of wastewaters were evaluated, including
power plant and refinery wastewaters. The range
of chemical compositions of these wastewaters
varied widely as shown in Table 1.
Table 1: Composition of Test Substrates
Parameter
pH
Specific Conductance (umhos @ 25C)
P Alkalinity (ppm as CaCO3)
M Alkalinity (ppm as CaCO3)
Sulfur (ppm as SO4)
Chloride (ppm as Cl)
Hardness (ppm as CaCO3)
Calcium (ppm as CaCO3)
Magnesium (ppm as CaCO3)
Iron (ppm as Fe)
Sodium (ppm as Na)
Potassium (ppm as K)
Aluminum (ppm as Al)
Manganese (ppm as Mn)
Nitrate (ppm as NO3)
Phosphate (Total, ppm as PO4)
Silica (ppm as SiO2)
Turbidity (ntu)
Mercury (ppt as Hg)
Study results with various contaminated
wastewaters demonstrated that the low ppt Hg discharge concentrations required for each
wastewater could be achieved with proper application of chemical additives. In many cases, the target discharge concentrations were achieved using
existing plant unit operations.
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Range of values
Low - High
3.2 - 9.0
828 - 40,300
0 - 920
0 - 3640
97 - 40,000
22 - 10,100
350 - 42,100
229 - 16,700
119 - 33,100
< 0.05 - 1310
24 - 1630
6.6 - 218
< 0.1 - 403
0.09 - 129
4.2 - 1,800
< 0.4 - 201
4.7 - 576
1.2 - > 4000
19.8 - 17,000
Figure 4 shows test results for studies done with a
pond wastewater. Untreated, this wastewater contained 47 ppt Hg. With chemical treatment and settling, Hg was reduced to 2.5 ppt. The improvement
in Hg reduction resulting from the MetClear treatment is clearly evident.
Technical Paper
Figure 4: Pond Wastewater Mercury Removal
Figure 6: FGD Wastewater Mercury Removal
Figure 5 shows results of tests with an FGD
wastewater that contained >14,000 ppt Hg. The
target Hg concentration of 1500 ppt was attained
by chemical treatment and settling. Increasing the
dosage of MetClear significantly improved Hg removal.
Another FGD wastewater containing 43,800 ppt Hg
was successfully treated to reduce Hg to < 150 ppt
using the MetClear product (Figure 7).
Figure 5: FGD Wastewater Mercury Removal
Figure 7: FGD Wastewater Mercury Removal
Figure 6 is a comparison of treatments with
MetClear to treatments with a competitive precipitant. In this FGD wastewater MetClear treatment
reduced Hg to below the target concentration of
1500 ppt while Competitor A’s product "leveled off"
at > 4000 ppt Hg.
As previously mentioned, the MetClear product is
very effective for removing other heavy metals in
addition to mercury. Figure 8 shows the effect of
MetClear treatments on the removal of mercury,
beryllium, cadmium, copper, vanadium, zinc, cobalt
and nickel.
Technical Paper
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Figure 8: MetClear Affinity for Heavy Metals
Key Treatment Considerations
When a mercury-contaminated wastewater
stream(s) is identified as a candidate for mercury
removal studies, several key steps must be followed
to determine the potential for successful treatment.
Periodic, routine and historical sampling and analysis of mercury contributing wastewater streams
must be implemented utilizing EPA approved protocols for sampling, handling and analysis. The U.S.
EPA has published guidelines for proper procedures
regarding this. Method 1669, titled “Sampling Ambient Water for Trace Metals at EPA Water Quality
Criteria Levels” is commonly used as a guide for
sampling techniques. Analytical techniques to
measure low parts per trillion levels were evaluated
and in a 2007 memorandum, guidelines for use of
these analytical techniques were disclosed11. Analytical method 1631E is currently the method of
choice for low level mercury analyses.
Common power plant FGD and ash pond systems
have a variety of incoming water quality characteristics as shown in Table 1. This variability is one of
the reasons why it is so important to routinely analyze and conduct evaluations in the laboratory and
on-site to ensure optimum removal is maintained.
Impact of Treatment System
Equipment Design
To maintain consistent, optimized mercury removal
across any treatment system, all reaction tanks,
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clarifiers, filtration equipment and in the case of oil
refining, both primary and secondary wastewater
systems, must be in excellent operating condition.
Removal of mercury, other heavy metals and total
suspended solids (TSS) are all impacted by the system design and the selection of chemical additives,
but that is not the whole answer for achieving effective removal. For pond systems, like bottom ash
ponds in coal fired power plants, it is also important
to ensure the right chemical feed points are chosen
and optimal dosages are maintained. Enough residence time under quiescent conditions is required
to facilitate settling of precipitated metals and other
suspended solids. Due to the large size of many
settling ponds, settling time is typically not a limiting
factor.
Pilot Studies
On-system application of the treatment programs
developed in laboratory jar testing is commonly
conducted through plant trials or pilot studies, leading to extended or continuous treatment. Several
applications are outlined below to demonstrate the
effectiveness of the MerCURxE program for removal
of total mercury. Often the requirements for meeting final discharge permit limits are accomplished
by treating each candidate stream individually.
Treated water may either be combined or discharged separately. In many cases, immediately
after the precipitation process, results show excellent removal even though the final discharge limits
are not met at that point in the system. Depending
Technical Paper
on the downstream design, further removal is obtained and lower mercury levels are realized.
FGD system # 1
This coal fired power plant FGD system has a traditional design with equalization, reaction tanks and
circular clarifier. The treatment program includes
the use of a coagulant, MetClear MR2405 and a GE
flocculant to remove TSS and mercury. Specific
regulatory discharge limits for mercury have not yet
been established at this site. Treatment results for
several heavy metals in this wastewater are shown
in Figure 9. Mercury removal of more than 99% on
average has been accomplished, from approximately 30 ppb to less than 0.2 ppb. The inlet loading of most other heavy metals is not significant
(less than 1 ppb), compared to iron and mercury.
Results indicate that boron, selenium and arsenic
are also removed by the combined treatment approach.
FGD System #3
This FGD wastewater treatment system includes
equalization, reaction tanks and a clarifier prior to
discharge. This system also utilizes a coagulant,
MetClear MR2405 and a GE flocculant to remove
mercury, other heavy metals and TSS. Results from
treatment are shown in Figure 10. Mercury removal
in excess of 99.91% was achieved with treatment.
Average influent mercury of 84,800 ppt was reduced to 78 ppt across the clarifier then to 18 ppt
through the sand filter. This site is also not currently
regulated for mercury removal.
Figure 10: FGD total mercury removal
Conclusion
Figure 9: FGD Wastewater Treatment System Results
FGD System #2
This FGD system includes equalization, desaturation, a primary clarifier, a chemical reaction tank, a
secondary clarifier and a continuous backwash
sand filter. This filtered effluent is discharged
through ash ponds. In this system only the secondary clarifier is treated with chemicals. Additives include lime for pH adjustment, MetClear MR2405, a
coagulant and a GE flocculant. Target levels for
mercury are less than 200 ppt Hg out of the treated
clarifier. Results have shown that mercury can be
removed from an inlet range of 230 to 350 ppt
down to as low as 65 ppt after the clarifier and as
low as 45 ppt after the filters, well below the target
goal of 200 ppt.
Technical Paper
The use of MerCURxE chemical technology has been
shown to be an effective method for removing mercury from several industrial wastewaters. Incorporating this type of treatment into an overall
wastewater treatment program should significantly
improve mercury removal. The industry generating
the wastewater, the design of the wastewater
treatment plant, the operating conditions of the
plant and the mercury concentration and speciation in the influent wastewater are all factors that
have an impact on the efficacy of a treatment program. Understanding the unique characteristics of
each system and the variability of the contaminant
loading is vital to the successful removal of mercury
from industrial wastewaters.
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References
1. www.epa.gov/camr/basic.htm “Basic
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infor-
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Program Review Meeting, August 12-12, 2003.
3. www.uwp.edu/departments/geosciences/merc
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Wet FGD Systems”
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Technical Paper