Trace Mercury Removal from Flue Gas

Desulfurization Wastewater

Mark Owens, P.E. Degremont Technologies, Richmond, VA

H. Robert Goltz, Ph.D. The Dow Chemical Company, Midland, MI

And

Dean Mingee, Robert Kelly, Ph.D. Degremont North American 

Research & Development, Richmond, VA

KEYWORDS: Mercury, Flue Gas Desulfurization (FGD) Wastewater, Trace Contaminant 

Removal, Selective Removal, Ion Exchange.

ABSTRACT: Discharges from Wet FGD Systems are facing new, very stringent, effluent 

limitations for mercury. To comply with these new limitations, an end-of-pipe system to 

remove trace mercury contamination to <12 parts per trillion was developed. This report will 

present the results of a successful pilot test performed in 2008.

INTRODUCTION

Many coal-fired power plants have installed 

or plan to install wet flue gas 

desulfurization (FGD) scrubbers in 

response to increasing pressure to control 

air emissions. The blowdown from these 

units contains elevated levels of solids, 

chlorides, sulfates, and heavy metals as 

well as nutrients (nitrates and/or ammonia)

This wastewater is generally treated in a 

physical/chemical system including 

desaturation of sulfates and chemical 

precipitation. These systems may or may 

not include filtration depending on effluent 

requirements. Physical/chemical treatment 

with sand filtration can remove mercury 

from FGD wastewater to concentrations as 

low as 2-10 parts per billion (ppb). 

In general, physical/chemical treatment of 

FGD blowdown wastewater has been 

sufficient to achieve discharge standards 

for many years. Many installations in the 

US have been operating from the early 

1990s. However, new and more stringent 

water quality based standards are being 

introduced, which require additional 

removal of specific contaminants such as 

mercury, selenium, boron, or nutrients. 

These standards are set based on the 

ability of the receiving water to accept the 

discharge without significant impact. They 

are not set based on any available 

technology. 
Mercury is of particular interest and 

concern because of its toxicity as well as 

the fact that it bio-accumulates. Due to 

water quality based standards such as the 

Great Lakes Initiative (GLI), some states 

are setting effluent mercury standards as 

low as 1-10 parts per trillion (ppt).

The information discussed in this paper is 

the result of collaborative research studies 

conducted by the Dow Chemical Company 

and Degremont Technologies in an effort 

to achieve trace mercury removal down to 

these new regulatory standards. The pilot 

test was performed at a power station in 

the eastern United States. 

Prior to pilot testing, a bench-scale 

treatability test was performed by 

Degremont’s North American R&D Center 

(DENARD) in Richmond, Virginia using the 

mercury selective resins. With preliminary 

treatability data in hand, the treatment train 

was developed and the necessary pilot 

equipment was assembled. 

The pilot treatment train included:

• Addition of organosulfide 

• Sand Filtration

• Cartridge Filtration (5 micron)

• Ultrafiltration (0.03 micron)

• Selective Ion Exchange

It is important to note that all of these 

technologies are well established and can 

be implemented commercially. The novel 

part of this system is the selective resin 

itself. 

The goals of the pilot study included:

• The determination of the lowest 

achievable effluent mercury 

concentration

• The determination of the most 

effective mercury-selective resin

• The determination of the 

effectiveness of differing levels of 

filtration

• The identification of potential 

operations and maintenance issues

The pilot operated from July through 

December 2008 and achieved all of the 

above goals. 

A number of interesting observations have 

been made during the course of these 

investigations. A patent disclosure has 

been filed on these observations. 

DEVELOPMENT OF MERCURY SELECTIVE RESINS

Modern ion exchange resins are made 

from styrene divinylbenzene plastic. This 

material is a tough and proven base for 

attachment of a variety of functional 

chemistries. Liquid styrene and divinyl 

benzene are combined with diluents, 

solvents, catalysts and stabilizers and 

formed into organic phase droplets by 

suspension polymerization processes. The 

suspension is heated to polymerize the 

styrene and DVB into a solid plastic 

sphere. 

The functional groups are added by first 

activating the benzyl rings of the plastic by 

chloromethylation with chloromethyl, 

methyl ether that adds a -CH2-Cl functional 

group. It is through this group that a wide 

variety of nucleophiles can be reacted to 

create the wide variety of selective media 

that are enjoyed today. 

For selective mercury removal, the 

chemical form of the mercury is a primary 

concern. Mercury is a very reactive 

molecule, often referred to as “sticky” in 
that it associates with a wide range of other 

molecules, forming a wide variety of 

complexes. In addition, mercury can exist 

in various forms such as Hg0 the zero 

valence or metallic form, Hg +2 the ionic 

form and organo-mercury such as 

methylmercury. Each of these forms of 

mercury has different physical and 

chemical properties that must be taken into 

account in the design of a selective media. 

A speciation study of the form of mercury 

that is present is recommended so that the 

right media is used for the right form of the 

molecule. Speciation studies are difficult to 

perform, as relatively few laboratories are 

capable of this kind of study at these very 

low levels. Speciation studies are subject 

to interpretation as the error in the analysis 

can be significant. In this case, the ionic 

form of the mercury was identified as the 

primary target for treatment. 

Since ionic mercury is a cation, removal 

with a cation exchange resin is logical. In 

the case of FGD wastewater, this 

treatment has not been successful, 

presumably due to the high salt 

background of the wastewater matrix. 

The literature describes Iminodiacetic acid 

(IDA) functionalized resins as having very 

high affinities for mercury. 

Figure 1A

IDA Functional Group

While this may be true for neat solutions, 

the high salt background of FGD 

wastewater likely causes interference with 

the IDA group causing lower affinity in 

testing. 

Another chemistry of interest is the Thiol 

functional group. The interactions of sulfur 

and mercury are used for mercury removal 

from vapor phase treatment of natural gas. 

Figure 1B

Thiol Functional Group

Other Thiol chemistries attach the S-H

directly to the benzyl ring changing the 

chemical properties of the functional group. 

Yet another functional group of interest is 

the Thiouronium chemistry. Here a 

thiourea moiety is the active group that 

forms an attractive charge transfer 

interaction. 

Figure 1C

Thiouronium Functional Group

A final note on selective media is that for 

removal of contaminants like mercury at 

parts per trillion levels, the total loading 

capacity of the media may not be the 

limiting factor. These media, when 

challenged with mercury at high levels 

(mg/L range) will load metal to wt/wt% 

levels. To even come close to

approaching this level of loading, the 

media would need to be exposed to billions 

of bed volumes of solution. Thus the 

media is less likely to become fully loaded 

with the trace contaminant and more likely 

to fail due to fouling. Thus the issues of 

concern are fouling resistance of the 

S

C

H2N

+ NH2

HOOCCH2NCH2COOH

S

H
media, an open pore structure of the media 

creating easy access and allowing fast flow 

rates and short contact times for cost 

effective system design. 

BENCH-SCALE TREATABILITY STUDY

Prior to performing on-site pilot testing, a 

bench-scale study was performed to 

screen various ion exchange resins for 

performance evaluation.

Bench-scale treatability for mercury with 

selective ion exchange resin was 

determined using a ‘shake flask’ technique. 

This method includes adding a measured 

mass of resin to a flask containing the 

wastewater to be treated. The flasks are 

left on a flask shaker for 24 hours. After 

the 24-hour shake period, the resin is 

allowed to settle and the supernatant is 

analyzed. 

The purpose of this test was not to 

determine the achievable mercury 

concentration, but to determine which resin 

achieved the best relative result. 

Table 1

Shake Flask Results

Resin Type

Ratio 

(g/mL)

Hg 

(ppt)

Percent 

Removal

Blank --- ---

Raw --- 126

Thiol Resin 0.01 7.98 94%

0.005 10.7 92%

Thiouronium 0.01 5.45 96%

Resin 0.005 8.61 93%

Silver Impregnated 0.01 73.8 41%

Alumina Resin A 0.005 101 20%

Silver Impregnated 0.01 87.9 30%

Alumina Resin B 0.005 89.4 29%

The above results clearly indicated the 

superior performance delivered by the 

Thiol and Thiouronium resins.

PILOT EQUIPMENT AND PROCESS FLOW DIAGRAMS

For the purposes of this study, the mercury 

present was loosely classified as ‘colloidal’ 

or ‘soluble.’ On this basis, the pilot 

equipment was divided into a chemical 

precipitation and filtration portion designed 

to remove the colloidal mercury followed by 

the selective ion exchange step to remove 

the remaining soluble mercury. 

Chemical Precipitation

Prior to filtration, the wastewater is dosed 

with an organosulfide precipitant. The 

purpose of this chemical is to coagulate 

mercury as a sulfide complex prior to 

filtration.

Filtration Removal of colloidal mercury by 

filtration is dependent on particle size. 

Accordingly, the pilot system was designed 

with serial filtration using decreasing pore 

size. In this way, the particle distribution 

could be loosely determined. 

Initially, the filtration sequence included a 

sand/anthracite filter followed by a 5-

micron cartridge filter as represented 

below.

Figure 2A

Initial Treatment Train

Organo

sulfide 

dosing 

tank

Sandanthracite 

filter

5 μm 

cartridge 

filter

Pump
However, initial analytical data suggested 

that colloidal mercury was passing through 

the 5-micron filter and contaminating the 

resin downstream. To eliminate the 

remaining colloidal mercury, a second 

cartridge filter rated at 0.35 microns was 

added as represented below. At this time, 

the sand filtration step was bypassed as 

redundant with the downstream filtration. 

Figure 2B

Revised Treatment Train

Even with this change, ongoing analytical 

data indicated that colloidal mercury was 

still passing through filtration to 

contaminate the resins. At this early stage 

of the pilot study it was decided to add an 

ultrafiltration step to replace the 0.35-

micron filter in order to completely remove 

the colloidal mercury. 

The UF membrane module was a hollow 

fiber, PVDF design, employing an outsidein flow configuration, with an effective pore 

size of 0.03 microns. The new pilot 

configuration is represented below:

Figure 2C

Final Treatment Train

Selective Ion Exchange

The selective ion exchange skid was 

comprised of six columns housing three 

different resins. The resin columns 

employed a packed-bed, down-flow design 

and were pump fed from a tank through a 

common distribution header. Each pair of 

columns could be connected in series 

allowing for evaluation of an effective resin 

depth of either 48” or 96”. 

Figure 3

Ion Exchange Beds

Resin 

Column

Flow Meter

Quick 

Connect

Effluent To drain

Sample 

Tap

ThioUr AThioUr B

L07 AL07 B

Thiol AThiol B

Organo 

sulfide 

dosing 

5 μm 

cartridge 

filter

Ultrafiltration 

system

Sand

filter

Pump

Break 

tank

Pump

Organo 

sulfide 

dosing 

tank

5 μm 

cartridge 

filter

0.35 μm 

cartridge 

filter

Pump
Based on this setup, the effluent from any 

individual column could be collected for 

analysis. 

PILOT PLANT OPERATION

The operation of the pilot proceeded 

without any major operational issues. The 

sand filter required periodic backwashing 

as expected. The cartridge filters also 

required periodic changeout. A clean in 

place system was included with the 

ultrafiltration skid and weekly chemical 

cleanings were performed. 

The resins were kept in place for the 

remainder of the test. Some compaction of 

the resin did occur within the columns. 

The one operational issue that was 

observed was a significant fouling of the 

ultrafiltration membranes. Even after 

physical/chemical treatment, the FGD 

wastewater remains very high in dissolved 

solids and makes a difficult matrix for 

ultrafiltration. Several different cleaning 

regimes were used before the appropriate 

cleaning regime was determined. 

SAMPLING AND ANALYTICAL METHODS

To obtain reliable data in the single digit 

part per trillion range, sampling and 

analytical analysis are extremely important. 

All sampling collection followed EPA’s 

“Sampling Guidance” (EPA Method 1669) 

to reduce or eliminate contamination. 

Using this method, samples were collected 

using the “clean hands/dirty hands” 

technique in fluoropolymer (FLPE) and 

glass sample collection bottles. Samples 

were shipped unpreserved via overnight 

delivery and then acid-preserved upon 

receipt by the analytical lab. 

EPA developed Method 1631 specifically 

to address the need for reliable 

measurement of mercury to concentrations 

in the range of 0.5 to 100 ng/L. Method 

1631 supports technology-based and water 

quality-based monitoring programs 

authorized under the Clean Water Act for 

mercury. Following EPA Method 1631, 

measurement of mercury is accomplished 

via the oxidation of mercury with bromine 

monochloride (BrCl), sequential reduction 

with ammonium hydroxide and stannous 

chloride to convert Hg(II) to volatile Hg(0) 

for ultimate detection via cold-vapor atomic 

fluorescence spectrometry (CVAFS).

PILOT TEST RESULTS

The initial results revealed limited removal 

(approximately 40%) of the total mercury 

through the treatment train. Subsequently, 

this hydraulic loading rate was considered 

too high for the influent pollutant load, 

which during that time ranged from 800 to 

1,700 ppt as highlighted in the table below. 

Lowering the hydraulic loading rate 

produced an immediate and significant 

improvement in treatment performance, 

yielding 65 to 80% removal of influent 

mercury for a similar influent concentration 

range. However, the effluent mercury 
concentration remained well above the 

ultimate objective of 12 ppt. 

It was suspected that the resin 

performance was being hindered by nonspecific fouling with particulate or colloidal 

mercury. A serial filtration study was 

conducted to characterize the form of 

mercury present. These results illustrated 

that a significant fraction of mercury 

thought to be soluble was in fact removed 

via filtration through a 0.2-micron filter, 

demonstrating the presence of a significant 

colloidal mercury fraction smaller than 0.2 

microns. 

Accordingly, the ultrafiltration stage was 

added to the pilot treatment train. The new 

configuration showed immediate 

improvement. 

Using this configuration, the results 

continued to show significant variability, 

presumably due to colloidal mercury 

remaining on the resin. However, the 

system performance improved over the 

next few weeks. Results are presented 

below:

Table 2

Microfiltration & Ion Exchange Results

Sample 

Date

Results

Influent

Influent 

Flow Effluent Resin/ 

Depth

Column 

Flow

ppt gpm ppt gpm

07/23/08 825 2.33 530 Thiol/96" 0.5

08/06/08 647 1.11 221 Thiol/96" 0.2

08/13/08 1640 2.59 337 Thiol/96" 0.2

08/19/08 375 2.50 56.8 ThioUr/48" 0.1

08/20/08 337 2.38 47.9 ThioUr/48" 0.2

08/20/08 385 2.38 42.3 ThioUr/48" 0.2

08/27/08 816 2.55 333 ThioUr/48" 0.2

09/03/08 1670 2.31 112 ThioUr/48" 0.2

09/10/08 2030 2.42 30.4 ThioUr/48" 0.2

09/17/08 1460 1.92 494* ThioUr/48" 0.2

09/24/08 1080 2.42 88.7 ThioUr/48" 0.2

10/01/08 553 2.51 45.6 ThioUr/48" 0.2

Min 337 1.11 30.4 0.1

Max 2030 2.59 530 0.5

Average 985 2 168 0.2
Table 3

Ultrafiltration & Ion Exchange

Sample 

Date

Results

Influent

Influent 

Flow Effluent Resin/ Depth Flow

ppt gpm ppt gpm

10/22/08 421 2.48 < 5 ThioUr/48" 0.2

10/29/08 167 2.6 < 5 ThioUr/48" 0.2

11/05/08 168 2.47 < 5 ThioUr/48" 0.2

11/12/08 588 2.22 < 5 ThioUr/48" 0.2

11/19/08 372 2.24 < 5 ThioUr/48" 0.2

12/09/08 130 2.09 < 5 ThioUr/48" 0.2

12/10/08 189 1.69 < 5 ThioUr/48" 0.2

Min 130 1.7 < 5 0.2

Max 588 2.6 < 5 0.2

Average 291 2.3 < 5 0.2

RESIN LOADING AND DISPOSITION

As mentioned earlier in the paper, bulk 

loading of the mercury was not the primary 

challenge for the media tested here. Both 

the Thiol and the Thiouronium 

functionalized resins were capable of 

removing mercury to the desired limits. 

Post mortem analysis of the resin showed 

that loading of other metals via the 

selective functional groups or nonselectively via inorganic precipitation or 

fouling may be a more important 

consideration for the long term application

of these media. Since variation in coal 

sources, variations in station operations 

and variations in pretreatment processes 

all affect the type and level of potentially 

interfering ions, pilot evaluation of the 

various media options is likely to be 

needed to sort through these issues. 

PILOT STUDY CONCLUSIONS

Achievable Effluent Mercury

The pilot was successful in removing trace 

mercury from FGD wastewater to

concentrations below 5 ppt . Thus the 

most important goal of the study was 

achieved. 

Most Effective Resin

The most effective resin was the 

thiouronium functional group. While the 

Thiol resin was also very effective, the 

performance of the Thiouronium resin was 

slightly more consistent. Only minimal/

incidental removal was achieved by the 

silver impregnated resin.

Effect of Filtration

The majority of the mercury removal (on a 

mass basis) was via filtration. While sand 

and cartridge filters removed the mercury 

to some degree, they did not remove all of 

the colloidal mercury. Complete removal 

of colloidal mercury was only achieved 

upon incorporation of the ultrafiltration 

treatment stage. It must be noted that the 

observed results are based upon the 

distribution and form of mercury present
within the FGD wastewater at the specific

facility tested. 

In any application where selective ionexchange is to be used downstream, it is 

necessary to remove ALL of the colloidal 

mercury to avoid its deposition on the

resin. 

The form and concentration of mercury is 

known to vary widely in FGD wastewater 

based on a number of variables such as 

coal composition and wet scrubber 

operational practices. Limited data from 

other pilot testing show removal of mercury 

to single-digit ppt concentrations without 

the use of ultrafiltration. Because of this, a 

bench scale analysis of particle size

distribution is recommended prior to pilot or 

full-scale testing of any trace mercury 

removal application.

Operational Difficulties

As previously mentioned, the only 

significant operational difficulty 

encountered during the pilot study involved

scaling of the ultrafiltration membrane. 

This issue was resolved by optimizing the 

chemical cleaning regime including reagent 

chemistry and dosage, length of cleaning 

cycle and cleaning frequency. 

Technology Status

All of the technologies included in the pilot 

test are well established and can be 

implemented commercially. With the 

proper application of the selective ion 

resin, it is possible to achieve mercury 

concentrations below 5 ppt in FGD 

Wastewater. With proper design work 

(including treatability testing), this 

technology is ready for commercial 

application.

REFERENCES

1. Goltz, H. Robert, “Trace 

Contaminant Removal from Ground 

and Waste Waters with Selective 

and Non-Selective Media” IWC-08-

76 (2008). 

2. Pudvay, Michael & Goltz, H. Robert, 

“Mercury Removal in Flue Gas 

Desulfurization Wastewater” IWC07-XX (2007)