Full Scale Operation of GE ABMet® Biological Technology for 

the Removal of Selenium from FGD Wastewaters 

Jill Sonstegard, Tim Pickett 

GE Water & Process Technologies 

265 Crossroads Square 

Salt Lake City, UT 84115, USA 

jill.sonstegard@ge.com

tim.pickett@ge.com

James Harwood 

GE Water & Process Technologies 

3239 Dundas Street West 

Oakville, Ontario L6M 4B2, Canada 

james.harwood@ge.com

Danny Johnson 

Progress Energy Carolinas, Inc. 

P.O. Box 1551 

Raleigh, NC 27602, USA 

danny.johnson@pgnmail.com

IWC-08-31 

Keywords: FGD wastewater, biological reduction, bacteria, selenium, mercury, nitrate, 

heavy metals 

Summary: To reduce sulfur dioxide emissions from the flue gasses produced while burning 

coal, Wet Flue-Gas Desulfurization (WFGD) systems are being employed at coal-fired power 

plants across the globe. These WFGD systems transfer sulfur and other compounds 

released during coal combustion from a gaseous phase to a liquid phase and then 

collectively contains them in a wastewater stream. These same facilities may also utilize 

Dibasic Acid (DBA) to increase WFGD efficiency. Treatment of the resulting wastewater in 

order to comply with increasingly stringent discharge limits presents itself as a formidable 

challenge. The untreated waste stream concentrations of soluble selenate can reach over 10 

ppm as well as contain elevated levels of mercury, arsenic and other solublized heavy 

metals, all of which require removal prior to environmental discharge. Two power plants at 

Duke Energy and Progress Energy in North Carolina have employed ABMet® biological 

technology for the removal of these contaminants from WFGD blowdown. This paper will 

discuss the fundamentals of the biological reduction process; its demonstrated removal 

efficiency of selenium, nitrate, and other trace metals; the design and commissioning of the 

first two facilities; as well as the data compiled during the initial months of full-scale operation 

with an emphasis on the Progress Energy Roxboro Station. 
‐‐ 2 ‐‐

BACKGROUND 

In 2002, the state of North Carolina adopted 

the Clean Smokestacks Act requiring power 

companies within the state to reduce their 

smog and haze-forming emissions by 

approximately 75% over the following 

decade. As part of this legislation, a 73 

percent cut in sulfur dioxide (SO2) 

emissions from 2002 levels is required by 

20131

. In order to achieve this target, 

Progress Energy and Duke Energy have 

both retro-fitted Wet Flue-Gas 

Desulfurization technologies into some of 

their coal-fired generating facilities. 

Progress Energy’s largest coal generating 

facility, the 2,443 MW, four-unit Roxboro 

Station2

 is the second of Progress’ stations 

to be retro-fitted with WFGD scrubbers 

under the new act. Located just outside of 

Roxboro, NC and about 75 miles north of 

Raleigh, NC, Roxboro is the tenth largest 

coal-fired station in North America, and the 

largest in North Carolina. In order to meet 

the standards of the act, Progress Energy 

turned to Babcock & Wilcox to provide their 

WFGD technology to achieve over 97% 

SO2 removal. At full design capacity, the 

WFGD scrubbers are expected to produce 

1,400 gpm of blowdown flow. 

Roxboro Station is located on the shore of 

Hyco Lake which is used to supply cooling 

water and WFGD make-up water to the 

plant. Hyco Lake also receives all of the 

plant’s water discharges, including the new 

blowdown from the scrubbers. The lake is 

home to a large fish population and is a 

very popular recreational destination in the 

summer months for anglers and cottagers. 

Special care needed to be taken to ensure 

that harmful levels of contaminants such as 

selenium, nitrate and other trace metals 

were eliminated from an environment that 

could affect the local fish and animal 

populations. 

While all of these contaminants are 

potentially harmful in high enough levels, 

selenium and mercury in particular must be 

controlled to very low levels as these 

elements can bio-accumulate in living 

tissue. Long-term exposure to elevated 

concentrations can cause serious health 

issues over several years. 

WET FLUE-GAS DESULFURIZATION

Wet Flue-Gas Desulfurization scrubbers, 

such as those from Babcock & Wilcox at 

Roxboro, remove SO2 from the flue gas by 

passing the flue gas through a large 

chamber (spray or absorber tower). 

WFGD systems utilize a limestone reagent 

mixed with water to form a slurry. As the 

gas travels through the tower, it is scrubbed 

with a spray made of the limestone mixture. 

Multiple levels of sprayers are utilized to 

obtain the most efficient contact between 

the limestone and the flue gas. As the flue 

gas contacts the spray, the limestone 

(calcium carbonate - CaCO3) in the 

limestone reacts with the SO2, removing the 

majority of the SO2 from the flue-gas before 

it exits the top of the stack. The rest of the 

slurry collects in the bottom of the tower 

where it is mixed and aerated in order to 

oxidize the bisulfite ions and produce 

calcium sulfate dehydrate (CaSO4•2H2O), 

or gypsum, which then precipitates out of 

solution. 

The slurry in the bottom of the tower is 

constantly recirculated back through the 

sprayers. A portion of the slurry mixture 

containing the precipitated gypsum is bled 
‐3‐

off of the bottom of the tower (blowdown). 

This blowdown is sent to a pair of 

Hydroclones to remove the bulk of the 

precipitated gypsum from the stream. The 

removed gypsum is then sent through 

vacuum belt filters to produce a dry cake for 

sale to a wallboard manufacturer. 

Figure 1 - Generic WFGD Schematic 

After the gypsum is recovered from the 

blowdown, the wastewater is still laced with 

various contaminants which originate from 

the coal itself and are also captured in the 

WFGD process. These contaminants must 

undergo further treatment before they are 

allowed to be discharged into the 

environment. 

TREATMENT OPTIONS 

Significant concentrations of many trace 

elements and heavy metals pose a unique 

challenge to treatment, as high levels of 

TDS, Cl-, Ca++, Mg++ and SO4= make 

barrier filtration using RO or NF difficult. To 

add to this challenge, the majority of the 

selenium present in the blowdown stream is 

in the form of soluble selenate (SeO4

2-). 

Several options were considered by 

Progress Energy to make their blowdown 

suitable for discharge, such as conventional 

physical/chemical systems, constructed 

wetlands and biological treatment. 

Without additional treatment processes, 

conventional physical/chemical systems 

using chemical precipitation and 

clarification have not been shown to 

consistently meet the required effluent 

criteria with regards to selenium in FGD 

applications. Progress Energy was also 

sensitive to consuming vast quantities of 

chemicals and disposing of large amounts 

of sludge. 

Constructed wetlands were strongly 

considered as a potential alternative. 

However, this approach would have 

required approximately 100 acres of land 

for the cells, dilution of the blowdown 

stream in order to mitigate potential chloride 

toxicity, and had not demonstrated the 

ability to remove selenium at the high 

removal rates required. 

Anoxic biological treatment had not yet 

been demonstrated in a full-scale WFGD 

application during the evaluation; however, 

it seemed to offer the best solution over the 

alternatives. With biological treatment, the 

foot-print would be <1 acre for a system of 

Roxboro’s size, the amount of chemical 

required would be minimal, sludge volumes 

would be reduced and chloride tolerances 

of the microbes could handle up to 20,000 

mg/l without adverse reactions. 

PRETREATMENT 

Even after gypsum recovery via 

Hydroclones, WFGD blowdown may still 
‐4‐

contain over 4,000 mg/l of TSS, and may 

also still be well over maximum operating 

specifications of 105 O

F. Both of these 

values need to be reduced in order to treat 

the blowdown biologically. Progress 

Energy employed the use of a 250 million 

gallon settling pond. The pond has a 

retention time of approximately 125 days 

and has four purposes: settle out 

suspended solids in the blowdown, equalize 

the blowdown to mitigate any fluctuations in 

the chemistry of the stream, cool the 

blowdown, and provide 30 days of 

blowdown storage capacity. 

Solids as mostly gypsum fines will settle-out 

in the pond over time. With this settling, it 

is expected that the pond will require 

dredging every 10-20 years depending on 

solids loading and Hydroclone 

performance. Decant is taken from the far 

end of the settling pond via a floating 

skimmer and sent to the biological 

treatment system. 

BIOLOGICAL PROCESS OVERVIEW 

 Max Min Avg 

Chlorides, mg/l 7,000 2,000 - 

TSS, mg/l 250 20 40 

Temp. OF 105 60 - 

pH 6.5 5.5 - 

Selenium, mg/l 3 1.2 - 

TDS, mg/l - - 13,500 

Table 1 – Design Bioreactor Influent 

Criteria 

Following the settling pond, water is sent to 

a series of fixed-film, biological reactors. 

The bioreactors utilize a mixture of 

specifically selected, naturally occurring 

bacteria. 

Many diverse species of bacteria have 

been proven capable of reducing dissolved 

selenium species (selenate and selenite) to 

particulate elemental selenium in an 

aqueous reductive environment3

 by linking 

the oxidation of organic substrates to the 

dissimilatory reduction of the selenium 

oxyanions4

. The oxidized selenium 

compounds serve as electron receptors 

during the electron transport process. This 

effectively reduces the oxidized selenium 

compounds to nanospheres of granulated 

elemental selenium which accumulate in 

and around the bacterial cells. The unique 

physical properties of these granules 

cannot currently be produced by purely 

chemical means and exhibit a higher level 

of stability than other chemically-induced 

elemental selenium formations5

. 

Within the reactors, the bacteria are fixed to 

a granular activated carbon (GAC) bed. 

The GAC is not used for absorption, but 

rather as a substrate for bacterial growth. 

This GAC matrix creates a fixed biological 

film which retains the biomass and prevents 

it from being washed out of the reactors. 

Achieving a reducing environment is critical 

to the success of the process. Wastewater 

entering the system has a positive ORP in 

the range of +200 to +300 mV. As 

wastewater travels through the bioreactors, 

the ORP drops across the system, causing 

several different reactions to occur in 

various locations throughout the carbon 

biomatrix. 
‐5‐

Figure 2 – Redox Profile 

As the ORP approaches 0 mV, 

denitrification occurs as the nitrate is 

reduced to nitrogen gas which is then 

released to the atmosphere. 

NO3- + org. C → NO2- + org. C → N2 + CO2 + H2O 

As the ORP drops further into the negative 

range, selenate and/or selenite is then 

reduced to an elemental state. 

SeO4

2- + org. C → SeO32- + org. C → Se0

 + CO2 + 

H2O 

Further reductions in ORP will yield sulfide 

production within the system, which will 

precipitate out other trace metals such as 

zinc, copper, nickel, lead and others as 

metal sulfides. These precipitates are 

retained within the biomatrix much the 

same as the reduced selenium. 

H2S + M2 → MS + 2H+, where M= Metal 

The precipitated constituents are retained 

within the biomatrix until the system is 

backflushed, leaving the effluent virtually 

free of these contaminants. 

Controlling the ORP in the process is 

achieved by feeding a proprietary 

molasses-based nutrient blend into the 

reactors as a carbon source for the 

bacteria. The nutrient is fed into the influent 

line of each bioreactor through an injection 

quill and static mixer to ensure that the 

system is evenly fed. The dosage rate of 

the nutrient is dependant upon several 

factors, including feed nitrate levels, 

required removal rates and desired ORP 

range. 

The bioreactors are designed for a plugflow, ensuring even distribution of the feed 

water and maximum contact for the 

bacteria. Roxboro utilizes four parallel 

bioreactor trains, each with two concrete 

bioreactors in series. Each of the 

bioreactors in the first and second-stages 

are identical. 

Material selection in this environment is 

important to the long-term operations of the 

plant. With such high levels of chlorides, 

carbon or regular (304, 316) series 

stainless steels are not appropriate due to 

the potential for corrosion. All process 

piping is non-metallic, typically PVC or 

HDPE depending on the line size and 

service. Teflon-coated butterfly valves are 

used for most valve applications and pumps 

have 2205 duplex stainless steel wetted 

parts. The bioreactors are lined with an 

HDPE sheet to ensure longevity of the 

concrete tanks. 

Wastewater from the settling pond is 

pumped via two centrifugal pumps (two 

duty, one on-line spare) to the first-stage 

bioreactors. It is then fed into the top of the 

ORP 

(mV) 

Process Electron 

Acceptors 

Conditions 

O 2

Oxic 

Or 

Aerobic 

NO 3 Anoxic 

Anaerobic 

+300 

+200 

+100 

0 

-100 

-200 

-300 

-400 

SO4

Carbonaceous 

Organics 

Fermentative 

Anaerobic 

1

2 

3 

4 

5 6

7 

8 

9 

1 – Organic Carbon Oxidation 

2 – Polyphosphate Development 

3 – Nitrification 

4 – Denitrification 

5 – Polyphosphate Breakdown 

6 – Selenium Reduction 

7 – Sulfide Formation 

8 – Acid Formation 

9 – Methane Formation 
‐6‐

bioreactors and flows evenly across the top 

of the GAC biomatrix through a distribution 

piping grid. Wastewater passes through 

the biomatrix and is collected via a piping 

collection grid located in a gravel subfill in 

the bottom of the reactor. The effluent from 

the first-stage bioreactors is pumped via a 

booster pump into the top of the secondstage bioreactors. Once the water flows 

through the second-stage bioreactor, it is 

sent via gravity to the ash pond discharge 

canal. 

Figure 3 - Bioreactor Cross-Section 

Periodically, the denitrification occurring 

within the system will cause nitrogen gas to 

build-up and form pockets within the GAC 

beds. These pockets can have a negative 

effect on the performance of the process by 

allowing for flow to short-circuit around 

them, and can increase the perceived 

head-loss across the bioreactors. A 

degassing sequence is employed in which 

a dedicated flush pump pulls water from the 

feed pond and sends it at a high flux rate 

into the bottom of the bioreactor via a 

dedicated piping distribution grid. The flush 

duration is long enough to loosen the media 

so as to allow for trapped nitrogen gas to 

escape, but short enough to remain below 

the headspace of the bioreactor. 

Differential pressure is monitored across 

the bioreactors using a combination of tanklevel transmitters and pressure transmitters 

located on the effluent piping of each 

bioreactor. Increases in differential 

pressure can be used as an indication of 

the accumulation of gas pockets within the 

beds. If a degas does not recover the 

differential pressure, an accumulation of 

solids in the bioreactors is likely and a 

backflush would be required. Degassing 

may also be done proactively to ensure 

optimum performance. 

Solids in the form of captured TSS, 

precipitated metals and biomass will also 

build up over time. When backflushed, the 

steps are very similar to the degas 

sequence where the same dedicated flush 

pump pulls water from the settling pond and 

sends it into the bottom of the bioreactor 

being flushed. During a backflush, the 

higher flow fluidizes the bed, dislodging the 

trapped solids, precipitated metals and any 

excess biomass, and carries it out of the 

bioreactor as overflow while retaining the 

activated carbon biofilm. The spent water 

is sent to the lined flush pond, similar to the 

settling pond but much smaller. The flush 

pond remains undisturbed, allowing the 

solids to settle and the clean water to 

decant off of the top of the pond back into 

the settling pond. The flush pond is sized 

to hold 20 years of accumulated solids. 

START-UP 

The first Roxboro WFGD started-up in April 

2007, followed by a second WFGD in 

November 2007. Blowdown from these two 

WFG scrubbers was stored in the settling 

pond while the water level came to a 

minimum operating height. The biological 

h1 – collection and flush 

distribution systems 

 in gravel subfill

h2 – ABMet®

Biomatrix 

h3 – biomatrix expansion 

volume

h4 – headspace

h5 – reactor cover
‐7‐

system at Roxboro was started-up in late 

February 2008, which necessitated 

changes in the seeding process. At the 

time of commissioning, the settling pond 

temperature was just below 50 OF, lower 

than the design value by 10 degrees. For a 

two-week acclimation period, a heated 

recirculation loop was used to maintain the 

reactor temperature at over 80 OF. After 

acclimation, raw feed water from the 

settling pond was introduced without 

supplemental heating. Even at low 

temperatures, the biological system was 

fully acclimated to the feed and was 

consistently producing effluent selenium 

values below design requirements by week 

5 of operations. 

OPERATIONAL OBSERVATIONS 

Through the beginning of June 2008, total 

selenium effluent values were consistently 

less than 10 μg/l with feed values averaging 

1,500 μg/l coming out of the feed pond. 

This equates to an average removal 

efficiency of 99.3%. Feed values for 

selenium have been as expected based on 

the design basis. The feed TSS loading 

has been well below design values, 

indicating that the Hydroclones and the 

settling pond are performing above 

expectations. Feed TSS has averaged <10 

mg/l throughout the spring of 2008. The 

feed pH has been well higher than 

expected, averaging slightly less than 7.5. 

The system was not backflushed until July. 

The low levels of TSS in the feed water 

helped to slow the need for a backflush by 

limiting the nutrient feed’s contribution to 

the solids in the carbon beds. Effluent 

selenium levels slightly increased in June, 

indicating that the system was in need of a 

backflush in order to remove the 

accumulated biomass and precipitated 

solids. 

Results of backflushing in July 2008 

showed a full recovery of the system, with 

effluent levels of selenium back below 10 

mg/l. 

CONCLUSIONS 

Overall, the biological system is performing 

as expected in terms of selenium reduction, 

and even better than expected when the 

start-up feed water temperature is 

considered. Several key points need to be 

taken away from this project: 

• Facilities with multiple scrubbers will 

have staggered start-ups and 

subsequent increases in blowdown 

flow. The system needs to have the 

flexibility to handle all flow scenarios. 

• The acceptable feed water 

temperature for acclimation and 

operations may be more forgiving 

than initially perceived, with the 

system starting operations in 50 OF 

water while the design minimum was 

originally 60 OF. 

• Accurate, factory certified ORP 

probes are critical. Verification with 

hand-held ORP meters can be done, 

provided that a meter is selected 

with the right electrode material 

(platinum). 

• Proper analytical techniques for lowlevel selenium need to be used. 

• A comprehensive sampling protocol 

with quick analysis turn-around is 

needed during the first few months of 

operation in order to ensure that the 

system is fully optimized for the best 

performance. 

• Changes in blowdown chemistry 

need to be monitored closely in order 

to adjust system parameters as 

needed. 
‐8‐

Figure 4 - Bioreactor Process Flow Diagram 

Feed 

Pond 

1st Stage 

Bioreactors 

Feed 

Pumps 

Booster Pumps 

Flush Pump 

2nd Stage 

Bioreactors 
‐9‐

Figure 5 – Roxboro Bioreactor System 
‐10‐

Figure 6 – Roxboro Bioreactor Selenium Analytical Data 

0

100200300400500600700800900

10001100120013001400150016001700180019002000

3/4 3/11 3/18 3/25 4/1 4/8 4/15 4/22 4/29 5/6 5/13 5/20 5/27 6/3 6/10 6/17 6/24 7/1 

Total Selenium ug/L

Bioreactor Feed Bioreactor Effluent
‐11‐

REFERENCES 

                                                      

1 North Carolina Department of Environment and Natural Resources – Division of Air Quality website: 

http://www.ncair.org/news/leg/cleanstacks.shtml 

2 Progress Energy Website: http://www.progressenergy.com/aboutenergy/powerplants/2008generatingplants.pdf 

3 Frankenberger, W T., Jr; Karlson, U. Microbial volatilization of selenium from soils and sediments. In: 

Frankenberger W T, Benson S. , editors; Frankenberger W T, Benson S. , editors. Selenium in the 

environment. New York, N.Y: Marcel Dekker, Inc; 1994. p. 369–387 

4 De Souza, M.P. et al. 2001. Identification and characterization of bacteria in a selenium-contaminated 

hypersaline evaporation pond. Applied and Environmental Microbiology. 67:3785-3794 

5 Oremland, R. S., et al. 2004. Structural and spectral features of selenium nanoshpheres produced by 

se- respiring bacteria. Applied and Environmental Microbiology. 70: 52-60