ABMet: Setting the Standard for Selenium Removal 

Jill Sonstegard, James Harwood, Tim Pickett 

GE Water & Process Technologies 

265 Crossroads Square 

Salt Lake City, Utah 84115 

jill.sonstegard@ge.com

james.harwood@ge.com

tim.pickett@ge.com

William Kennedy, PE 

Orion Engineering, PLLC 

b.kennedy@orionengr.com

IWC-10-XX 

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

Summary 

As public awareness of environmental concerns has risen, increasingly stringent discharge 

regulations are being imposed. Coal-fired generating stations have become the subject of 

such regulations for both air and water emissions. Release of sulfur dioxide from coal-fired 

boilers can be mitigated through the employment of wet forced-oxidation Flue Gas 

Desulfurization scrubbers; however, these systems can produce a wastewater stream 

containing selenium, nitrate and other constituents, which necessitate further treatment 

prior to discharge. 

Removal of selenium in its oxidized forms to concentrations of less than 10 parts per billion 

(ppb) has proven to be a formidable challenge for conventional physical and chemical 

systems. Although historically proven, natural insitu reduction of selenate/selenite in wetland 

systems requires a lengthy contact time and may allow the selenium to remain in the local 

ecosystem. A more efficient approach to natural remediation technology is the utilization of 

fixed-film, packed-bed bioreactors, which exploit site-specific, naturally-occurring, nonpathogenic microbes in an optimized, self-contained system. These systems have been 

designed to remove selenium in a two-to-sixteen hour empty bed contact time while 

sequestering selenium in a low-volume sludge that may be easily filtered for disposal. The 

self-perpetuating microbial biocatalysts require only a nominal nutrient supplement and 

power supply, thereby resulting in very low operating and maintenance costs. In addition to 

selenium and nitrate removal, other constituents and trace metals can be co-precipitated, 

further increasing the overall benefit and value of the system. 

Three prominent United States utilities have pioneered the employment of this revolutionary 

treatment system at five coal-fired generating stations. This paper will discuss the 

fundamental concepts of biological reduction, Scanning Electron Microscope imaging of 

selenium nanospheres; ABMet’s demonstrated removal efficiency of selenium; and the 

design, cycling, performance data and operating and maintenance costs of the full-scale 

systems. 
2

Background 

The United States Environmental 

Protection Agency (EPA) is required by the 

federal Clean Water Act to develop 

guidelines addressing priority constituents 

being discharged into public waters. 

While all priority constituents are 

potentially harmful in high enough levels, 

selenium in particular is controlled to very 

low levels, as these elements may bioaccumulate in living tissue. The current 

EPA National Recommended Water 

Quality Criteria 

(www.epa.gov/waterscience/criteria) are 

designed to assess and quantify the total 

recoverable selenium concentrations in a 

body of water and have an established 

chronic value of 5 ppb (μg/L) and an acute 

value of 20 ppb Se. States currently have 

the option to accept the recommended 

guidelines or to adopt more stringent 

regulations of their own. 

Recent environmental publicity and 

litigation have encouraged the EPA to 

propose new guidelines for constituents 

such as selenium. The new EPA guidelines 

will have a significant impact on coal-fired 

power plants. Currently many of those 

plants abiding by the SOx restrictions 

(implemented by the Clean Air Act of 

1990), have been retrofitted with wet Flue 

Gas Desulfurization (FGD) units. These 

FGD systems are designed to introduce an 

alkaline sorbant, typically consisting of 

lime or limestone in a spray slurry form, 

into the exhaust of an existing coal-fired 

boiler. The alkali reacts with the SO2 gas 

and is collected in a liquid form as calcium 

sulfite or calcium sulfate slurry. The slurry 

is discharged to a treatment system 

where the gypsum (which when removed 

by centrifugation) can be recovered as a 

saleable product. 

The resulting pre-treated FGD wastewater 

(blowdown) can contain elevated levels of 

mercury, nitrate and selenium as well as 

other metals and metalloids found in coal. 

Besides the extreme variability in effluent 

quality due to the variety of coal sources, 

limestone sources, and scrubber 

operation, FGD blowdown poses a unique 

set of challenges for treatment. Chloride 

levels can run over 25,000 ppm 

exacerbating scaling and corrosion issues. 

Temperatures in excess of 140°F and 

scrubber additives such as dibasic acid 

(DBA) both require treatment prior to 

discharge. Scrubber systems can go offand on-line with the power units as the 

need for power and routine maintenance 

mandates, resulting in large fluctuations in 

the quantity of wastewater produced. 

Analytical analysis in these applications is 

subject to interferences from high Total 

Dissolved Solids (TDS), bromine, chlorides, 

sulfur, argon, iron, manganese, copper 

and zinc, as well as having the target 

discharge limits so close to analytical 

detection limits that an acceptable margin 

of error can mean the difference between 

meeting or exceeding permit levels. 

Fixed-Film Biological Process Overview 

Following a settling pond or clarification to 

remove excess suspended solids (which 

often exceed 5,000 ppm after gypsum 

removal), the blowdown can be sent to a 

series of fixed-film, packed-bed, anoxic 

bioreactors. These reactors are filled with 

a permanent porous substrate such as 

granular activated carbon (GAC) and are 

seeded with a naturally-occurring, 

beneficial, consortia of bacteria. 

In the proprietary technology discussed in 

this paper, the bacteria used for seeding 

have been isolated from previouslycontaminated sites and chosen 

specifically for use in FGD systems 

because of their hardiness in the extreme 

water chemistry as well as for their proven 

efficiency for selenium respiration and 

reduction. The bacteria form a fixed 

biofilm throughout the GAC substrate 

material (termed biomatrix), forming a 
3

cooperative microbial community. In 

addition to increasing the efficiency of 

selenium removal, this communal 

structure also serves as a shelter, 

protecting the bacteria from chemical and 

physical upsets. These bacteria are 

facultative anaerobes, which flourish in 

oxygenated environments; however, when 

oxygen is depleted the bacteria utililize 

other compounds for respiration, resulting 

in the dissimilatory reduction of other 

oxianions present in the water. See Table 

1. 

Flow 

Through 

Bioreactor 

Final eAcceptor Approximate 

ORP 

Oxygen > 0 mV 

Nitrate < 0 mV 

Nitrite < -50 mV 

Selenate < -100 mV 

Selenite < -150 mV 

Sulfate < - 200 mV 

Table 1. Fixed Film Bioreactor 

Gradational ORP Zones 

A reducing environment is required to 

facilitate selenium reduction. The plugflow design of the system allows 

gradational zones of decreasing ORP to 

form in layers throughout the biomatrix. 

As the ORP approaches 0 mV, 

denitrification occurs with the nitrate 

being reduced to nitrogen gas and 

released to the atmosphere. Complete 

nitrate reduction is achieved in the system. 

NO3 + organic C → NO2 + organic C → 

N2 + CO2 + H2O 

Deeper in the bioreactor bed, the ORP 

drops lower; dissolved selenate and/or 

selenite is reduced to an elemental state, 

precipitates out of solution and is filtered 

from the effluent by the biomatrix. 

SeO4 + organic C → SeO3 + organic C →

Se + CO2 + H2O 

Reduction and precipitation of selenium 

occurs within a two-to-sixteen hour empty 

bed contact time (EBCT). EBCT is the 

residence time of the water assuming that 

there is no substrate, biofilm or solids 

present in the bioreactor. 

Periodic degassing is required to release 

the gasses from the bioreactor bed. This is 

achieved with a small-volume, high flow of 

stored effluent water back through the 

biomatrix, which releases the gasses 

trapped within. 

Collection of the precipitated constituents 

is achieved by a backwash cycle, which is 

strong and long enough to fluidize the 

biomatrix. This allows the release and 

collection of the contaminants and spent 

biomass. Attrition of the GAC has been 

insignificant at of the full-scale facilities to 

date and does not contribute to the 

minimal sludge volume. 

A proprietary molasses-based nutrient is 

fed into the reactors as a carbon source 

for the bacteria (this carbon is a food 

source for the bacteria, and is not to be 

confused with the Granular Activated 

Carbon substrate which serves as the 

permanent structure for the biofilm). The 

nutrient is fed into the influent line of the 

bioreactors utilizing a static mixer to 

ensure the system is evenly fed. 

Bioreactor Engineering & Design 

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

water and maximum contact with the 

bacteria throughout the biomatrix. The 

system utilizes parallel trains with each 

train consisting of a first- and secondstage bioreactor cell. The systems are 

scalable to virtually any flow size by the 

addition (or reduction) of trains and by 
4

varying the volume of the individual cells. 

Each of the bioreactors in the first- and 

second-stage are identical to each other, 

which allows for alternate cycling and 

continuous operation during degas and 

backwash cycles. 

Material selection in an FGD system is 

essential to the reliable, long-term 

operation of the plant. Elevated chloride 

levels in the water preclude the use of 

carbon steel or low alloy stainless steels 

due to the potential for corrosion. Process 

piping is non-metallic, typically fiberglass 

reinforced plastic (FRP) or high-density 

polyethylene (HDPE). Coated butterfly 

valves are used for most applications and 

pumps have 2205 duplex stainless steel 

wetted parts or are rubber-lined to ensure 

longevity. 

Water is fed into the top of the bioreactors 

and flows evenly across the top of the 

biomatrix through distribution piping. The 

water passes down through the biomatrix 

and is collected via a manifold located in a 

gravel subfill in the bottom of the reactor. 

The effluent from the first-stage 

bioreactors is pumped into the top of the 

second-stage bioreactors. See Figure 1. 

Periodically, the denitrification occurring 

within the system will cause gases to build 

up and form pockets within the biomatrix. 

These pockets can have a negative effect 

on the process by allowing the water flow 

to channel, resulting in reduced microbial 

contact and increasing head-loss across 

the bioreactors. A degassing sequence is 

employed in which a backwash pump 

pushes water at a high flow rate into the 

bottom of the bioreactor via a dedicated 

manifold. The flush duration is long 

enough to lift the biomatrix and allow the 

entrained gases to escape, but short 

enough to avoid wasting any water out of 

the bioreactor. 

Figure 1. Simplified Bioreactor CrossSection 

Head-loss through the biomatrix is 

monitored across the bioreactors using 

level transmitters and a baseline level 

point. If a degas does not recover the 

head-loss, an accumulation of solids in the 

bioreactors is likely and a backwash is 

required. 

Solids in the form of captured total 

suspended solids (TSS) from the 

wastewater, precipitated selenium, metals 

and excess biomass will build up over 

time. A high-flow backwash (similar to the 

degas) fluidizes the biomatrix, dislodging 

the trapped solids and carrying them out 

of the bioreactor as overflow. The 

biomatrix remains in the bioreactor, 

leaving the active bacteria population for 

continued operation. The resulting sludge 

is de-watered and disposed of. 

Full-Scale Operations 

Four full-scale systems treating FGD 

blowdown have been in operation for 

several years, the first being 

commissioned in early 2008. Each 

wastewater treatment system (WWTS) has 

utilized a slightly different philosophy in 

the implementation of fixed-film biological 

technology. These facilities cover a wide 

range of possible flow and constituent 

loadings. See Table 2. 

Distribution systems in 

gravel subfill 

Biomatrix 

 Biomatrix expansion 

 volume 

 Headspace 
5

Table 2. Plant Capacities 

The first two plants require reduction in 

levels of selenium, arsenic and mercury, 

while the second two employ biological 

treatment only for selenium removal, 

using other upstream processes for all 

other constituents of concern. 

These specific systems were designed to 

handle chloride levels up to 20,000 ppm, 

suspended solids up to 100 ppm, nitrate-N 

loading less than 100 ppm. 

Plants 1 and 2 send the blowdown (postgypsum recovery) to large, lined settling 

ponds for solids removal, temperature 

reduction and equalization prior to 

biological treatment. These ponds have 

capacities of several months and are 

designed to only require solids removal 

approximately every 10 years at design 

condition. Plant 2 is also adding a sulfidebased reagent chemical for mercury 

reduction into the settling pond prior to 

the biological treatment system. 

Of significant note is the behavior of the 

feed water temperature coming out of the 

settling pond at both plants. Due to the 

size of the ponds, the water generally 

cools to ambient temperature. This results 

in a major feed-water temperature swing 

throughout the year. Summer water 

temperatures at both Plant 1 and Plant 2 

can approach 95OF. Winter temperatures 

have been observed as low as 42OF for 

several weeks at a time. Despite the 

variation, effluent results have remained 

consistent throughout the year with 

average performance values remaining 

stable. See Tables 3 and 4.

Table 3. Plant 1 One-Year Avg. Full-Scale 

Data (ug/L) and % Removal Rates 

Table 4. Plant 2 Four-Month Avg. FullScale Data (ug/L) and % Removal Rates 

Plants 3 and 4 direct the blowdown (posthydrocyclone) through conventional 

physical/chemical processes prior to 

biological selenium removal. These 

processes begin with gypsum 

desaturation by adding hydrated lime, 

which elevates the pH to between 8.5 and 

9.2. Ferric chloride is added to facilitate 

iron hydroxide co-precipitation of 

dissolved metals, followed by coagulation 

and clarification. 

Table 5. Plant 3 One-Year Avg. Full-Scale 

Data (ug/L) and % Removal Rates 

 Flow # of Trains 

Plant 1 2.02 MGD 4 

Plant 2 0.25 MGD 2 

Plant 3 0.86 MGD 8 

Plant 4 0.59 MGD 6 

Plant 5 0.86 MGD 6 

Bio Bio 

% 

Bio 

 Influent Effluent Removal 

Tot Se 1,812.9 13.7 99.2% 

Diss Se 1,807.5 11.8 99.3% 

Bio Bio 

% 

Bio 

 Influent Effluent Removal 

Tot Se 727.1 12.9 98.2% 

Diss Se 700.9 11.3 98.4% 

Bio Bio 

% 

Bio 

 Influent Effluent Removal 

Tot Se 214.4 9.2 95.7% 

Se (IV) 57.1 0.8 98.6% 

Se (VI) 181.8 1.3 99.3% 
6

Table 6. Plant 4 One-Year Avg. Full-Scale 

Data (ug/L) and % Removal Rates 

All four plants have consistently met their 

discharge requirements during the 

operation of the fixed-film WWTS. As with 

most scrubber systems, the majority of the 

selenium concentration is present in the 

form of selenate. While many other 

technologies struggle with the removal of 

selenium in the Se (VI) state, the fixed-film 

biological plug-flow system removes both 

Se (IV) and Se (VI) efficiently. See Graphs 1, 

2 & 3. It should be noted that this data, 

while from a single station, coincides with 

performance from the three other fullscale operating units, as well as data 

generated from pilots conducted at five 

other FGD-scrubbed sites. 

Operating and Maintenance Costs 

Costs for a fixed-film biological treatment 

system are minimal in comparison to 

other available technologies. Electrical 

consumption is low. Operation and 

maintenance costs for these facilities 

range from $0.35 to $0.46 per 1,000 

gallons of water treated. For a 0.6 MGD 

biological treatment system described 

earlier in this paper, operating costs are 

approximately $400,000 USD per year for 

direct labor, routine maintenance, and 

nutrient. 

Fixed-Film Bioreactor Re-start after 

Cycling Event 

Plant 4 is a five-unit FGD-scrubbed 

generating plant. It is designated as a 

cycling facility, indicating that load will 

vary from hour-to-hour and day-to-day 

based on demand. During the fall of 2009, 

mild weather that reduced industrial 

demand and the availability of other 

generating capacity on the grid both 

created a situation causing the station to 

be off-line for an extended period of time. 

During this outage period, the WWTS did 

not receive scrubber purge water for 48 

consecutive days. 

The WWTS operators initially were 

unaware that they would be experiencing 

an extended outage. As such, the pumps 

were simply turned off instead of the 

system being prepared for long-term shutdown. After several days, it became 

apparent that the outage would continue 

for an undetermined length of time. 

Process water was periodically drained 

from process vessels to run through the 

bioreactors. This procedure was 

conducted every four days, and lasted 

approximately two hours. Rather than 

allowing the biomatrix to become 

dormant, nutrient was dosed into the 

system during this procedure. The 

process water cycle also allowed for the 

flushing of accumulated biological waste 

products and prevented the ORPs from 

dropping to levels below negative 400 mV. 

Process water was used primarily because 

it was readily available, but also in an 

effort to maintain higher TDS levels. As the 

outage extended, it became necessary to 

begin using service water (filtered river 

water) to maintain the periodic flow to the 

bioreactors. During the outage the 

bioreactors reached ambient 

temperatures of between 55 and 60 °F. 

When the station went back on-line, the 

WWTS was started up just as if it had gone 

through a one-day shutdown. Pumps 

were turned on and the automatic control 

system resumed. An accelerated 

backwash schedule of one-cell-per-day 

was the only change to standard 

operating procedures. One week after 

restarting the system, the selenium 

Bio Bio 

% 

Bio 

Tot Se 334.1 7.1 97.9% 

Se (IV) 7.0 1.5 78.4% 

Se (VI) 257.8 4.4 98.3% 
7

concentration in the first scheduled 

effluent sample was less than 2 ppb. 

Weekly samples for the remainder of the 

month never exceeded selenium 

concentrations of 3.5 ppb. 

There was an initial concern that the 

cycling nature of Plant 4 would have an 

adverse impact on both the 

physical/chemical and biological portions 

of the WWTS; however, frequent shortterm shutdowns of WWTS due to a lack of 

purge water had no noticeable impact on 

system performance. The long-term 

unplanned outage demonstrated the 

resiliency of the bioreactor system to be 

rapidly brought back on-line with minimal 

effort. An understanding of the biological 

technology by an experienced staff 

provided for a seamless cycling of the 

system. 

 

SEM Imaging of Selenium Nanospheres 

To demonstrate the formation of 

elemental selenium nanospheres in the 

fixed-film biological process, SEM imaging 

was performed on bacteria samples 

supplemented with 200 ppm of selenium 

were grown at GE’s Global Research 

Center in Schenectedy, NY. Microbial 

samples were collected on polycarbonate 

membranes and then fixed with 2% 

gluteraldehyde in 0.1M sodium cacodylate 

buffer. The preserved samples were 

dehydrated in ethanol, critical point dried, 

platinum coated and then imaged in the 

Zeiss Supra FE Scanning Electron 

Microscope (SEM). Images were generated 

using 5kV and 15kV energy beams with 

the Scanning Electron 2 detector. 

Using a Scanning Electron Microscope, 

Backscattered Electrons (BSE) imaging 

showed a contrast of chemical 

composition where selenium appears as 

bright white spheres. Energy Dispersive 

Spectroscopy (EDS) analysis showed 

selenium as bright particles vs. gray rods 

of bacteria. See Figures 3 & 4. 

The resulting images depict lateexponential-phase gram-negative rods 

with elemental selenium nanospheres 

(Se(0)). The formation of extracellular, 

insoluble selenium nanospheres is a 

dissimilatory bi-product of anaerobic 

cellular respiration and can be seen 

adjacent to the cell walls both singularly 

and in clusters. See Figures 4 & 5. Dr. 

Ronald Oremland proposes that these 

nanospheres are composed of interconnecting three-dimensional nets of 

selenium, in which both the chain and ring 

structural aspects are maintained. These 

red amorphous nanospheres are believed 

to be more structurally stable than 

chemically derived morphology. 

Oremland also noted that extracellular 

nanosphere accumulation was far more 

common than intracellular elemental 

accumulation in anaerobic selenium 

respiring organisms. 

Conclusions 

As more and more coal-fired utilities 

implement wet FGD scrubbers as a 

method to control SOx emissions, the need 

for advanced wastewater treatment is 

becoming a necessity. Regulating bodies 

are continuously implementing tighter 

discharge criteria requiring new and 

advanced technology. Historic physical 

and chemical treatment processes have 

not proven themselves capable of 

consistently removing selenium and heavy 

metals from complex waters to the low 

levels achieved by biological reduction. 

Successful long-term operation coupled 

with consistent low-level selenium 

removal demonstrates that the fixed-film, 

packed-bed WWTS is a proven technology 

both in function and design. 
Figure 2. Typical Fixed-film Bioreactor Process Flow Diagram 

FGD Blowdown 

FeedPumps

2ndStagePumps

2ndStageFeed Tank

BackwashWasteStorage Tank

ReactorA1ReactorB1ReactorC1ReactorD1ReactorE1ReactorF1

ReactorA2ReactorB2ReactorC2ReactorD2ReactorE2ReactorF2

Backwash/ Effluent Storage Tank

Nutrient

Storage 

Tank

Nutrient

Pumps

FGD Blowdown 

FeedPumps

2ndStagePumps

2ndStageFeed Tank

BackwashWasteStorage Tank

ReactorA1ReactorB1ReactorC1ReactorD1ReactorE1ReactorF1

ReactorA2ReactorB2ReactorC2ReactorD2ReactorE2ReactorF2

Backwash/ Effluent Storage Tank

Nutrient

Storage 

Tank

Nutrient

Pumps
9

Graph 1. Fixed-film Bioreactor Full-scale Selenium Feed and Removal Concentrations 

Graph 2. Exponential View of Graph 1: Fixed-film Bioreactor Full-scale Selenium Feed and 

Removal Concentrations 
10

Graph 3. FGD Purge Selenite and Selenate Concentrations 
11

Figure 3. SEM Imaging of Anaerobic Selenium Respiring Bacteria and Selenium 

Nanospheres 

Figure 4. SEM Imaging of Anaerobic Selenium Respiring Bacteria and Selenium 

Nanospheres
12

Figure 5. Elemental Analysis of Selenium Nanospheres imaged in Figure 4 
13

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