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# Design and Start-up of a Full-scale Biological Selenium Removal System for Flue Gas Desulfurization (FGD) Wastewater from a Power Generating Station

Antonio Lau 1 

## , Michael Pudvay 1 

## , John Bradberry 2 

## , Ray D’Alesandro 2 

## , Steve Frank 2 

## , Anthony Garaventa 2 

## , Shelley Wojciechowski 2 

## , Enos Stover 3 

## , Ross Stover 3 

## , Chris Campana 3 

## , Robert Bernstein 4 

## , Harry McNamee 4



### 1 – Infilco Degremont, Inc., 8007 Discovery Drive, Richmond, VA 23229 2 – GenOn, 1750 Power Plant Road, New Florence, PA 15944 3 – Stover & Associates, Inc., PO Box 2056, Stillwater, OK 74076 4 - URS Corporation, 510 Carnegie Center, Princeton, NJ 08540

Keywords: Flue Gas Desulfurization, Wastewater, Heavy Metals, Denitrification, Selenium, Reduction

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Table of Contents

Abstract 3

1. Introduction 4 FGD Wastewater Treatment 4 The iBIO ® Biological Wastewater Treatment Process 5

2. Microbial Reactions for Denitrification and Selenium Removal 8

3. Process Design Considerations 8 3.1 Selenium Removal Targ et Concentration 8 3.2 Integration into Existing WWT Facilities 9 3.3 Carbon Source Selection 9 3.4 Treatment System Robustne ss Considerations 10 3.5 Final Process Flow Diagram 10

4. Full-scale Start-up Cons iderations 12 4.1 FGD Wastewater Characteristics 12 4.2 Biological Seed Acclimation 12

5. Full-Scale Results 13 5.1 Oxidation Reduction Pote ntial Monitoring 13 5.2 Denitrification Performance 16 5.3 Selenium Removal Performance 17

6. Conclusions 18

7. References 18

8. Acknowledgements 19



### Figures Figure 1.1 Schematic Diagram of a FGD Scrubber System 4 Figure 1.2 Schematic Diagram of the iBIO ® Biological Treatment System 5 Figure 1.3 Denitrification and Sele nium Removal Major Components 7 Figure 3.1 Process Flow Diagram of the Installed Selenium Treatment System 11 Figure 4.1 Biological Seed Acclimation Unit 13 Figure 5.1 Oxidation Reduction Potential (ORP) Chart 14 Figure 5.2 Oxidation Reduction Potential (ORP) in Reactors 15 Figure 5.3 Nitrate/Nitrite Removal from FGD Wastewater (Laboratory Data) 16 Figure 5.4 Selenium Removal from FGD Wastewater 17



### Tables Table 1.1 Major Process Equipment List - iBIO ® Treatment System 6 Table 4.1 FGD Wastewater Characteristics – Design Basis 12 Table 5.1 Total Selenium Concentration through Wastewater Treatment 18 Table 5.2 Total Selenium Removal Across Wastewater Treatment Units 18

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Abstract

This paper describes the design, star t-up and commissioning of a full-scale biological treatment system that was in stalled at a coal-fired power generating station to remove selenium and nitrates from a Flue Gas Desulfurization (FGD) blowdown stream. The new patented iBIO ® wastewater treatment (WWT) system, which was first pilot-tested at power gener ating stations, is based on a suspendedgrowth continuously stirred-tank anaerobic reactor that has been seeded with a source of commonly available anaerobic microorganisms which were subsequently acclimated to remove selenium from the FGD wastewater. This new innovative WWT system was integrated into the existing FGD wastewater treatment units at the power generating station to provide the additional selenium removal capability from the wastewater.

This demonstrated biological selenium removal treatment system provides a robust and cost-effective alternate technology to coal-fired utilities that use FGD wet scrubbers to control their sulfur oxide (SOx) emissions with the additional need to consistently meet the more stringent lower-level selenium effluent limits.

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Flue Gas

FGD Scrubber Gypsum

Blowdown Effluent

Phys

‐ Chem (Existing) 

### Figure 1.1 Schematic Diagram of a FGD Scrubber System

Wastewater Trea tment Systems Anaerobic Biological (Expansion) Aerobic Biological &
Sand Filtration ( Existin g) Limestone 

### 1. Introduction

Air emissions from coal-fired power plants require the installation of various types of air pollution control system technologies. One such technology is the use of wet scrubbers for the removal of sulfur dioxide (SO 2 )

from stack emissions. The wet scrubbing of flue gases with lime or limestone slurries is a proven and commercially established process to
decrease SO 2 emissions. However, inherent with this treatment system is the blowdown of a slurry stream that
contains elevated concentrations of suspended and dissolved solids, chlorides, sulfates, heavy metals (chromium, mercury, selenium, etc.), and nitrates. FGD systems available in the marketplace have different combinations of characteristics (such as coal type, scrubber type, additive use, operational strategies, etc.) that result in extremely varied wastewater
characteristics.

This paper discusses the design and start-up of a full-scale biological wastewater treatment plant expansion implemented at a power generating
station to remove nitrates and residual oxyanions (specifically, selenites and selenates) present in the wastewaterafter undergoing an initial physicalchemical treatment process for the removal of total suspended solids (TSS) and heavy metals from the FGD

blowdown stream.

Figure 1.1 shows a schematic diagram of the FGD scrubber system and the new biological wastewater treatment plant expansion for the blowdown stream. The new biological treatment system was retrofitted into the existing wastewater treatment

processes.

Vent Gases

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Figure 1.2 Schematic Diagram of th e Complete Biological Treatment System for Selenium Removal The new biological wastewater treatment system is a patentedanaerobic activated sludge suspendedgrowth system developed to remove nitrates and heavy metals (specifically, oxidized ions such as
selenates, selenites, chromates, vanadates, etc.).

The biological treatment system uses anaerobic microorganisms commonly available from industrial and municipal wastewater treatment plants and requires a carbon source to conduct
their microbial activities. This treatment system was designed
to handle contaminant fluctuations from the FGD physical-chemicalwastewater treatment system. Pilotscale demonstrations of this process were previously described by Stover et. al. (Reference 1, IWC 07-49) and Teng et. al. (Reference 2). Figure 1.2

and Table 1.1 show a schematic diagram and equipment list, respectively, of the complete biological treatment process which
includes the “anoxic/anaerobic”
section as well as the “aerobic” polishing treatment system.

ble 1.1 Process Equipment List

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Table 1.1 Major Process Equipment List for the Complete Biological Treatment System

Figure 1.3 shows the major components for the denitrification and selenium removal steps which consist of an Anoxic Reactor, Anaerobic Reactor, Anaerobic Clarifier and a Gravity Sand Filter. 1 Anoxic Reactor for Denitrification

2 Anaerobic Reactor for Heavy Metals Reduction

3 Anaerobic Clarifier

4 Anaerobic Sludge Recycle Pump

5 Anaerobic Sludge Waste Pump
6 Sludge Holding Tank

7 Sludge Dewatering Pump
8 Continuous Backwash Sand Filter

9 Aerobic Reactor
10 Aerobic Reactor Aeration Blower

11 Aerobic Clarifier

12 Aerobic Clarifier Sludge Recycle Pump
13 Aerobic Clarifier Sludge Waste Pump

14 Continuous Backwash Sand Filter

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Carbon Source Figure 1.3 Denitrification and Selenium Removal Major Components



## To AEROBIC

Biological Polishing

Treatment

S y stem Nutrients

Sludge Handling

System Anoxic Reactor
“Denitrification”

Anaerobic Reactor
“Selenium”

Anaerobic Clarifier

Gravity Sand Filter FGD
Wastewater
afterPhysicalChemical
Treatment



## Coagulant

Flocculant Sludge Recycle

Sludge Waste

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2. Microbial Reactions for Nitrate and Selenium Removal by the Biological Treatment Process

Soluble selenium in FGD wastewaters is primarily present in the form of selenates (SeO 4 2- ) and selenites (SeO 3 2- ). Selenites, to some extent, can be removed through chemical precipitation, leaving selenates as the primary selenium species remaining in the wastewater. Biological processes have proven to be a viable and robust option for removal of selenates from wastewaters (References 1,2,4). A number of microorganisms are able to utilize the oxygen in selenate for energy and growth in a manner similar to the way denitrifying microorganisms utilize the oxygen from nitrates (NO 3 - ). The selenium removal bacteria are commonly known as the “sulfur reducing bacteria” (SBR) and they only operate in a reducing
environment. The selenium removal process can be represented as follows:

SeO 4 2- (selenates) + Carbon Source + Microorganisms ------> Se 0 (solid) + CO 2 + H 2 O + Biomass

Similarly, the denitrification reaction carried out by heterotrophic bacteria known as “denitrifiers” operate in an
anoxic environment and can be represented as follows: NO 3 1- (nitrates) + Carbon Source + Microorganisms ----  N 2 (gas) + H 2 O + Alkalinity + Biomass

While the biological reduction of nitrates results in gaseous nitrogen formation, the biological reduction of
selenates and selenites results in the formation of elemental selenium (Se 0 ),

which is insoluble and can be removed along with the biological solids. Microorganisms gain more energy from using the oxygen in nitrate than they do from using the oxygen in selenate, and as such, nitrates must
first be reduced to near zero levels
before selenate reduction will take place. Both types of microorganisms (SRB and denitrifiers) were found to be capable of reducing the oxidized selenium compounds to elemental selenium. The process configuration used in this new biological system optimizes these microbiological reactions by providing two separate
reactors to allow the environmental
conditions to be better established in each stage. First, the biological
reduction of nitrates is conducted in the Anoxic Reactor, which is followed by a second Anaerobic Reactor for the biological reduction of selenates and selenites where more reducing (i.e., more negative Oxidation Reduction Potential (ORP)) conditions are created for the SRB microorganisms.

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3. Process Design Considerations

3.1 Selenium Removal Target
Concentration

The power generating station had a target to reduce the concentration of Total Selenium from 1.3 mg/L in the
untreated FGD blowdown stream down to <0.21 mg/L in the effluent of the wastewater treatment system.

3.2 Integration into Existing
Wastewater Treatment Facilities

The power generating station had an existing physical-chemical treatment system which included the typical precipitation of metals at an elevated pH followed by an aerobic biological treatment system which consisted of sequencing batch reactors, mainly for organics reduction. As a final polishing step, the Total Suspended Solids (TSS) of the wastewater is reduced by flowing through multimedia pressure filters.

In this project, it was possible to insert the new anaerobic biological treatment process after the existing physical-chemical treatment system to be followed by the existing aerobic treatment process to remove the residual organic carbon and ammonia nitrogen to the required permit limits. As part of the process design for the new biological treatment system, a six-month laboratory study was
conducted to simulate and validate the selenium removal performance using
actual FGD wastewater from the power generating station. 3.3 Carbon Source Selection

FGD wastewaters do not have sufficient amount of organic carbon to support the microbiological reactions to denitrify and reduce selenium. Therefore, an external source of organic carbon is required to be added to the anaerobic treatment system.

Molasses is known to support biological selenium reduction, but it
can be difficult to handle due to its high viscosity, and can vary in composition, even from a single provider. It is suspected that the sugar fraction of molasses is primarily responsible for supporting selenium reduction, and as such, sugar (sucrose) was used in our initial pilot

studies.

A commercially-available liquid carbon source (carbohydrate-based product) was also evaluated in the laboratory and was selected for this project. Thisliquid carbon product is nonflammable, and according to the manufacturer, has both a consistent compositional make-up and a long shelf-life. The material flows readily at temperatures as low as 16°F, with a viscosity of abou t 100 cp at this temperature, and therefore is easier
to handle than molasses. The material is approximately 50% by weight carbohydrate, with a COD concentration of 670,000 mg/L, which is higher in concentration than sugar solutions, which have a typical
maximum concentration of 25 to 35 percent by weight.

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Discussion on the selection of the appropriate carbon source for this process was provided by Lau, et. al. (Reference 3, IWC 11-33, and Reference 5) which describes a pilot study conducted to confirm the applicability of the liquid carbon

source.

3.4 Treatment System Robustness
Considerations

Consideration was taken in the design of the biological treatment system to provide a design that was both robust and operationally flexible, while still being able to meet the stated effluent requirements. Some critical items that were incorporated are the following: (1) Equalization Tank to provide sufficient wastewater holding time to
allow for minor mechanical repairs/changes, (2) instrumentation redundancies for critical parameters, such as the ORP and pH sensors, (3) bypassing loops for the anaerobic
reactors and anaerobic clarifier.

3.5 Final Process Flow Diagram

The final Process Flow Diagram shown in Figure 3.1 provides the major components of the new biological treatment system installed at the power generating station for selenium removal. The design flow rate of this treatment system is 410 gpm (0.6 MGD). The major process components include:  Chemical addition systems for the carbon source, macronutrients (urea, phosphoric acid), micronutrients, coagulant (ferric chloride) and flocculant (polymer).

 Anoxic and Anaerobic Reactors
for denitrification and selenium
reduction.

 Anaerobic Clarifier for the separation of the anaerobic
biomass from the treated wastewater followed by a gravity sand filter for additional polishing of the suspended
solids.

 The reactors and the clarifier vessels are equipped with a foam control system which
recirculates the contents of the vessel and sprays it on top of
the tank.

 The vent gases are collected
and sent to a biofilter for biological conversion of sulfides
into sulfates.

 A “Sludge Storage Tank” was
included to maintain an on-site inventory of acclimated
bacteria.

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Figure 3.1 Process Flow Diagram of the Installed Selenium Treatment System

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4. Full-Scale Start-Up Considerations

4.1 FGD Wastewater Characteristics

The FGD wastewater characteristics used for process design is shown in Table 4.1



### Table 4.1 FGD Wastewater Characteristics Design Basis 4.2 Biological Seed Acclimation

The biological treatment system utilizes anaerobic microorganisms that have been acclimated to the existing
FGD wastewater. This treatment process does not require the purchase of expensive proprietary microorganisms supplied by some vendors for the specific removal of selenium compounds. This treatment process, in contrast, uses typical anaerobic microorganisms easily
available and found in industrial or municipal wastewater treatment sludges, usually obtained for a minor
cost to the client.

For this project, the source of the
anaerobic bacteria was from anaerobic sludges of a WWT facility treating food manufacturing wastewaters. The
bacteria were acclimated to the actual FGD wastewater in a separate “Biological Seed Acclimation Unit” which consisted of a 6,000 gallons anaerobic reactor followed by an anaerobic clarifier (see Figure 4.1).

The major factor in the acclimation process was the controlled increase of
the chloride concentration in the mixed liquor from ~500 mg/L up to ~20,000 mg/L while maintaining the proper Oxidation Reduction Potential (ORP) in the reactor. The bacterial acclimation process was initiated ~3 Parameter Concentration (mg/L)

Ammonia (NH 3 -N) 8

Nitrate (NO 3 -N) 10

Total Kjeldahl Nitrogen (TKN) 62

Sulfate (SO 4 ) 3,789

Chemical Oxygen

Demand (COD) 897

C-BOD 5 53

Total Suspended Solids (TSS) 91

Total Selenium 1.3

Chloride (Cl

- ) 20,000

Total Dissolved

Solids (TDS) 28,907

pH 6.6

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months prior to the full-scale start-up date.



### Figure 4.1 Biological Seed Acclimation Unit

5. Full-Scale Results

5.1 Control of the Oxidation
Reduction Potential (ORP) in the

Reactors
One of the critical parameters in the operation of the selenium removal
reactors is the Oxidation Reduction Potential (ORP) of the mixed liquor.

The ORP is an analytical parameter reported in milliVolts (mV) which is

obtained when the ORP sensor is placed in contact with the liquid being
measured. The ORP value defines whether the reactor is in an oxidizing
environment or a reducing environment. An oxidizing reactor environment (or an oxygenated environment) is indicated by a positive ORP value (ORP>0 mV); the higher the positive ORP value, the stronger is the oxidizing environment. In contrast, a reducing reactor environment (or an oxygen depleted environment), is indicated by a negative ORP value (ORP<0 mV); the more negative the ORP value, the stronger is its reducing environment.
The Selenium Removal Anaerobic Reactors require a “ reducing

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environment ” for the bacteria to accomplish their tasks. This ORP scale is graphically shown below in Figure 5.1. 

### Figure 5.1 Oxidation Reduction Potential (ORP) Chart

Positive ORP Values => Oxidizing Environment

(ORP > 0 milliVolts)

Negative ORP Values => Reducing Environment

(ORP < 0 milliVolts)

-150 mV (minimum level for selenium removal)

-200 to -300 mV (Target Operation)

-300 to -350 mV (Conservative Operation)

The purpose of monitoring ORP in the
Selenium Anaerobic Reactors is to ensure that the oxidized species of selenium (i.e., selenates and selenites), are reduced to elemental

selenium. Selenates and selenites are soluble compounds and they are not sufficiently removed in the existing physical-chemical treatment system to meet the required lower selenium effluent limit. Therefore, the
wastewater is treated in the new
Selenium Removal Anaerobic Reactors
under a reducing environment where the selenate and selenite compounds are converted by the bacteria into
their insoluble elemental selenium
form. The elemental selenium precipitates out of solution and stays with the biomass as a particulate
material and it leaves the treatment system via the Anaerobic Clarifier waste biomass. As a requirement prior to precipitation of the oxidized selenium compounds, the nitrates present in the wastewater must be reduced to nitrogen gas.

During the start-up period, the strategy was to operate the Anaerobic
Reactors at a controlled ORP value of
at least -300 mV. This was a
conservative ORP value with sufficient flexibility to allow the removal of selenium to meet the target effluent
selenium concentration if the maximum system design value of 1.3 mg/L Total Selenium is reached in the
untreated FGD wastewater. Once the WWT Operator develops sufficient expertise with this biological treatment system, an optimization process will be implemented to adjust

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the ORP value that will result in a more cost-effective operation.
To achieve a negative ORP value, the addition of the correct proportions of a carbon source, macronutrients (urea, phosphoric acid) and micronutrients is necessary. These chemicals are
added to the Anaerobic Reactor to serve a two-fold purpose: (1) they are first needed for proper biological growth, and (2) they are needed to support the biological reaction mechanisms necessary to reduce nitrates, selenites, and selenates in
the Anaerobic Reactors. Sulfate reduction to hydrogen sulfide occurs simultaneously when the oxidized selenium species are converted to elemental selenium. Thus, vent gases containing sulfides are collected and
sent to a biofilter for oxidation to
sulfates.

Figure 4.3 below shows the ORP level
achieved in both Anaerobic Reactors during the full-scale start-up period.
The data shows that the ORP was
maintained between -300 mV to -400 mV, as originally planned.



### Figure 5.2 Oxidation Reductio n Potential (ORP) in Selenium

Anaerobic Reactors

‐ 600 ‐ 500 ‐ 400 ‐ 300 ‐ 200 ‐ 100 0 2/1/2012 3/1/2012 4/1/2012 5/1/2012 6/1/2012OxidationReductionPotential(mV)

Sampling Date 

# Oxidation Reduction Potential in Selenium

Reactors

Anoxic Reactor

Anaerobic Reactor

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Figure 5.3 Nitrate/Nitrite Re moval from FGD Wastewater (Laboratory Data) 5.2 Denitrification Performance

Currently, the FGD wastewater at this power generating station has a very
low concentration of nitrates and nitrites in the wastewater (<10 mg/L). Daily analytical monitoring showed that the nitrate values were <1 mg/L
in the effluent stream of the new biological treatment system indicating complete denitrification. The full-scale data confirmed the laboratory finding
that was obtained when the same
wastewater was tested over a sixmonth period. The laboratory data showed that the treatment system successfully removed the (nitrate/nitrite) concentration from ~22 mg/L (average) down to <0.2 mg/L as shown in Figure 5.3 below.

5.3 Selenium Removal Performance

The reduction of total selenium throughout the wastewater treatment process is shown in Figure 5.4 and
Tables 5.1 and 5.2 below. The
influent selenium concentration from the FGD blowdown ranged from 0.1 to 0.7 mg/L and the final effluent from the complete WWT system had a total selenium concentration range of 0.02 to 0.22 mg/L. The new Selenium Treatment System, when operating at steady-state conditions, does produce
an effluent wastewater that meets the targeted total selenium concentration of <0.21 mg/L.

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Total selenium removal across the major wastewater treatment processes is shown in Table 5.2. The physical-chemical treatment process consisting of pH elevation and coagulant/polymer a ddition was able
to remove 51% of the total selenium. This result is expected because historical data from this power generating station indicates that the selenium species in the influent
wastewater is ~90% in the selenite form. During the start-up period, the
selenium removal across the Anaerobic Treatment System was 36% and across the complete WWT system was 69%. These results were obtained during the start-up period while the system is not yet stabilized at the optimized operating and performance conditions. Higher removals of selenium are expected as the system gets optimized.



### Figure 5.4 Selenium Remo val from FGD Wastewater

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

2/6/2012 3/6/2012 4/6/2012 5/6/2012 6/6/2012TotalSelenium(mg/L)

Sampling Date 

# Total Selenium Concentration

Surge Tank
Phys

‐ Chem Effluent

Final Effluent

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Table 5.1 Total Selenium Concentration through Wastewater Treatment

Sampling Point Total Selenium Concentration Range (mg/L)

FGD Blowdown - Influent to WWT 0.12 - 0.68

After Physical-Chemical Treatment 0.06 - 0.28

After Anaerobic Biological Treatment 0.02 – 0.23

Table 5.2 Total Selenium Removal Across Wastewater Treatment Units

Process Average Total Selenium Removal (%)

Physical-Chemical Treatment 51

Anaerobic Biological Treatment 36

Across All WWT Units 69



## 6. Conclusions

As coal-fired power generating stations face stricter effluent regulations, there is a need for advanced wastewater treatment technologies to meet the lower, stricter limits. The selenium removal anaerobic biological wastewater treatment system installed at this power generating station has demonstrated that it can meet the targeted selenium concentration of <0.21 mg/L in the effluent stream. In fact, total selenium concentrations of <0.1 mg/L were obtained on many operating days.

In summary, the application of a suspended-growth activated sludge treatment system which includes anoxic, anaerobic and aerobic reactor stages proved to be successful for the removal of selenium compounds from FGD blowdown wastewaters from a power generating station.



## 7. References

1. Stover, E.L., Pudvay, M.L., Kelly, R.F., Lau, A.O., Togna, A.P., “Biological Treatment of
Flue Gas Desulfurization Wastewater,” IWC 07-49,
68 th International Water Conference, Orlando, FL (2007).



### 2. Teng, J., Kelly, R., Lau, T., Mehta, S., Berry, M. “iBIO ® Biological Treatment System
for Flue Gas Desulfurization Wastewater,” Air Quality

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Conference, Baltimore, MD (2008).



### 3. Lau, A.O., Labban, R., Mehta, S., Togna, A.P., “Evaluation
of Carbon Sources for the
Anaerobic Treatment of Flue Gas Desulfurization (FGD) Wastewaters for Heavy Metals Removal,” IWC 11-33,

72 nd International Water Conference, Orlando, FL (2011).



### 4. Sandy, T., C. DiSante,
“Review of Available Technologies for the Removal of Selenium from Water”, Final Report prepared for the
North American Metals Council, CH2M Hill (June 2010).



### 5. Onnis-Hayden, A., Du, Y., Yu, S., Ledwell, S., Togna, P., Gu, A.Z., “Characterization Of
Alternative Carbon Sources For Denitrification,” proceedings of the 84 th

Annual Water Environment Federation Meeting and Exhibition, Los Angeles, CA (2011).



## 8. Acknowledgements

We would like to acknowledge the following individuals for their analytical and operations support during this project: John Williamson, Laboratory Manager, and Denise Horner, Sr. Chemist, both from Degremont North America Research & Development Laboratory; Brittany Looke, Chemist, and Cody McLachlan, Lead Operator, from the Stover Group.