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iBIO TM Biological Treatment System for Flue Gas Desulfurization 

Wastewater

Jason (Xinjun) Teng, Research Engineer, Southern Company, 600 18th Street North, Birmingham, AL 35291 

Tel: 205-257-5951 Fax: 205-257-5367 email:xteng@southernco.com; Robert Kelly, Director of Research, 

Degremont Technologies North American R&D Center, Richmond, VA 23219; Tony Lau, Environmental 

Technology Manager, Infilco Degremont, Inc., Richmond, VA 23229; Sunil Mehta, Senior Research Engineer, 

Degremont Technologies North American R&D Center, Richmond, VA 23219; Mark Berry, Manager of 

Environmental Controls R&D, Southern Company, Birmingham, AL 35291

Abstract: Wet Flue Gas Desulfurization (FGD) systems are being installed at coal-fired power plants to 

improve air quality by removing sulfur dioxide from flue gas emissions. The resulting FGD blowdown 

wastewaters are known to contain elevated levels of chlorides (4,000 – 20,000 mg/L), trace concentrations of 

heavy metal contaminants such as chromium, mercury, and selenium, often high levels of nitrates (10–700 

mg/L), and very high levels of total dissolved solids (20,000 to 60,000 mg/L). Of particular concern in recent 

years is the removal of nitrogenous contaminants such as nitrates (NO3

-

) and ammonia, as well as various heavy 

metals, with selenium (Se) of particular concern as the Selenate (SeO4

2-

) form is not efficiently removed via 

conventional physical-chemical treatment practices. 

An innovative and patent-pending biological treatment system, iBIOTM technology, developed by Degremont 

Technologies – IDI, provides an enhanced treatment option for the removal of residual selenium compounds 

from FGD wastewaters. Using fundamental microbial biochemical principles, a (1 – 2 gpm) pilot-scale iBIOTM 

treatment system was designed and installed at a steamed power plant of Southern Company. A description of 

the iBIOTM system is presented herein along with results from the pilot-scale studies that targeted the removal of 

nitrates, selenium and chromium from the FGD wastewater. These results were used to develop design 

parameters for a full-scale iBIOTM system. The FGD wastewater in this study contained significant 

concentrations of nitrates (10-316 mg/L), total selenium (0.5-3.7 mg/L), and total chromium (~0.02 mg/L). The 

results of the study demonstrated that the iBIOTM system can achieve an average effluent concentration of NO3

-

< 1.0 mg/L, total Se < 0.2 mg/L and total Cr < 0.005 mg/L. The data shows that all of the selenate species is 

removed by the iBIO TM system, while trace concentrations of residual Selenite and unknown selenium 

compounds were found to remain in the treated effluent. 

Keywords: Flue Gas Desulfurization, wastewater, iBIOTM, Biological Nutrient Removal, Selenium, Selenate, 

Selenite, Heavy Metals

INTRODUCTION

Coal-fired power plants produce a significant portion of the electricity for the United States. Air emissions from 

coal-fired power plants require air pollution control systems. This has resulted in the implementation of wet 

scrubbers for removal of sulfur dioxide (SO2) from stack emissions. The wet scrubbing of flue gases with lime 

or limestone slurries is a proven and commercially established process for decreasing SO2 emissions. The 

resulting flue gas desulfurization (FGD) wastewater can contain elevated concentrations of solids, chlorides, 

sulfates, heavy metals, and nitrates. FGD systems have unique combinations of characteristics (such as 

scrubber type, absorber type, additive use, operational issues, etc.) that result in extremely varied wastewater 

characteristics.

Traditionally, FGD wastewater treatment has primarily consisted of physical/chemical processes for the 

removal of total suspended solids (TSS) and heavy metals. Due to regulation under the Clean Water Act 

(CWA), effluents discharged into aquatic systems may require additional treatment beyond traditional 

physical/chemical systems.

Biological treatment for heavy metals removal for FGD waters is an emerging technology. Suspended-growth 

activated sludge systems are the most commonly employed biological treatment processes for the removal of 

targeted organics and inorganics from various types of wastewater. Biological treatment has proven effective 

for removal of particular heavy metals in FGD wastewater, such as selenium, by reduction and precipitation 

reactions.
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Infilco Degremont, Inc. (IDI) initiated a pilot scale field demonstration study in order to provide complete and 

effective treatment of FGD wastewater. IDI’s patent pending biological treatment process, iBIOTM, has the 

following objectives when treating the FGD blowdown water: 1) Nitrate removal – Denitrification, 2) Selenium 

Removal, and 3) organics and Ammonia removal.

EXPERIMENTAL

IDI’s iBIOTM biological FGD wastewater treatment system is an activated sludge suspended-growth system. 

The biological treatment system consists of a series of reactors designed for biological removal of nitrate, heavy 

metals, ammonia, and organics from the FGD wastewater before discharge.

The overall treatment process consists of the following steps:

Urea, micro-nutrients and carbon source addition.

The removal of nitrate in the anoxic reactor.

Sedimentation, thickening, and clarification of anaerobic/anoxic activated sludge.

The removal of selenium, chromium by sulfate reducing microorganisms in the anaerobic reactor.

Return activated sludge from the anaerobic/clarifier to anoxic reactor.

Intermediate filtration before aerobic reactor.

Removal of total organic carbon (TOC) and ammonia in the aerobic reactor.

Waste activated sludge from the anaerobic/anoxic and aerobic clarifiers. 

Sedimentation, thickening and final clarification of aerobic activated sludge before final filtration.

Effluent filtration before final discharge. 

The biological treatment process flow diagram is presented in Figure 1.

Figure 1. Biological treatment- Process Flow Diagram

Equalization Tank (100 gallon)
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An equalization tank is provided to balance the wastewater quality and flow either from the pilot physicalchemical treatment process or the scrubber pond effluent. A constant flow will enhance plant performance. The 

tank is equipped with an influent pump.

Anoxic Reactor (1000 gallon)

The influent through the equalization tank is loaded into the anoxic reactor. In the anoxic reactor, nitrates are 

removed through de-nitrification processes via microorganisms. Selenate, selenite, and sulfate are also reduced 

in this process. Macro- and micro-nutrients and a carbon source (sugar) are added to the anoxic reactor to allow 

for the biological remediation of constituents in this FGD wastewater. 

Anaerobic Reactor (1000 gallon)

The wastewater leaves the anoxic reactor and flows by gravity into the anaerobic reactor that favors sulfate 

reducing bacteria (SRB) that can transform constituents such as selenate, senenite, and sulfate to less dissolved

forms. 

Anaerobic Clarifier (325 gallon)

The effluent from the anaerobic reactor flows into a clarifier, where mixed liquor suspended solids (MLSS) are 

settled out. Part of settled sludge is recycled back to the head of the anoxic reactor as return activated sludge 

(RAS) and part of sludge is removed as waste activated sludge (WAS). 

Aerobic Treatment (500 gallon)

After leaving the anaerobic clarifier, the wastewater flows into the aerobic reactor. In this reactor, aerobic 

organisms can degrade and decrease the residual organic carbon compounds and ammonia prior to discharge. 

Aerobic Clarifier (325 gallon)

The wastewater from aerobic reactor then passes into an aerobic clarifier, where the solids are settled out. Part 

of settled sludge is recycled back to the head of the aerobic reactor as return activated sludge (RAS) and part of 

sludge is removed as waste activated sludge (WAS). 

Effluent Filtration

The clarified water then flows by gravity through sand filters where suspended solids concentrations can be 

decreased before final discharge.

RESULTS AND DISCUSSION

Composite sampling of equalization tank was performed to determine the chemical composition of pre-treated 

scrubber wastewaters. Analyses included pH, ammonia (NH3), Nitrate (NO3

-

), Nitrite (NO2

-

), TSS, COD, TOC, 

total and dissolved selenium, and chromium. These analyses were performed throughout this study

Influent Characteristics

FGD wastewater samples from the scrubber pond effluent (from recycle line back to scrubber) were collected 

three times per week during this study. Constituents measured in the scrubber pond effluent wastewaters are 

shown in Table 1. Constituent loadings (lbs/day) to the pilot-scale biological treatment system varied 

throughout this study and these data are presented as averages and minimum and maximum concentrations in 

Table 1. The flow rate ranged from 0.4 gpm to 2.0 gpm with an average of 1.1 gpm.

Denitrification

The purpose of the anoxic reactor (Reactor A) is to denitrify nitrites and nitrates (NOx) and convert them to 

diatomic nitrogen gas (N2). Sugar was added as a supplemental organic carbon source to enhance denitrification 

performance of the system. Application rates of sugar were critical to the performance of these systems, and 
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indicate that these systems require appropriate and consistent amendments of sugar to maintain the performance 

of these systems. 

Table 1.Influent Characteristics

Parameter Units Avg Max Min

pH 6.70 7.51 5.90

Ammonia mg/L 1.85 10.30 0.30

Nitrate mg/L 140 316 3.60

Nitrite mg/L 0.9 3.4 0.06

TSS mg/L 42 77 18

COD (Total) mg/L 149 395 35

TOC mg/L 17 116 1.16

Selenium (Total) mg/L 1.01 3.49 0.12

Selenium (Dissolved) mg/L 0.72 1.43 0.09

Chromium (Total) mg/L 0.07 0.41 0.01

Chromium (Dissolved) mg/L 0.02 0.17 0.01

The evaluation of the anoxic reactor is based on the removal of nitrite and nitrate concentrations from influent 

FGD waters. The removal of nitrite and nitrate concentrations was compared to system parameters including: 

Food to Microorganism ratio (F/M), temperature, pH and Carbon to Nitrogen (C/N) ratio. 

The sum of nitrite and nitrate concentrations from effluent samples of the anoxic reactor ranged from 0.3 mg/L 

during periods of normal operation to 242 mg/L during periods of non-normal operation or upset conditions as 

shown in Figure 1.

Figure 1 Nitrite and Nitrate Removal 

0

50

100

150

200

250

300

350

10/15 11/14 12/14 1/13 2/12 3/13 4/12 5/12 6/11 7/11 8/10 9/9 10/9

Nitrate + Nitrite (mg/L)

Nitrate+Nitrite Influent Nitrate+Nitrite Effluent
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Lower removals of nitrite and nitrate were due to process upsets. Process upsets consisted of inadequate loading 

of sugar due to freezing problems associated with the delivery system. Process upsets in a biological system are 

highly detrimental due to the recovery time of the affected bacteria and may require 3-4 weeks before 

performance is consistently achieved after re-establishing normal operating conditions. During periods of 

normal operation, the anoxic reactor was successful in decreasing nitrate concentration from ~200 mg/L down 

to < 1 mg/L (>99.5% removal). A description of the various process upsets is provided below.

Impact of the Active Population of Denitrifying Bacteria on Denitrification

The impact of the active population of denitrifying bacteria, MLVSS, on nitrate removal was reviewed. As the 

MLVSS concentration in the reactor increased, the percentage of nitrate removal in the reactor also increased. 

Table 2 and Figure 2 illustrate that at MLVSS concentrations above 3000 mg/L, nitrate and nitrate removals 

exceed 99.5%. 

Table 2 Impact of Biomass (MLVSS) on Denitrification Performance

Figure 2 Impact of Biomass (MLVSS) on Denitrification Performance

y = 8E-05x + 0.5942

R = 0.56

0%

20%

40%

60%

80%

100%

120%

0 1000 2000 3000 4000 5000 6000 7000

MLVSS, mg/L

NOx Removed, %

Impact of the Food to Microorganisms (F/M) Ratio on Denitrification

An evaluation of Food-to-Microorganisms (F/M) ratio shows that as the F/M ratio decreases, the percentage of 

nitrate/nitrite removal increases. Nitrite and nitrate removal percentages of >99.5% removal were observed 

when F/M ratios were below 0.5 and removal efficiencies started to decrease above the F/M ratio of 0.5. This is 

shown in Table 3 and Figure 3.

MLVSS Influent NOx Effluent NOx NOx Removed

mg/L mg/L mg/L %

< 1500 147 92 41.9%

1500 - 3000 162 14 91.4%

3000-4500 183 0.7 99.6%

> 4500 128 0.6 99.4%
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Table 3 Impact of Food to Microorganism Ratio on Denitrification Performance

F/M Influent NOx Effluent NOx NOx Removed

(g COD/g VSS/d) (mg/L) (mg/L) (%)

< 0.25 128 0.6 99.4%

0.25 to 0.5 180 0.8 99.6%

0.5 to 0.75 155 8.4 92.9%

0.75 to 1.0 158 65 77.1%

> 1.0 143 103 24.7%

Figure 3 Impact of Food to Microorganisms (F/M) Ratio on Denitrification Performance

y = -0.668x + 1.2142

R = 0.75

0%

20%

40%

60%

80%

100%

120%

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

F/M NOx Removed, %

Impact of pH on Denitrification Performance

Denitrification can occur over a wide range of pH values. Denitrification is relatively insensitive to acidity; 

however it does decrease under low pH values. The preferred pH range for proper floc formation by facultative 

anaerobes is 6.5- 8.5, which is also ideal for the denitrification process. To ensure adequate enzymatic activity 

of facultative anaerobes, the pH in the Anoxic Reactor was maintained at ~7.0 throughout this pilot study. 

Since the denitrification process can produce an alkalinity source, (3.57 mg alkalinity as CaCO3 is produced per 

mg of nitrate-nitrogen reduced to nitrogen gas) pH of the FGD wastewater can increase when nitrate 

concentrations are removed within the reactor. However, during the period of process upsets and/or lowinfluent-nitrate concentrations, a supplemental alkalinity feed system (sodium hydroxide) was initiated to 

maintain the anoxic reactor pH at ~7. 

Figure 4 Impact of pH on Denitrification Performance
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y = 0.7148x - 4.1619

R = 0.72

-40%

-20%

0%

20%

40%

60%

80%

100%

120%

140%

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

pH

NOx Removed, %

Impact of Temperature on Denitrification Performance

Nitrate removal rates were affected significantly by water temperature within the reactor. It indicates that a 10° 

C drop in water temperatures will decrease the kinetic rate by one-half and may also apply to microbial reaction 

kinetics. The impact of temperature on nitrate/nitrite removal is shown in Table 4.4. A significant decrease in 

performance was observed at lower temperatures especially when lower biomass (MLVSS) concentrations were 

measured in this reactor. 

Table 4 Impact of Temperature on Denitrification Performance

Temperature Influent NOx Effluent NOx NOx Removed *SNDR T

(F) (mg/L) (mg/L) (%) (g NOx /g VSS/d)

< 65 158 94.8 45% 0.04

65 to 70 178 21.8 84% 0.05

70 to 75 141 9 90% 0.06

75 to 80 123 1 99% 0.07

* Specific Nitrate Denitrification Rate

Impact of Oxidation Reduction Potential (ORP) on Denitrification

Generally, in the absence of oxygen and in the presence of nitrate/nitrite ions, bacterial degradation of carbon 

(e.g., sucrose) occurs when operating conditions have a redox potential <-100 millivolts (mV). Nitrate removal 

rates were the greatest (>96% nitrate removal) when ORP values were <-300 mV. A significant decrease in 

performance was observed when ORP values were measured > -150 mV as shown in Figure 5 and Table 5.

Table 5 Impact of ORP on Denitrification Performance

ORP NOx Removed

(mV) (%)

> -100 11.84%

-100 to -200 83.55%

-200 to -300 91.43%

< -300 96.63%
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Figure 5 Impact of ORP on Denitrification Performance 

y = -0.0016x + 0.3823

R = 0.67

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

120%

-500 -400 -300 -200 -100 0 100 200 300

Impact of Carbon Source and C/N Ratio on Denitrification Performance 

Addition of an organic carbon source (e.g., sucrose) is required for denitrification because there is insufficient 

biodegradable-carbon available in the FGD wastewater to support denitrification processes. The quantity of the 

organic carbon source, rather than the quantity of nitrite or nitrate ions, is considered to be an important factor 

in denitrification kinetics. The larger the quantity of degradable organic carbon sources, the higher quantity of 

electron acceptors such as nitrite and nitrate ions is needed to maintain microbial respiration. Theoretically, 

complete denitrification usually occurs when the carbonaceous biochemical oxygen demand (cBOD) to nitriteions-and-nitrate-ions ratio is ~ 3:1.

Sugar was chosen as the supplemental carbon source due to its low cost, and ease of application in a full-scale 

system. The impact of supplemental carbon on the denitrification rate was evaluated based on the chemical 

oxygen demand (COD) to NOx ratio (C/N). An evaluation of the C/N ratio shows that as the C/N ratio 

decreases from 6:1, the percentage of nitrate/nitrite removal decreases. Results are shown in Figure 6 and 

Table 6.

Table 6 Impact of Carbon to Nitrogen ratio on Denitrification Performance

C/N Ratio NOx Removed

(%)

< 5.0 68%

5.0 to 6.0 86%

6.0 to 7.0 92%

>7.0 94%

Figure 6 Impact of Carbon to Nitrogen Ratio on Denitrification Performance 
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0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

120.0%

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

C/N

NOx Removed, %

Selenium Removal Performance

The dissolved selenium leaving the Anaerobic Reactor ranged from 0.081 mg/L to 1.25 mg/L with an average 

value of 0.32 mg/L, as shown in Figure 7. The average influent dissolved selenium concentration was 1.16

mg/L during the pilot study. These average values apply to the complete study period, including data that was 

taken when the pilot unit was experiencing upsets.

The results also show that the biological system is capable of producing an effluent dissolved selenium 

concentration < 0.1 mg/L when operating conditions are optimized (i.e. ORP and pH at the targeted values) and 

influent selenium concentration is lower than 1mg/L. When the pilot is operated under normal conditions, the 

effluent selenium was kept lower than 0.4mg/L consistently even when the influent selenium was as high as 

3.7mg/L. The selenium speciation analysis further showed that the biological system completely removed the 

selenates (below detection limit) from the wastewater (table 7). 

The variation in effluent dissolved selenium concentrations is correlated to the denitrification performance in 

the Anoxic Reactor. It was noted throughout the study that the presence of nitrite or nitrate ions in the 

Anaerobic Reactor inhibits the respiration of sulfur and selenium reducing bacteria, and hence it reduces the

performance of selenium removal. 

Table 7. Influent and Effluent selenium speciation

IBIO 

Influent

IBIO 

Effluent

Total Se 390.7 227

Diss Se 375.7 61.4

Se(IV) 45.4 40.6

Se(VI) 231.7 ND

other Se 98.6 20.8

Figure 7 Total and dissolved selenium removed by the IBIOTM system
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0

0.5

1

1.5

2

2.5

3

3.5

4 1/10 2/10 3/10 4/10 5/10 6/10 7/10 8/10 9/10 10/10 11/10 12/10 1/10 mg/L

Total Se Influent Total Se Effluent

Dissolved Se Influent Dissolved Se Effluent

An evaluation of ORP shows that the lower ORP the reactor achieved, the lower selenium concentration 

in the effluent. The lowest selenium concentrations (0.097 to 0.12 mg/L) in effluent occurred when ORP 

is <-400mV. The correlation between ORP and intermediate clarifier effluent soluble selenium are shown 

in Table 8 and Figure 8.

Table 8 Correlation between ORP and Intermediate Clarifier Effluent Soluble Selenium

Concentration

ORP (mV) Soluble Selenium (mg/L)

> -150 0.76

-150 to -250 0.38

-250 to -350 0.25

< - 350 0.22

Figure 8 Correlation between ORP and Intermediate Clarifier Effluent Soluble Selenium

Concentration 

y = 0.0012x + 0.7433

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-500 -400 -300 -200 -100 0

ORP (mV)

IC Effluent Se (Dis) concentration (mg/L)
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CONCLUSIONS

The pilot plant study performed at the Yates Station demonstrated that IDI’s FGD wastewater treatment 

provides complete and comprehensive solution to achieve NPDES discharge criteria. The iBIOTM, biological 

treatment, can effectively remove dissolved selenium to <0.2 mg/L. Anaerobic treatment provides selenium

removal at ORP values <-350 mV. Furthermore, the reduced environmentalconditions required for selenium 

reduction, were also found to provide the reducing conditions that precipitated other heavy metal species such 

as hexavalent chromium and vanadium. Aerobic biological treatment was demonstrated as a polishing process 

to effectively remove residual organics and ammonia-nitrogen.