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Memorandum

Date: September 30, 2015

From: Phillip Flanders, OW/OST/EAD

Ron Jordan, OW/OST/EAD

Re: Variability in Flue Gas Desulfurization Wastewater: Monitoring and Response

1 Introduction

EPA is promulgating new effluent limitations and guidelines (ELGs) for the steam electric power 

generating industry for some process wastewaters including flue gas desulfurization (FGD) 

wastewater from wet scrubbers. Some commenters stated that FGD wastewater is too variable to 

establish national standards due to changes in fuel type, plant operations, or other factors. 

However, steam electric power plants are sophisticated operations and they have demonstrated 

the ability to properly monitor, operate, and maintain FGD wastewater treatment systems.1 EPA 

included costs in its costs model to account for necessary equipment, chemicals, and operators to 

ensure that the FGD wastewater treatment system operates effectively. This memorandum 

discusses monitoring and response strategies that a plant could implement to address issues with 

FGD wastewater variability that were raised by commenters. Sections 2 and 3 contain 

background information on FGD scrubber chemistry and flue gas variability. Section 4 discusses 

chemical precipitation and biological treatment of FGD wastewater. Sections 5 and 6 discusses 

the monitoring capabilities that plants either have or will install and what plants can to do

respond to changing conditions. 

In wet FGD scrubbers, the flue gas stream contacts a liquid stream containing a sorbent, which 

causes the mass transfer of pollutants from the flue gas to the liquid stream. The chemistry of 

these scrubbers is discussed in the following section. During the scrubbing process, metals and 

other constituents that were not removed from the flue gas stream by the electrostatic 

precipitators (ESPs) may transfer to the scrubber slurry and leave the FGD system via the 

scrubber blowdown (i.e., the slurry stream exiting the FGD scrubber that is not immediately 

recycled back to the spray/tray levels). The scrubber blowdown is typically transferred

intermittently from the FGD scrubber to the solids separation process (typically hydrocyclones

and belt filter presses) and then to wastewater treatment. See the Technical Development 

Document for the Effluent Limitations Guidelines and Standards for the Steam Electric 

 

1 For example, the FGD wastewater treatment technologies operating or being installed at some plants includes chemical 

precipitation/iron coprecipitation (e.g., Pleasant Prairie, Miami Fort, Keystone), a combination of both chemical and biological 

treatment (e.g., Allen, Belews Creek, Mountaineer), and mechanical vapor-compression evaporation (e.g., Merrimack, Mayo, 

Petersburg). Many other plants are also operating these or similar treatment technologies.
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Generating Point Source Category (Technical Development Document) for additional 

information.

The BAT FGD wastewater treatment system described in the preamble for the final ELGs and 

modeled in the Incremental Costs and Pollutant Removals for the Final Effluent Limitations 

Guidelines and Standards for the Steam Electric Power Generating Point Source Category 

report (DCN SE05831) consists of chemical precipitation followed by biological treatment. The 

chemical precipitation stage employs hydroxide precipitation, sulfide precipitation, and iron 

coprecipitation. Biological treatment consists of an anoxic/anaerobic fixed-film bioreactor 

system that is designed to remove selenium, nitrate/nitrite, and heavy metals. Thus, the system

as a whole is designed to remove suspended solids, particulate and dissolved metals (such as 

mercury and arsenic), soluble and insoluble forms of selenium, and nitrate and nitrite forms of 

nitrogen. In addition, the system is preceded by an equalization tank with 24-hour hydraulic 

residence time. See Section 7 of the Technical Development Document for further information.

2 FGD Scrubber Chemistry

Wet FGT scrubbers contact flue gas with a stream of sorbent to remove SO2 from the flue gas 

and convert it a solid that is removed by the solids dewatering equipment (Buecker, 2006). The 

sorbent is typically a slurry of lime (Ca(OH)2) or limestone (CaCO3) in water. The 

lime/limestone serves as a source of calcium and as a source of alkalinity. Scrubbers are 

typically operated under acidic conditions around pH 5. When SO2 dissolves into the sorbent 

water it forms sulfurous acid (H2SO3):

SO2(g) + H2O ⇌ H2SO3 (1)

Sulfurous acid dissociates and releases protons (H+), which is the source of most of the acidity in 

the scrubber absorber tank. Lime and limestone are only poorly soluble under neutral or acidic 

pH and the acidity from sulfurous acid is necessary to dissolve these solids. The dissolution of 

lime (2) or limestone (3) produces hydroxide ion and serves to provide alkalinity and to prevent 

the pH from becoming too acidic and causing corrosion of the absorber tank. Note that under 

acidic conditions carbonate can be lost from solution as carbon dioxide gas (4), which would 

compete with absorption of SO2. 

Ca(OH)2 ⇌ Ca2+ + 2 OH-

(2)

CaCO3(s) + H2O ⇌ Ca2+ + HCO3

- + OH-

(3)

CaCO3(s) + H+ ⇌ Ca2+ + H2O + CO2(g) (4)

The so-called “scrubbing” reaction expresses the combination of dissolution of SO2 and lime (5)

or limestone (6) in acidic conditions resulting in sulfite (SO3

2-

) and calcium ions (Ca2+). 

Ca(OH)2(s) + SO2(g) + 2H+ ⇌ Ca2+ + SO3

2- + H2O (5)

CaCO3(s) + SO2(g) + 2H+ ⇌ Ca2+ + SO3

2- + H2O + CO2(g) (6)

Scrubbers can operated with a forced, inhibited, or natural oxidation system. Forced oxidation 

scrubbers use powerful blowers to force air into the scrubber absorber tank slurry from the 
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bottom of the scrubber tower. This introduces oxygen (O2) into the slurry where it can oxidize 

sulfite into sulfate:

2SO3

2- + O2 ⇌ 2SO42-

(7)

How effectively the sulfite is oxidized into sulfate influences the solids that are produced in the 

scrubber. Forced oxidation scrubbers are usually designed to produce gypsum:

Ca2+ + SO4

2- + 2H2O ⇌ CaSO4∙2H2O(s) (8)

However, a mixed sulfite-sulfate solid (9) and calcium sulfite (10) can also be produced. 

2Ca2+ + SO3

2- + SO42- + 1⁄2H2O ⇌ (CaSO3∙CaSO4)∙1⁄2H2O(s) (9)

Ca2+ + SO3

2- + 1⁄2H2O ⇌ CaSO3∙1⁄2H2O(s) (10)

Combustion of coal in the boiler is the source of most of the other constituents of the FGD 

absorber tank slurry. The chlorine content of the coal converts to hydrogen chloride gas (HCl) 

and is readily absorbed into the slurry. If the boiler conditions are sufficiently oxidizing chlorine 

gas (Cl2) can also be produced, which can then be captured in the scrubber. (Hatt, 2015)

Similarly, if conditions in the absorber tank slurry are sufficiently oxidizing, Clcan be oxidized 

to dissolved Cl2, which forms hypochlorous acid (HOCl) and can lower the pH:

Cl2 + H2O ⇌ HOCl + H+

(11)

Similar reactions would be expected for bromine. Bromine is naturally present in many coals, but 

it can also be sprayed onto the coal in the form of calcium bromide. This added bromine is 

intended to curtail mercury emissions. (Reisch, 2015) Mercury is present in flue gas as 

particulate-bound mercury, elemental mercury, and oxidized forms such as mercuric chloride 

(HgCl2). Particulate mercury can be removed by the ESPs. Oxidized mercury is a soluble ion 

(Hg2+) in water and can dissolve in the absorber slurry. Elemental mercury is poorly captured by 

these scrubbers and its volatility can lead to mercury emissions from the scrubber. (Miller, et al., 

2006; Brown, DeVault, & Johnson, 2013) Other trace elements found in the slurry include 

selenium and arsenic.

The limestone feed can also be a source of contaminants in the absorber slurry. Limestone can 

be a source of iron, silicon in compounds such as quartz, and magnesium. (Buecker, 2006) Some 

scrubbers add dibasic acid (a mixture of acids such as adipic acid) to improve SO2 removal. See 

Section 6 of the Technical Development Document for the final ELGs for additional information. 

3 Flue Gas Variability and Scrubber Operation

Flue gas entering the FGD scrubber will vary somewhat based on the coal burned and the 

operation of the boiler. However, with proper information about characteristics of the coal 

burned and effective communication between the coal pile, boiler, and wastewater treatment 

system operators the effects of this variability can be mitigated. As previously mentioned, the 

coal itself is the primary source of sulfur, chlorine, mercury, selenium, and arsenic in the 
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absorber slurry. Plant staff operating the coal pile and the boiler know the electricity generating 

load(s), what coal type they are burning, the source of the coal, and the sulfur and chlorine 

content of the coal. The coal characteristics could alter the characteristics of the FGD purge 

stream, however with this information the wastewater treatment system operators can use this 

information to better prepare for any variability in the characteristics of the FGD purge stream.

4 FGD Wastewater Treatment System

Plants that install the technology basis selected as BAT for the final ELGs will install a chemical 

precipitation system followed by anaerobic/anoxic biological treatment. A brief description is

included here. For further details see Section 7 of the Technical Development Document.

Following solids separation, the FGD purge stream is sent to the chemical precipitation system. 

In the chemical precipitation system plants add lime (calcium hydroxide), ferric (or ferrous) 

chloride, and organosulfide to precipitate some metals. Lime is added to elevate the pH of the 

wastewater to a designated set point, helping precipitate metals into insoluble metal hydroxides 

that can be removed by settling and filtration. Ferric (or ferrous) chloride is used to 

coprecipitate additional metals and organic matter. The ferric chloride also acts as a coagulant, 

forming a dense floc that enhances settling of the precipitated in the downstream clarification 

step. Generally, sulfide precipitation improves the removal of some metals, such as mercury, by 

creating metal sulfide compounds that are less soluble than metal hydroxide compounds. . 

Chemical precipitation can be designed to add all three chemicals to the same reaction tank or to 

add each chemical in separate tanks. By reducing the concentration of dissolved metals, 

adjusting the pH or alkalinity, and reducing the total suspended and dissolved solids 

concentrations of the FGD wastewater, the chemical precipitation system step acts to condition 

the wastewater for optimal treatment in the biological system, and additional chemical 

pretreatment can be done if necessary. This could include chemical addition to remove oxidants 

and lower ORP (e.g., sodium bisulfite, or use of ferrous chloride instead of ferric chloride).

2

 A 

separate denitrification step can also be included to decrease nitrite/nitrate concentrations prior to 

the biological stage.

3

Anaerobic biological systems reduce certain pollutants (e.g., selenium, nitrates) more effectively 

than has been possible with surface impoundments, chemical precipitation, or aerobic biological 

treatment processes. Anaerobic fixed-film bioreactors consist of a packed bed of granular 

activated carbon (GAC) or other permanent porous substrate that is inoculated with specific

naturally occurring microorganisms. Power plants also add a nutrient feed such as a molassesbased feed to the wastewater before it enters the bioreactor. The nutrient feed provides a source 

of carbon and other minerals to the microorganisms. Microorganisms require both an electron 

acceptor and a carbon source in order to grow. Microorganisms that are contained in biological 

systems for FGD wastewater can use several compounds including nitrate, nitrite, selenite and 

selenate as electron acceptors. This is why they can reduce nitrate and nitrate to nitrogen gas and

 

2 EPA’s costs include equipment for sodium bisulfite addition. However, the data in the record indicates that such chemical 

addition would be needed only infrequently, if at all. See, e.g., data submitted by Duke Energy for Allen and Belews Creek 

Stations (March 2014 data submittal showing when Allen added sodium bisulfite; no use of the chemical identified in Belews 

Creek data).

3 EPA included costs in its estimates for denitrification systems at plants with greater than 100 ppm nitrate-nitrate in their 

untreated FGD wastewater.
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selenate and selenite to elemental selenium. However, microorganisms use electron acceptors 

preferentially in order of their reduction potential. Acceptors with high reduction potentials are 

“easier” for the microorganisms to reduce and thus are consumed first. Table 1 shows the 

biological system ORP as reported by Sonstegard et al. that corresponds to the reduction of each 

relevant electron acceptor. The electron acceptors in Table 1 are listed in order of reduction 

potential, microorganisms will consume them in order from the top of to the bottom. That is 

oxygen will be depleted first, then nitrate, then nitrite, then selenate and so on.

Table 1. Approximate ORP of electron acceptors relevant to biological treatment of FGD 

wastewater. ORP is related to the reduction potential of the acceptor. (Sonstegard, et al., 2010)

Electron Acceptor Approximate ORP

Oxygen > 0 mV

Nitrate < 0 mV

Nitrite < -50 mV

Selenate < -100 mV

Selenite < -150 mV

Sulfate < -200 mV

The bioreactor contains different zones that have differing ORP levels, similar to those shown in 

Table 1. Plants operate the bioreactors to achieve a sufficiently negative ORP in the bioreactor 

cells to provide the optimal environment to reduce selenium to its elemental form. The ORP can 

be controlled by the amount of nutrient that is fed to the system. Additionally, the system 

maintains a pH level in the bioreactor between 6.0 and 9.0 S.U. because extreme high or low pH 

levels could affect the performance of the microorganisms and potentially allow nondesirable 

microorganisms to propagate. Selenate and selenite is reduced to elemental selenium by the 

microorganism and forms solid elemental selenium nanospheres which are retained within the 

biomass. The microorganisms can also reduce some sulfur compounds (e.g. sulfate) to sulfide 

and improve removal of other metals, including arsenic, cadmium, nickel, and mercury, by 

forming metal sulfides within the system.

Periodically, the bioreactor must be backflushed to remove the solids and inorganic materials 

that have accumulated within it. The flushing process involves flowing water upward through the 

packed bed, which dislodges the particles fixed within the activated carbon. The backflush water

is sent to the chemical precipitation system to remove the solids (and adhered selenium) within 

the clarifier. The backflush does not result in a discharge separate from the biological system 

effluent, because it is completely recycled through the treatment system.

As the microorganisms denitrify the wastewater, nitrogen and carbon dioxide gases form, which 

periodically build up and form pockets within the bioreactor. As a result, water flows through

constricted channels, reducing microbial contact and increasing pressure drop across the 

bioreactor, an overall negative effect on the system. (Sonstegard, et al., 2010) To remove these 

gas pockets, plants occasionally perform a degassing operation by transferring water backwards 

through the cells, similar to a backflush, but only long enough for the gas to “burp” out of the 

system. This type of flush is long enough to lift the biomatrix and release entrained gases, but 

short enough to avoid flushing any water out of the bioreactor.
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Properly run biological treatment systems balance the amount of biomass in the system with the 

incoming load of nitrates and selenium. They do this by adjusting the nutrient feed and the 

duration and frequency of the backflush. Insufficient nutrient addition will limit the growth of 

the microorganisms so there will not be enough biomass to reduce all of the selenium in the 

wastewater. In contrast, too much nutrient can lead to conditions that are too strongly reducing 

and lead to the generation of sulfate and ammonia. Additionally, too much biomass will clog this 

system, lead to high pressure drops and inefficient operation, and potentially allow breakthrough 

of solids, releasing pollutants in the treatment system effluent. The following sections discuss 

the monitoring capabilities that plants have and suggested response strategies for situations 

where FGD wastewater characteristics change. Attachment 1 presents an evaluation of data for

Duke Energy’s Allen Steam Station and Belews Creek Steam Station. 

5 Monitoring Capabilities

5.1 Coal Characteristics 

EPA recognizes that a plant may decide to change the fuel it is burning and that the decision to 

change the fuel could be spurred by several factors; however, with good communication among 

plant staff there is no reason it should be “unexpected.” First of all, the plant cannot switch to a 

different fuel unless it has already purchased it and received it onsite. Thus, the plant’s coal 

buyers and coal pile operators (and probably other plant staff) are aware that this different coal 

(or pet coke) is present at the plant. Then, the coal pile operator must be directed to switch to the 

different coal, which the boiler operators are likely to be aware of. Plants usually have detailed 

information on the coal they are burning sometimes on a day by day basis including coal basin 

and mine of origin, chlorine content, and sulfur content.4 Some plants may decide they do not 

need to closely track this information if they do not anticipate their coal selection will affect 

treatment system performance. However, plants can ensure they have this information if needed, 

thus the act of changing coal type or source should not lead to unexpected conditions in the 

treatment system. 

5.2 Flue Gas SO2 Content

The loads and sulfur content information could be used to determine if the scrubber should 

expect changes in the SO2 in the flue gas. Plants may also be using flue gas analyzers that can 

detect SO2 in the flue gas prior to the scrubber. The relative amounts of SO2 and O2 in the 

scrubber are important because O2 drives the oxidation of sulfite to sulfate (7). It is essential at 

plants that produce gypsum for sale that enough air be supplied to the scrubber to completely 

oxidize all of the sulfite. A rule of thumb is that there should be four parts oxygen per part SO2

(Buecker, 2006). If there is insufficient O2 some of the solids produced with be the mixed 

sulfite-sulfate solid (9) and calcium sulfite (10). The relative amounts of SO2 and O2 in the 

scrubber should also affect the ORP in the scrubber slurry because O2 drives the oxidation of 

sulfite to sulfate (7). Scrubbers are designed to handle boilers burning a certain load of coal with 

certain specifications including sulfur content and chlorine content. For example, the blowers 

are designed to provide enough O2 to oxidize the sulfite to sulfate. So, if a large load of high 

 

4 See data submitted by Duke Energy for Allen and Belews Creek Stations (March 2014 submittal).
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sulfur coal being is burned, it follows that there will be too much SO2 in the flue gas, which will 

then lead to more sulfite in the absorber reaction than can be oxidized. In this case the ORP will 

probably be low because there is insufficient oxidant (O2) to oxidize all of the sulfite to sulfate. 

The opposite case would be a small load of low sulfur coal might be more likely. With less SO2

than designed for the O2 will overwhelm the decreased concentration of sulfate and increase the 

ORP as the scrubber tank slurry becomes oxygenated. Plants may be able to turn down the 

blowers to reduce the amount of air delivered to the slurry and mitigate the increase in ORP. 

5.3 FGD Solids Composition

Plants can use thermogravimetric analysis to determine the composition of the solids produced 

by the scrubber. (Buecker, 2006) Thermogravimetric analysis works by heating a dried sample

of the scrubber solids and monitoring the mass of the sample as the temperature increases. 

Scrubber solids lend themselves well to this analysis because the three main component solids 

decompose at distinct temperatures. The composition of the solids indicates how complete the 

oxidation of sulfite to sulfate was in the scrubber. Plants that sell their solids as gypsum likely 

already conduct this analysis. 

5.4 ORP and pH monitors

Plants can install relatively simple and inexpensive pH and ORP probes that can continuously 

monitor pH and ORP. Many plants likely already have the capability to monitor pH in the 

scrubber installed as they target their lime/limestone feed rate to maintain the absorber tank 

slurry pH around 5 S.U. Some plants have also started to monitor ORP in absorber vessel.

5

Plants also monitor pH of the wastewater influent to the chemical precipitation system to control 

chemical addition rates to ensure that the plant is achieving the target pH in the reaction tank. 

The pH is also monitored and can be adjusted upstream of the biological reactors (as part of the 

chemical precipitation system) to provide optimum conditions for microbial health and selenium 

reduction in the bioreactors. An ORP sensor included downstream of the bioreactors is employed 

to adjust the nutrient feed. (YSI Environmental, 2005)

5.5 Nitrate/Nitrite

In addition to sending samples to analytical laboratories, plant operators can conduct field tests 

to monitor the nitrate/nitrate concentration in the feed water to the biological system using a 

simple, commercially-available kit consisting of test strips or a wet chemical test based on 

cadmium reduction. Such field tests can provide immediate indication of significant changes in 

the influent or effluent nitrate concentrations.

5.6 Strong Oxidizers

In some cases, an excessive concentration of oxidants will drive the ORP to increase to high 

level. The primary indication would be seen as a high ORP measured by an ORP sensor. At 

 

5 See, e.g., pH and ORP data submitted by Duke Energy for Allen and Belews Creek Stations and by American Electric Power 

for the Mountaineer Plant (March 2014 submittals).
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least one firm has developed a wet-chemical titration method that can also be used to determine a 

“total oxidant loading” even though the chemical identity of the oxidant is not known. One 

method uses ferrous ammonium sulfate and cerium (IV) sulfate to determine the oxidant loading.

(Babcock and Wilcox, 2010) Iodometric titration could also be an effective method. These are 

tests that could be conducted on-site at a plant by a technician to confirm the presence and 

quantify the magnitude of free oxidants in the influent wastewater. 

5.7 Total Suspended Solids

Plants can measure total suspended solids (TSS) using simple standard measurement devices. 

High TSS in the bioreactor influent suggests that the chemical dosage or residence time in the 

chemical precipitation system is insufficient or there may be an upset condition in the clarifier. 

High TSS in the bioreactor effluent suggests that solids are breaking through the bioreactor, 

which would be one indication that the backflushing operation needs to be modified (e.g., more 

frequent, longer duration). Plants could also use turbidity meters as an alternative method to 

monitor suspended solids. Conductivity meters could be used to measure total dissolved solids 

along with TSS.

5.8 Pressure Drop across the Biological Treatment System

Plants can use pressure gauges to measure the drop in pressure between the influent and the 

effluent to the biological treatment system also known as head loss. The pressure drop can be 

diagnostic of the conditions within biological treatment system because it contains packed beds 

with continuously growing biofilms. 

5.9 Temperature

Plants can use temperature probes and meteorological information to monitor the temperature of 

the influent to the biological treatment system and the outdoor ambient temperature. GE has 

evaluated their biological treatment system at temperatures ranging from 41 to 105 degrees F. 

GE demonstrated their plants are able to effectively treat their FGD wastewaters once optimized 

for the plant’s operations within this temperature range. EPA includes costs for a heat exchanger 

for certain southern plants determined to require cooling of FGD wastewater prior to biological 

treatment during the summer months when FGD wastewater can reach temperatures greater than 

105 degrees F. Based on comments, EPA also includes costs for plants to house the biological 

reactors in a building in cold weather climates.

5.10 TDS and Chlorides

Plants use probes to measure total dissolved solids (TDS) and chloride concentration in the FGD 

purge stream. Most plants use chlorides level to determine when to purge wastewater to the 

treatment system, ensuring the chlorides concentration in the absorber remains below operational 

limits established to prevent corrosion in the FGD system. In addition, some plants have become 

aware that their operational limits have not provided the corrosion protection they anticipated

and in some cases are establishing lower chlorides operational limits to better protect the 

scrubber metallurgy. GE has determined that the microorganisms in the bioreactor can function 
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without any degraded performance in FGD wastewater with chlorides as high as 35,000 ppm

(DCN SE04208). This was determined in laboratory testing based on requests from power 

companies that were expecting to be operating at those high chloride levels. High TDS 

concentrations may dehydrate the microorganisms in the system. From a biological perspective, 

it is the total concentration of dissolved species (where chloride is one of many dissolved species 

present in FGD wastewater) surrounding a cell that will determine the osmotic balance and 

whether the cell becomes stressed by hypertonic conditions. Outside of FGD applications, the 

vendor has demonstrated biological treatment at TDS levels containing 150,000 mg/L (DCN 

SE04208). For comparison, the data in the record for FGD wastewater shows that, in general, 

TDS concentrations in FGD wastewater are typically double the chloride concentration. Using 

that rule of thumb, the microorganisms in the bioreactor are expected to remain viable and 

effectively treat FGD wastewater at chloride levels much higher than 35,000 ppm.

6 Response Strategies

6.1 Increased Scrubber ORP

High ORP levels in the influent to the biological treatment system can be problematic when they 

alter the ORP within the bioreactor cells and result in there being insufficient effective bed 

contact time at the appropriate ORP level to sufficiently reduce the nitrates/nitrites and to reduce 

selenate and selenite to elemental selenium. However, plants that monitor ORP in the absorber 

or in the FGD purge will have sufficiently advanced warning to respond to high ORP levels. 

Plants with anoxic/anaerobic biological treatment systems already monitor the ORP within the 

bioreactor and use that data to control the nutrient feed, as an indicator of the health of the 

microorganisms, and to monitor the overall performance of the bioreactor. EPA also included 

costs for plants to install an ORP probe in the FGD purge line prior to the equalization tank.

This additional ORP probe serves a couple purposes. First, it acts as an early warning system (in 

addition to the ORP monitoring plants some already do for the FGD absorber) for the treatment 

system operators. Because of the hydraulic residence time in the chemical precipitation stage of 

the treatment system (including the equalization tank with 24-hour HRT), knowing the ORP of 

the influent purge gives operators at least 2 days advance notification before the wastewater 

reaches the bioreactor, and possibly double that amount of time.6 This gives operators ample 

opportunity to take steps to adjust the ORP of the influent wastewater, adjust nutrient addition 

rate or, if necessary, stimulate additional microbial activity in the bioreactor. Second, high ORP 

is an indicator that high concentrations of free oxidants may be present in the influent 

wastewater. Thus, measurements from this ORP probe could be used, along with direct analyses 

of free oxidants if operators choose, to determine whether the plant should add chemicals to 

condition the wastewater. EPA included costs for a chemical addition system that will add a 

chemical reductant (e.g. sodium bisulfite) into the chemical precipitation system when needed to 

respond to elevated ORP levels. The reductant will react with oxidants present in the wastewater 

and lower the ORP into an acceptable level for biological treatment. Alternatively, plants could 

add ferrous chloride as instead of ferric chloride in the chemical precipitation system. Ferrous 

chloride will act react with oxidants while ferric chloride does not.

 

6 See, e.g., the HRT for the chemical precipitation stage at Duke Energy’s Allen and Belews Creek Stations. 
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Additionally, it is possible to respond to elevated ORP levels by increasing the feed rate of the 

nutrient mix. As mentioned above, the nutrient mix provides the carbon necessary for the 

biomass to grow. By increasing the nutrient feed, microbial activity increases and the biomass 

will grow large enough to have sufficient capacity to reduce the increased load of electron 

acceptors. If the treatment system is not managed properly (including managing ORP, free 

oxidants and nutrient feed), the microorganisms may not be able to reduce all of the electron 

acceptors in the wastewater; they will preferentially reduce electron acceptors that are “easy” to 

reduce (i.e. have high reduction potential) and may not have sufficient biomass and microbial 

activity to fully reduce selenite and selenate which are more “difficult” to reduce (i.e. have low 

reduction potentials). 

See Section 7.1.3 of the Technical Development Document for additional discussion of how

plants can implement a proactive oxidant monitoring and mitigation strategy. 

Plants are finding that by controlling scrubber ORP they can also reduce the incidence of 

corrosive conditions within the absorber. Maintaining closer control of scrubber ORP to remain 

within desired operating ranges to minimize corrosion has the added benefit of controlling 

influent purge ORP within the range that facilitates biological treatment (without the need for 

further conditioning such as sodium bisulfite addition) and reduces fluctuations in the influent 

purge ORP. A Babcock and Wilcox study suggested the highly oxidizing (i.e. high ORP) 

conditions in the reaction tank can lead to formation of manganese (IV) oxides that form 

galvanic corrosion cells when they deposit on the walls of the tank. (Bonnin-Nartker, et al., 

2012) They also noted that high concentrations of peroxodisulfate, also linked to high ORP, are 

found in scrubber tanks that are aggressively corroded. 

6.2 Increased Nitrate/Nitrate in Bioreactor Influent

Plants monitor influent nitrate levels and use that information to control the amount of nutrient 

feed for the bioreactor.7 The carbon-to-nitrogen ratio is an important parameter for 

microbiological activity and the nutrient feed provides carbon and other nutrients for the 

microbes. As nitrate concentrations rise, the nutrient feed is increased to maintain the desired 

C:N ratio. 

The treatment system for any individual plant will be designed to remove pollutants to a desired 

target level, based on the expected range of influent pollutant concentrations. In the case of the 

biological treatment stage, the expected influent nitrate concentrations would be a factor taken 

into consideration. There is no real upper bound to the influent nitrate concentration that could be 

treated by the BAT technology since the bioreactor can be sized to provide whatever effective 

bed contact time is needed. Alternatively, the bioreactor can be designed to treat a specified 

maximum expected nitrate concentration, and plants with wastewater containing even higher 

influent nitrate concentrations can pretreat the wastewater prior to the bioreactor using a 

denitrification step. This is the approach EPA used to estimate costs for the ELGs, basing the 

cost for the anoxic/anaerobic biological stage on influent nitrate concentrations of 100 mg/L. The 

 

7 An alternative approach for controlling nutrient feed to have the control system use measurement data from the ORP probe to 

automatically adjust the feed rate. This approach works because the ORP within bioreactor cells changes as the microorganisms 

reduce nitrates and selenium compounds. Thus, this approach feeds the nutrient as needed to achieve the target ORP levels.
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data in the record indicates that most power plants have influent nitrate concentrations 

substantially lower than 100 mg/L. For those plants with influent nitrate concentrations higher 

than 100 mg/L, EPA included costs for a denitrification step in the treatment process prior to 

bioreactor 

6.3 Increased TSS in Bioreactor Influent and Effluent

High TSS in the bioreactor influent suggests that the chemical dosage or residence time in the 

chemical precipitation system is insufficient for the clarifier to effectively remove solids, or that 

there may be an upset condition such as turnover of the clarifier bed. The chemical dosage or 

hydraulic flow rate could be adjusted to improve performance of the clarifier. 

High TSS in the bioreactor effluent suggests that solids are breaking through the bioreactor. One 

possibility is that the amount of biofilm that has accumulated in the bioreactor might exceed the 

capacity that the granular activated carbon (GAC) bed can contain. A portion of these biosolids 

are sloughed off the GAC and exiting the system in the effluent. The amount of biosolids 

residing within the bioreactor is controlled by the bioreactor backflush cycle. During the 

backflush, some of the biosolids are removed and sent to the influent of the chemical 

precipitation stage. If a plant discovers that amount of biosolids is becoming too high, the 

backwash duration or frequency could be increased to control the amount of biosolids to a lower 

level. Plants should also ensure that they are “burping” the system with sufficient frequency to 

prevent the buildup of gas which might occupy space in the bioreactor and block the growth of 

biomass or facilitate channeling that could allow solids to be released into the effluent.

6.4 Increased Pressure Drop across the Biological System

Increasing pressure drop indicates that flow through the bioreactors has been constricted. This 

can mean that the amount of biomass in the system has grown too large and the biofilms on the 

GAC particles are restricting flow or that too much gas has been generated and the gas bubbles 

are restricting flow. If there is too much biomass, plants can respond to the increased pressure 

drop by increasing the duration or frequency of backflush which removes some of the biomass 

and sends it to the chemical precipitation stage of the treatment system. If there is too much gas

building up, plants can increase the duration or frequency of the “burping” cycle that removes 

entrained gases from the packed beds. 

6.5 Ammonia Generation and Presence in the Biological System Effluent

Based on discussions with GE, ammonia is not generated from the biological treatment system 

but it can be a byproduct attributable to the nutrient blend used to feed the microorganisms. That 

is, nitrate and nitrite should be reduced to nitrogen gas (N2) and not ammonia. The presence of 

ammonia could indicate that the bioreactors have become too strongly reducing, and the ORP in 

the effluent would be very low. Plants could decrease the rate of nutrient feed or increase the 

backflush to compensate for this. Additionally, GE noted that the nutrient blend can be 

formulated to minimize the addition of ammonia to the treated FGD wastewater effluent. 

Nutrient dosages can be tightly controlled through proper monitoring to ensure over-dosing 

events are minimized and excessive ammonia additions prevented.
Page 12 of 46

The ELGs do not establish a limit on ammonia. Should a plant an NPDES permit limit for

ammonia experience elevated effluent concentrations it should investigate reformulating the 

nutrient mix. It could also decide to install additional treatment to remove excess ammonia, 

which would not substantially increase the overall cost for the treatment system. Note, however, 

that EPA has not identified any plants with an ammonia limit for their FGD wastewater. 

6.6 Presence of Organics (BOD) in the Biological System Effluent

Elevated effluent BOD indicates that the amount of biomass in the bioreactors has grown too 

large and some is sloughing off the packed bed and leaving with the effluent.8 The amount of 

biomass contained within the bioreactor is controlled by backwashing the system, which flushes 

a portion of the biomass from the reactor. The backwash is returned to the chemical 

precipitation stage so that the bioreactor solids can be removed along with the chemical 

precipitation treatment solids in the clarifier. Plants can backwash the bioreactors at a sufficient 

frequency to ensure that the level of organics in the effluent is minimal.

6.7 Increased Effluent Concentrations of Nitrate-Nitrite or Selenium

Significant increases in effluent nitrate-nitrite or selenium concentrations would be an indication 

that the treatment system is overloaded or not operating properly. A situation where the 

treatment system is merely overloaded should not occur if operators properly monitor the 

influent wastewater characteristics (e.g., pH, ORP, flow), monitor bioreactor performance (e.g., 

pH, ORP, pressure drop), adjust chemical dosages as needed to reflect wastewater characteristics 

and treatment system parameters (e.g., lime, organosulfide, ferric chloride, polymers, nutrients, 

sodium bisulfite), and maintain effective communication with other plant operators to remain 

aware of changes at the coal pile, boiler, or air pollution control equipment.

Should a treatment system upset or other degraded performance occur, operators can take action 

as described in this memorandum, including redirecting the effluent back to the treatment system 

for further treatment (see Section 6.9) or ensuring the wastewater has undergone sufficient 

pretreatment prior to the bioreactor (see Sections 6.1-6.4).

9 A significant increase in effluent 

nitrate-nitrite concentrations can be resolved by increasing the nutrient dose in the bioreactor. 

Such a situation should not occur, however, if operators have been properly adjusting their 

nutrient feed to correspond to measurements of influent nitrate-nitrite and ORP (including 

monitoring the nitrate concentrations and ORP in the bioreactor). To address an upward spike in 

 

8 Plants experiencing this condition where they are not adequately retaining the biomass within the treatment system may also 

observe an increased concentration of total selenium in the effluent because the elemental selenium is contained within the 

biomass. The selenium present in such instances is expected to be largely in particulate form and the amount of dissolved 

selenium in the effluent would not be affected. This is an indication that the microbiological organisms are effectively removing 

selenium from the wastewater, but that the treatment system is not being properly operated to backflush the resulting bioreactor 

solids back to the chemical precipitation stage.

9

Increased effluent concentrations of arsenic or mercury would suggest that the chemical precipitation stage is not being operated 

so that chemical additives are added at the dosage necessary to remove dissolved or small-sized particulate forms of the 

pollutants, or that the clarifier may not be operating properly. It can also be an indication that bioreactor solids are not being 

adequately backflushed back to the chemical precipitation stage and are being released in the effluent. The mercury analyzer 

included in the costs for the chemical precipitation stage would provide information to operators on how the effluent quality of 

wastewater exiting that stage of the treatment system. 
Page 13 of 46

effluent selenium concentrations, operators should check flows and pressure drop for the 

bioreactor to determine if bed permeability has been compromised. It may be necessary to adjust 

backwash frequency or duration to remove solids, including conducting a more vigorous 

backwash to restratify the bed to ensure channelization is not occurring. Operators should also 

check to see if there has been a change in ORP or pH of the wastewater exiting from the 

chemical precipitation stage (i.e., the bioreactor influent). Additional actions operators can take 

include checking the nutrient feed rate (e.g., confirm the feed system has not run out of the feed 

chemical or has a clogged line or strainer restricting the feed rate) and changing the hydraulic 

retention time to increase the effective bed contact time by slowing the flowrate through the 

treatment system or placing more bioreactor cells (i.e., beds) in service.

6.8 Plant or Treatment System Shutdown Periods or Reduced Operation

Short shutdowns should have no effect on biological treatment systems. Extended shutdowns can 

be managed to have little or no effect on effluent quality and, based on the data in the record, can 

be managed so that plants will be able to comply with the ELG effluent limitations at all times. 

Based on discussions with GE, there are many cases where anaerobic systems run on seasonal 

industrial operations and can experience shutdowns for as long as 6 months. A relatively small 

amount of feed water (either plant service water or FGD wastewater stored prior to the 

shutdown10, along with some nutrient added) added weekly to the biological reactor will keep the 

microorganisms alive, albeit at a lower rate of activity. A constant recycle through the system 

can also be done but is not necessary. 

The biomass can remain in a less active (nearly inactive) state for long periods of time and 

resume normal metabolic function in a relatively short time once wastewater is again available 

(DCN SE04208). The biological treatment stage can quickly return fully to normal operation. 

According to GE, the biological treatment system can return to operation within a period of days 

if the actions described above are taken to sustain the microbes during shutdowns. Additionally, 

if the shutdown was extended and microbial activity has been at a lower rate for a long period of 

time, operators can spur increased activity and biomass growth in advance of the generating unit 

startup using the actions described above (see Section 4) or can slowly ramp up the rate at which 

fresh influent wastewater is introduced into the treatment system.11 EPA reviewed effluent data 

in the record for plants operating combined chemical precipitation and biological treatment for 

FGD wastewater to evaluate how cycling operation (i.e., changes in electricity generation rate) 

and short or extended shutdown periods may affect the ability of plants to meet the BAT effluent 

limitations. These data demonstrate that cycling operations and shutdown periods, whether short 

or long in duration, are manageable and do not result in plants being unable to meet the ELG 

effluent limitations. See Attachment A for more information.

 

10 Although service water can be used to maintain the microbial activity, if a plant knows in advance that it will undergo an 

extended shutdown it can stockpile a large volume of FGD wastewater for this purpose. In addition to filling the treatment system 

equalization tank to its maximum operating level, plants typically have large maintenance tanks (normally empty) that are used to 

hold the liquid drained from absorber vessels when the FGD system undergoes maintenance that requires entry into the scrubber 

itself (e.g., repair work to spray headers or mist eliminators).

11 Operators can also recycle the bioreactor effluent back to the influent of the treatment system (usually, to an equalization tank 

at the beginning of the treatment system) although this typically should not be necessary.
Page 14 of 46

6.9 Effluent can be recycled

If, for whatever reason, the treatment system were to become overloaded or suffer a treatment 

system upset condition and become unable to reduce the pollutant concentrations below the 

effluent limits, the plant can recycle the effluent wastewater back to the influent of the treatment 

system for reprocessing (e.g., redirect the effluent to the equalization tank). This allows the 

wastewater to be treated a second time through the chemical precipitation and biological 

treatment system. Recycling the effluent can also provide time for the bioreactor to grow 

additional biomass to remove all of the nitrate-nitrite and selenium from the wastewater. 

Another benefit of effluent recycling is to reduce variability of the flow rate due to the 

discontinuous generation of the FGD purge. This recycle flow, in addition to the equalization 

tank reducing the variability of influent pollutant concentrations and flow, can contribute to more 

steady operation of the treatment system (and additional source to sustain microbial activity) in 

times when the FGD purge varies substantially due to large changes in the plant’s electricity 

generation (either due to cycling operation or shutdown of one or more generating units).

There are a number of parameters plants can monitor to know when recycling the effluent may 

be warranted. Nitrates, pH, bioreactor pressure drop, wastewater flow and ORP are frequently, if 

not continuously monitored. Plants also frequently monitor TSS, and turbidity and conductivity 

are typically continuously monitored. EPA included costs for plants to include a mercury 

analyzer so they can know the mercury content of their effluent without having to wait for the 

results for samples shipped to laboratories for analysis. In addition, there are several vendors that 

have developed selenium analyzers specifically for the purpose of in-plant analysis of FGD 

wastewater. Should any of these parameters being monitored indicate that treatment system 

performance and effluent quality may be compromised, the plant could recycle the treatment 

system effluent back to the treatment system to be reprocessed.

6.10 Notification of Key Changes in Plant Operations

Effective communication among plant staff is an important component of optimizing treatment 

system performance. Although treatment system operators have sufficient mechanisms available 

to recognize and react to changes in FGD purge quality that could potentially affect the treatment 

system and effluent quality, their ability to do is further improved if they have been alerted that a 

change in operations is planned or occurring at the plant (e.g. switching to a different type or 

source of coal, significant change in generating load, change in scrubber operations, etc.). Such 

notification also alerts them to the possibility that characteristics such as flow, pH, or ORP may 

change. Knowing this, the treatment system operators can respond by monitoring influent 

characteristics and adjusting dosages of chemical additives (e.g., sodium bisulfite) and ensuring 

that the biomass has adequate capacity to reduce all of the nitrate, nitrate, selenite and selenate 

present in the wastewater. As explained elsewhere in this memorandum, there is sufficient 

hydraulic residence time in the FGD scrubber and chemical precipitation treatment stage 

(including the equalization tank) that treatment system operators have several days before any of 

the FGD purge affected by the changes in plant operations would reach the bioreactor. Should 

any of the parameters being monitored for the FGD purge (e.g., ORP, nitrates) or chemical 

precipitation stage indicate that it is necessary, treatment system operators can increase the 
Page 15 of 46

nutrient feed or introduce a chemical additive containing selenium to spur an increase in 

biomass. 

EPA reviewed effluent data in the record for plants operating combined chemical precipitation 

and biological treatment for FGD wastewater to evaluate how changing the type of coal, or the 

source for the coal, may affect the ability of plants to meet the BAT effluent limitations. 

Additionally, EPA evaluated the data to determine whether the periods when plants changed 

their coal were correlated to changes in effluent selenium concentrations. These data demonstrate 

that in most instances changing the type or source of the coal burned did not have an effect on 

effluent concentrations of selenium. Furthermore, the data support EPA’s determination that any 

effects that would result from changing the coal burned are manageable, as described in Section 

6 of this memorandum and should not result in plants being unable to meet the ELG effluent 

limitations. 

For two of the plants, these data represent 15 periods when the plants switched to a non-standard

coal (as defined by the operating company) and then back again to the standard coal, providing 

data for a total of 30 times when the plants changed the type of coal used. For a third plant, EPA 

did not have information for specific dates when the coal type or source changed; therefore, the 

data for this plant was evaluated more broadly to identify whether there were any periods when 

effluent concentrations for selenium changed substantially. The data for this plant represented 

the use of coals from many sources and multiple coal basins. See Attachment B for more 

information.

7 Considerations for New FGD Wastewater Treatment Systems

While EPA has based the effluent limitations and standards for selenium and nitrate/nitrite (as N) 

for FGD wastewater based on the performance of the Allen and Belews Creek FGD biological 

treatment systems, EPA does not contend that every plant in the industry can simply take the 

design parameters from those two plants, install the biological treatment system, and meet the 

effluent limitations. Each plant will need to work with engineering and design firms to assess the 

wastewater characteristics present at their plant to determine the most appropriate technologies 

and design the system accordingly meet the effluent limitations. 

Plants have time to conduct the necessary studies to properly design the chemical precipitation 

and biological treatment systems to have sufficient capacity to handle the variability of the FGD 

purge stream at a particular plant. As described in section XVI.A.1 of the preamble for the final 

rule, the permitting authority must make the final effluent limitations applicable on or after 

November 1, 2018 but no later than December 31, 2023. This means that plants have between 3 

and 8 years to design and build their FGD wastewater treatment systems.

During that time, plants should conduct studies investigating the variability of their FGD purge 

stream in order to better inform their design process. Plants can use the monitoring techniques 

described in Section 5 and other appropriate analytical methods to measure the flow rate, 

selenium concentration, nitrate/nitrate concentrations, and any other values necessary for design 

of the system. Plants should acquire this information over a long enough period of time that will 

include variability in plant operations such as shutdowns, fuel switches (preferably for all fuel 
Page 16 of 46

types burned at the plant), variability in electricity generating loads, periods with high ORP, etc. 

Armed with this information, the plant can work with a consulting firm to identify the “worst 

case” scenario and design a sufficient FGD wastewater treatment system that can operate under 

the worst case conditions and achieve the effluent limits. The system should be designed to 

incorporate the response strategies identified in this memo (which are included in EPA’s cost 

estimates for the BAT system).

Plants will need to design and operate the systems appropriately to be able to comply with the 

effluent limitations. EPA acknowledges that operating the systems will be require skilled 

operators and that much of the industry might not have much previous experience with an active 

treatment system such as the biological system. EPA acknowledges that operators must gain the 

technical experience of operating any new system, including a biological treatment system. EPA 

considered the cost of installation and operations in its approach to estimating industry-level 

costs for implementing the rule, as described in Incremental Costs and Pollutant Removals for 

the Final Effluent Limitations Guidelines and Standards for the Steam Electric Power 

Generating Point Source Category (DCN SE05831). EPA notes that plants will have the 

additional time before the effluent limits apply to them to install the treatment system and 

properly train operators. 

8 Conclusion 

While the data from Allen and Belews Creek Stations do show that the selenium concentration 

can sometimes become elevated, they do not demonstrate that the limits of the final rule cannot 

be met or that the biological system cannot accommodate changes in fuel type or load. 

Additionally there are reasonable actions that could have been taken to improve the performance 

of the treatment system. For the most part the biological treatment systems at both Allen and 

Belews Creek have performed very well, as demonstrated by the data. 

EPA notes that neither the Allen nor Belews Creek Stations are currently subject to an 

enforceable selenium effluent limit. Accordingly, Duke has not needed to optimize the 

performance of the biological treatment systems at the Allen and Belews Creek Stations and they 

have not provided any evidence that they adjusted operations to do so. Given the information that 

is available to operators at both plants, such as unit loads, fuel type, fuel sulfur content, ORP, 

pH, and nitrate/nitrate content of the FGD blowdown, Duke’s operators can adjust the operation 

of the biological treatment to account for fuel switches. Operators may find that some other 

parameters, such as the quality of gypsum produced and efficacy of SO2 removal in the scrubber, 

may also prove to be effective indicators of characteristics of the incoming wastewater. At the 

very least, enhanced communication between plant staff operating the boilers, the air pollution 

control equipment and the wastewater treatment plant would provide sufficient notice and 

coordination of operational changes to ensure that the wastewater treatment plant can be adjusted 

to handle changing conditions in advance.

9 References

Babcock and Wilcox. (2010). Redox Titration Method for Determination of Total Strong 

Oxidizer in WFGD absorber (BWRC-TC-1715-00).
Page 17 of 46

Bonnin-Nartker, P., Brown, S. R., DeVault, R. F., Jevec, J. M., Milobowski, M. G., Rogers, K. 

J., . . . Ulbricht, S. P. (2012). Corrosion in Alloy Wet FGD Absorber Reaction Tanks. 

Power Plant Air Pollutant Control "MEGA" Symposium (pp. 1-9). Baltmore, MD: 

Babcock & Wilcox Power Generation Group.

Brown, S. R., DeVault, R. F., & Johnson, D. B. (2013, July 1). ORP as a Predictor of WFGD 

Chemistry and Wastewater Treatment. POWER Magazine.

Buecker, B. (2006, September 1). Scrubbing: Optimizing Flue Gas Desulfurization Technologies 

Is Essential. POWER Magazine.

Hatt, R. (2015, February 1). Operational Considerations When Burning Higher-Chlorine Coal. 

POWER Magazine.

Miller, C. E., Feeley, T. J., Aljoe, W. W., Lani, B. W., Schroeder, K. T., Kairies, C., & 

McNemar, A. T. (2006). Mercury Capture and Fate Using Wet FGD at Coal-Fired 

Power Plants. U.S. Dept of Enery, National Energy Technology Laboratory.

Reisch, M. S. (2015, March 16). Bromine Comes To The Rescue For Mercury Power Plant 

Emissions. Chemical & Engineering News, 93(11), pp. 17-19.

Sonstegard, J., Harwood, J., Pickett, T., & Kennedy, W. (2010). ABMet: Setting the Standard for 

Selenium Removal, DCN SE02040.

Takeno, N. (2005). Atlas of Eh-pH diagrams Intercomparison of thermodynamic databases. 

Geological Survey of Japan Open File Report No.419. National Institute of Advanced 

Industrial Science and Technology.

YSI Environmental. (2005). Measuring ORP on YSI 6-Series Sondes: Tips, Cautions and 

Limitations.
Page 18 of 46

Attachment A: Evaluation of the Effect of Plant or Treatment System Shutdown Periods or 

Reduced Operation on Effluent Selenium

This attachment presents data showing how changes in electricity generation, including cycling 

operation and unit and plant-wide shutdowns, affect the effluent concentrations of selenium in 

FGD wastewater following treatment by chemical precipitation and anoxic/anaerobic biological 

treatment technologies. Data for three coal-fired steam electric power plants are presented: Duke 

Energy’s Allen and Belews Creek Stations and American Electric Power’s (AEP) Mountaineer 

Plant. The data shown reflect treatment system performance over a period of two years at one 

plant, and more than four years for each of the other plants. 

Both baseload and cycling power generation operations are included in the data and this dataset 

provides the ability to evaluate effluent selenium concentrations under steady power output, 

increasing and decreasing power output, shutdown and restart of one or more generating units at 

a plant, and complete plant-wide shutdown followed by restart of one or more generating units. 

In addition, the long periods of time covered by the data from these plants reflect operation 

during different seasons and therefore reflect the effects of both hot and cold weather on both 

power output and the wastewater treatment systems themselves. As such, this dataset provides a 

comprehensive look at effluent selenium concentrations under a wide variety of conditions

encountered at power plants. 
Page 19 of 46

Belews Creek

Belews Creek has two coal-fired generating units and is a baseload plant that generally operates

at a high output for long periods of time. The operating data for Belews Creek represents more 

than 4 years of operation, covering the period September 1, 2009 to December 31, 2013. There 

are some extended periods when electricity generation is substantially reduced due to one of the 

generating units being shut down; however, there are few periods when all units at the plant are 

shut down and these episodes are very short duration. During this approximately 4-year period, 

Belews Creek Unit 1 was shutdown 16 times; the shutdown duration ranged from 1 to 76 days. 

Belews Creek Unit 2 shutdown 29 times; the shutdown duration for this unit ranged from 1 to 43 

days. There were only 4 times when both generating units at Belews Creek were shutdown, with 

the plant-wide shutdown durations ranging from 1 to 8 days. The duration of the unit-level and 

plant-wide shutdown periods at Belews Creek are shown in Figures A1 – A3 below. Figure A4 

presents the effluent selenium concentrations along with electricity generation (MW).

Figure A1. Duration (days) of shutdowns of Belews Creek Unit 1 during the time period for 

which EPA has selenium effluent concentration data (September 1, 2009 to December 31, 2013).
Page 20 of 46

Figure A2. Duration (days) of shutdowns of Belews Creek Unit 2 during the time period for 

which EPA has selenium effluent concentration data (September 1, 2009 to December 31, 2013).

Figure A3. Duration (days) of plant-wide shutdowns of Belews Creek during the time period for 

which EPA has selenium effluent concentration data (September 1, 2009 to December 31, 2013).
Page 21 of 46

Figure A4. Total power generation and selenium effluent concentration at Belews Creek. For 

selenium concentration both detect and non-detect data are shown.
Page 22 of 46

Allen Station

Allen has five coal-fired generating units with total capacity of 1155 MW. The generating 

capacity for the units are as follows: Unit 1 – 165 MW; Unit 2 – 165 MW; Unit 3 – 275 MW; 

Unit 4 – 275 MW; and Unit 5 – 275 MW. Allen is a cycling/peaking plant with large swings in 

electricity generation. The operating data for Allen represents more than 4 years of operation, 

covering the period September 1, 2009 to December 31, 2013. One or more generating units 

were often shutdown during this period and there were frequent periods when all units at the 

plant are shutdown. In addition, there were 41 times when all five generating units at Allen were 

shutdown, with the plant-wide shutdown duration ranging from 1 to 44 days. The average plantwide shutdown period was 11.5 days (median 9 days). The duration of the plant-wide shutdown 

periods at Allen is shown in Figure A5 below. Figure A6 presents the effluent selenium 

concentrations along with electricity generation (MW). The change in electricity generation over 

time at Allen is also illustrated in Figure A7.

Figure A5. Duration (days) of plant-wide shutdowns of Allen during the time period for which 

EPA has selenium effluent concentration data (September 1, 2009 to December 31, 2013).
Page 23 of 46

Figure A6. Total power generation and selenium effluent concentration at Belews Creek. For 

selenium concentration both detect and non-detect data are shown.

Figure A7. Power generation at Allen including unit specific generation from 2009 to 2013.
Page 24 of 46
Page 25 of 46

Mountaineer Plant

Mountaineer has a single 1,300 MW coal-fired generating unit and is a baseload plant. Although 

the plant typically operates at a relatively high output, there are many periods when the output 

increases substantially. There are few times when the plant is entirely shutdown and these 

episodes are infrequent. The operating data for Mountaineer represents two years of operation, 

covering the period January 1, 2012 to December 31, 2013. During this 2-year period 

Mountaineer was shutdown 7 times; the shutdown duration ranged from 2 to 104 days. The 

duration of each shutdown period at Mountaineer is shown in Figure A8. Figure A9 presents the 

effluent selenium concentrations along with electricity generation (MW-hr).

Figure A8. Duration of plant-wide shutdowns at Mountaineer between January 1, 2012 and 

December 31, 2013.
Page 26 of 46

Figure A9. Power generation and effluent selenium concentration at Mountaineer between 

January 1, 2012 and December 31, 2013.
Page 27 of 46

Evaluation of the Belews Creek and Allen Plants’ Selenium and Generation data

The goal of this analysis is to investigate whether there is any statistical relationship between 

selenium levels and plant behavior related to cycling between units. The data for Belews Creek 

and Allen contain information on effluent selenium concentration and generation at the unit 

level. It is therefore possible to perform statistical tests of hypotheses about the relationship 

between plant behavior and selenium levels. Two central hypotheses are: (1) do units cycling on 

after shutdown have a statistically significant impact on the likelihood that the selenium 

concentration will be higher than the limits in the final ELGs; and (2) do large swings in 

generation impact the likelihood of a selenium concentration above the ELG limits? The 

following discussion applies probit regression analysis to these hypotheses.

Effluent Selenium Concentration Greater than the ELG limits

Probit regression analysis models the impact of several independent variables on a binary 

dependent variable. The dependent variable of interest is the existence of a selenium 

concentration value greater than the ELG limits. Additionally, we must investigate whether any 

conclusions can be extracted because, between the two plants, there are only 13 total selenium 

concentration daily observations higher than the ELG daily maximum limit. For this question, 

we use a probit model of the following format:

1 2 3 4 5 6 SeExceed f MW NonDetect spring summer fall Date         ( ( ) )       Difference e

The above equation models the probability of a selenium effluent concentration greater than the 

ELG limits occurs on several independent variables, where Φ represents the cumulative 

distribution function (CDF) of the standard normal distribution and e represents a mean zero, 

normally distributed error term. Several alternatives are used for f(MW) to investigate different 

hypotheses about what causes selenium greater the limits. First, we use an indicator variable 

equal to one if one of the plant’s units was shut down in the previous period, but is active in the 

current period. Since we are also interested in shifts in generation, we also look at the percent 

change in the generation between the current and previous period. To further examine rapid 

changes in generation, we use an indicator variable that indicates if the percent change in 

generation between periods is in the 90th percentile of all changes between periods. 

For all of these regressions, we control for the time between periods on the right hand 

side of the regression, to control for differences in the time between periods across observations. 

Approximately 58% of the samples occur within 3 days of another sample (31% within 1 day), 

and 80% are within two weeks. Some are spread even farther apart, with the longest lag at 60 

days. To account for unobserved differences over time, such as different patterns of electricity 

demand in the summer, as well as potential differences in dispatch order across seasons, we use 

seasonal indicator variables. Finally, we also use an indicator variable to control for non-detect 

observations, where the sampling instruments were not sensitive enough to detect any selenium 

concentrations. 
Page 28 of 46

Table A1 presents the results of the probit regressions that explore selenium exceedances. The 

first two rows of coefficients contain the estimated coefficients for the cycling-related variables. 

The columns correspond to either Belews Creek or Allen plants. As denoted by the lack of stars 

next to the coefficients, the results do not indicate any significant relationship between 

exceedances of selenium and either extreme changes in MW (Diff MW 90), a unit being 

shutdown in the previous period (Post Shutdown), or the percentage change in capacity 

generated.12 As stressed earlier, these data are relatively thin with respect to exceedances, so that 

not all variables could be used in the final regression. However, based on these data, we cannot 

conclude that shutdowns, or large changes in generation, are statistically related to selenium 

exceedances.

Table A1. Results of probit regression analysis comparing effluent selenium concentration 

values greater than ELG limits at Belews and Allen with shifts in generation including 

shutdowns and seasonal change.

Large Change in MW Post Shutdown Percent Capacity

Belews Allen Belews Allen Belews Allen

Diff MW 90 0.524 0.788

(0.494) (0.539)

Post Shutdown -0.0579 0.0253

(0.665) (0.634)

% Capacity -0.0122 0.0110

(0.0100) (0.0152)

spring -0.283 + -0.304 + -0.401

(0.604) (0.608) (0.633)

summer + + + + + +

fall -0.112 + -0.180 + -0.400 +

= (0.463) (0.452) (0.497)

Date1Diff 0.0315 0.0147 0.0423 0.0217 0.0400 0.0711*

(0.0289) (0.0247) (0.0272) (0.0222) (0.0268) (0.0387)

nonDetect -1.443** + -1.427** + -1.530**

(0.621) (0.613) (0.652)

+ Variable omitted due to multicollinearity concerns

Standard errors in parentheses, constant results excluded

*** p<0.01, ** p<0.05, * p<0.1

 

12 We also explored a variable that counts the number of units shutdown in the previous period for Allen, which has 5 units. That 

variable was not significant either. Additionally, we also explored lags of up to three periods for the capacity, shutdown, and 90th

percentile (Diff MW 90) variables, with similarly insignificant results. 
Page 29 of 46

Attachment B: Evaluation of the Effect of Changing Coal on Effluent Selenium 

1 Introduction

This attachment summarizes data for Duke Energy’s Allen and Belews Creek Stations, and for 

American Electric Power’s Mountaineer Plant. For Allen and Belews Creek, these data represent 

15 periods when the plants switched to a non-standard coal (as defined by the operating 

company) and then back again to the standard coal, providing data for a total of 30 times when 

these two plants changed the type of coal they burned. Importantly, any effect that the fuel 

switches at these plants may have had on the effluent concentrations is reflected in the dataset 

EPA used to calculate the BAT effluent limitations for selenium and nitrate-nitrite in discharges 

of FGD wastewater. For Mountaineer, EPA did not have information for specific dates when the 

coal type or source changed; therefore, the data for this plant was evaluated more broadly to 

identify whether there were any periods when effluent concentrations for selenium changed 

substantially. The data for this plant represented the use of coals from many sources and multiple 

coal basins. This attachment examines the data for these three plants and demonstrates that fuel 

switches in and of themselves do not lead to selenium and nitrate/nitrite discharges above the 

effluent limitations. 

The data for Allen and Belews Creek do not demonstrate that coal switches consistently lead to 

elevated concentrations of selenium and nitrate/nitrite, nor that changing the type or source of 

coal burned would lead to discharges at concentrations above the effluent limitations. On the 

contrary, the data show that in almost all cases, the plant would meet the selenium effluent 

limitations following fuel switches. As shown below, of the 14 coal switch events at Allen,

13

only one measured value during one of the fuel switches exceeded the level for the daily 

maximum limitation and the average of all 13 observations for that month was below the 

monthly average limitation. During the 16 times that Belews Creek switched coals,14 the data 

demonstrate that the plant would meet the final limitations in all cases. The one observation at 

Allen that exceeded the daily maximum limit was associated with the use of Illinois Basin 

bituminous coal. EPA closely reviewed the types of standard and non-standard coal burned by 

the plants and found that both the Allen and Belews Creek plants burned Illinois Basin coal (and 

other coals) multiple times without selenium concentrations exceeding the effluent limitations. 

Furthermore, EPA has reviewed the report for a pilot test of the biological treatment technology 

at a plant that burns Illinois Basin coal and found that the effluent concentrations for selenium 

and nitrate-nitrate were consistently below the effluent limitations. The data for Mountaineer 

support the findings for Allen and Belews Creek, showing that although the plant obtained coal 

from multiple sources and changed the type of coal it was burning, there is no indication that 

changing coals would affect treatment system performance and prevent a plant from meeting the 

effluent limitations. In fact, all effluent concentrations for the entire 2-year period at 

Mountaineer were substantially below both the daily maximum and monthly average effluent 

 

13 Allen switched to a non-standard coal 7 times, and after each of these events then switched back to the plant’s standard coal. 

That resulted in a total of 14 times that the plant changed from one type of coal to another. 

14 Belews Creek switched to a non-standard coal 8 times, and after each of these events then switched back to the plant’s standard 

coal. That resulted in a total of 16 times that the plant changed from one type of coal to another.
Page 30 of 46

limitations for FGD wastewater. For a detailed discussion of each plant, see sections 2 through 4

below.

2 Fuel Switches at Allen Station

The FGD treatment system at Allen has a 5-day hydraulic residence time, with the wastewater 

taking 4 days to get through the chemical precipitation stage and another day through the 

bioreactor. Thus, there should be no observed effect on effluent concentrations for at least 5 

days after making a coal switch. Allen switched to a non-Central Appalachian (non-CAPP) coal 

seven times during 2010 through 2013, using four Illinois Basin (ILB) coals. The periods of the 

fuel switch are illustrated below in Figures B1 and B2.

 

2.1 Coal Switch A-1 (10/1/2010-10/10/2010; ILB Charger (50%)) 

The effluent selenium concentrations were below both the daily and monthly effluent limitations 

during the coal switch and in the month following the end of the switch. Furthermore, there was 

no observed increase in effluent selenium concentrations during that period. The treatment 

system was able to achieve the effluent limitations even though the data show a substantial 

increase in selenate and total selenium in the bioreactor influent (which continued until 

10/14/2010) and the bioreactor influent nitrate concentrations appeared to be slightly elevated. 

2.2 Coal Switch A-2 (11/30/2010-12/8/2010; ILB Charger (75%))

There was no increase in effluent selenium concentrations during the coal switch and in the 

month following the end of the switch. All effluent selenium observations were below both the 

daily and monthly limits. No changes were observed for selenium, selenate, selenite, nitrates, or 

ORP for the FGD purge, bioreactor influent, or bioreactor effluent.

2.3 Coal Switch A-3 (3/1/2011-3/9/2011; ILB Charger (100%))

There was no increase in effluent selenium concentrations during the coal switch and in the 

month following the end of the switch. All observations were below both the daily and monthly 

limits. There was an increase in nitrates in the bioreactor influent but no subsequent increase in 

the nitrates in the effluent. No change was observed for the ORP of the bioreactor influent. 

Selenate and total selenium concentrations increased in the bioreactor influent and selenate 

concentration (but not selenite or total selenium) increased in the FGD purge; however, the 

selenium was removed by the treatment system as noted by the lack of increase in effluent 

selenium.

2.4 Coal Switch A-4 (4/29/2011-5/25/2011; ILB Gibson County (100%))

All values during this period were below both the daily and monthly limits. The sample collected 

approximately 10 days after the switch was ND at <5 ug/L. There was one observation 

approximately one month after the coal switch that was higher than others in that time period; 

however, its value is below both the daily and monthly limits. No other elevated effluent 

concentrations were observed during or after the coal switch. Increased selenate was present in 
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the FGD purge (but no significant change in selenite or total selenium), but no unusual values 

were observed for bioreactor ORP or influent/effluent nitrates.
Page 32 of 46

Figure B1. Effluent selenium concentration at Allen Steam Station. Colored vertical bars 

represent periods when the plant was not burning the standard coal (Central Appalachian coal 

(CAPP)). The legend shows the dates for those non-standard coal periods, along with the type of 

coal. At Allen, the non-standard coal is always Illinois Basin coal: ILB-G (Gibson County), ILBS (Somerville), ILB-Sh (Shay), or ILB-C (Charger).
Page 33 of 46

Figure B2. Close-up illustration of effluent selenium concentration at Allen Steam Station for 

periods with fuel switching. Colored vertical bars represent periods when the plant was not 

burning the standard coal (Central Appalachian coal (CAPP)). The non-standard coal is always 

Illinois Basin coal: ILB-G (Gibson County), ILB-S (Somerville), ILB-Sh (Shay), or ILB-C 

(Charger).
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2.5 Coal Switch A-5 (11/28-2011-12/31/2011; ILB Somerville (50%))

There was an increase in selenium effluent concentrations for the Nov/Dev 2011 fuel switch, as 

the values begin to rise 12/12/11 and the high values ending 12/27/11. Only 1 of the 13

observations in December was higher than the daily limit (23.8 on 12/27/11) and the average of 

all observations was below the monthly limit. Bioreactor influent nitrates and ORP begin rising 

in early December and by mid-December bioreactor influent nitrates are an order of magnitude 

higher than before the coal switch and ORP is approx. 500 mV (as compared to approx. 260-350 

mV in the weeks preceding the coal switch). Increased selenate and total selenium were observed 

in the FGD purge and bioreactor influent. There was a very large spike in effluent nitrate 

concentrations during this period (e.g., 10 mg/L on 12/15/11 and continuing through the month). 

Note there is a corresponding spike in effluent selenium, with 21.7 ug/L effluent selenium on 

12/16/11 (1 day after the 10 mg/L effluent nitrate value) and 23.8 ug/L on 12/27/11; increased 

selenate in the bioreactor effluent correspond to these increased effluent selenium concentrations. 

There is also a very high process monitoring result for effluent nitrate (not an NPDES sample) on 

12/21/11. These effluent nitrate and selenium values indicate that the plant did not adequately 

respond to the increased load of nitrate and selenate in the bioreactor influent. Section 6 of the 

memorandum describes response actions plants can take to address episodes such as this. Note 

that the plant does not an effluent limit for selenium for FGD discharges and therefore did not 

need to act to maintain the effluent concentrations below a specified limit.

2.6 Coal Switch A-6 (7/5/2012-9/8/2012; ILB Shay #1 (20-50%))

There were some increases in selenium effluent concentrations in August and September 2012, 

beginning approximately in the middle of a coal switch period, but all values for July through 

September were below the daily maximum limit and the average for observations for each of the 

months were below the monthly average limit (8 values in July, 13 values in August, and 5 

values in September). The bioreactor influent nitrate levels were higher during this period. Duke 

Energy added sodium bisulfite to the treatment system 8/4/12-9/21/12, indicating they were 

trying to mitigate the presence of free oxidants in the FGD purge.

As expected, the total electricity generation load (Figure B3, top panel) at Allen Station 

increased during the summer months, peaking around mid-July. A peak in nitrate as N loading to 

the bioreactor trailed by approximately 1 month, peaking around mid-August (Figure B3, middle 

panel). The biological treatment system was able to remove the increased nitrate loading 

throughout the time period shown in Figure B3 and the concentration of nitrate in the effluent 

remained near or below the detection limit. 

The ORP in both absorbers increased dramatically during this period around mid-July from 

around 150-250 mV to around 550 mV corresponding with the peak in electricity generation and 

remained high through August (Figure B3, bottom panel). Absorber 3 reached a peak ORP of 

almost 800 mV. The effect of the mid-July shift in ORP can be seen in the analytical data for the 

FGD purge stream. The top panel of Figure B4 shows both solid and dissolved manganese 

concentration in the purge, biological system influent and effluent. In the reduced form of 
Page 35 of 46

manganese (the +2 oxidation state), manganese exists as a dissolved ion, Mn2+. In the oxidized 

form (the +3 and +4 oxidation states) manganese exists as precipitated solids, mostly MnO2. 

Prior to the shift when the absorber ORP was lower, the manganese in the purge was almost 

entirely dissolved which corresponds to Mn2+. However, after the increase in ORP in mid-July, 

the manganese is almost entirely in the solid phase corresponding to MnO2. The shift in 

oxidation state of manganese from Mn2+ to MnO2 solids demonstrates that the shift in ORP in the 

absorbers does affect the oxidation state of the chemical constituents in the purge stream, as you 

would expect. However, both before and after the shift in ORP the treatment system is able to 

remove manganese to below 1mg/L. The increase in ORP actually improves removal of 

manganese because the solids are removed by the clarifier in the chemical precipitation system.

Similar effects were observed for selenium. Prior to the ORP shift in mid-July, the purge stream 

contained both dissolved and particulate selenium and the concentrations of selenite, Se(IV), and 

selenate, Se(VI), were roughly the same. The biological treatment system influent total and 

dissolved selenium concentration were roughly the same because particulate selenium was 

removed by chemical precipitation. Se(IV) and Se(VI) concentrations in the bottom panel of 

Figure B4 in the biological treatment influent are approximately the same as in the purge. The 

samples used to analyze for Se (IV) and Se(VI) are filtered and thus represent dissolved 

concentrations. After the shift in ORP, the selenium changes to mostly dissolved and almost no 

removal occurs in the chemical precipitation system. The selenium in the purge and biological 

treatment influent changes oxidation state to mostly Se(VI) and little Se(IV), consistent with high 

ORP in the absorbers. The Se(VI) concentration in the biological system influent increased from 

around 100 ug/L to around 1000 ug/L after the ORP shift. 

Importantly, throughout this entire time period the biological treatment system performed well. 

The system was able to remove nitrate and selenium both before and after the mid-July shift in 

ORP. The effluent from the biological treatment system effluent selenium effluent concentration 

never exceeded 15 ug/L. The system performed well both before and after the shift in ORP even 

though the Se(VI) concentration surged from around 100 ug/L to around 1000 ug/L. 

High concentrations were observed on 11/26/12 (17.2 ppb) and 12/16/12 (24.7 ppb), months 

after the last coal switch ended on 9/8/12. The plant was burning the standard CAPP coal during 

this period. 

2.6.1 Coal Switch A-7 (1/21/2013-2/18/2013; ILB Shay #1 (26-100%)) 

Although the increased selenium concentrations during this period may ultimately have been 

affected by the fuel switch, the fuel switch itself was not the original cause for a rise in effluent 

concentrations. The coal switch to ILB-Shay began 1/21/13. It should take some time for the 

switch to affect characteristics in the scrubber and begin changing characteristics of the 

wastewater purge to treatment. However, even if the purge characteristics were affected 

immediately, because of the treatment system HRT it should take at least 5 days before changes 

would be observed in the effluent concentrations. However, the observations actually begin 

rising before the coal switch, with 11.9 ug/L on 1/18/13. The concentration continued to rise to 
Page 36 of 46

27.6 ug/L on 1/23/13 (2 days after the coal switch, a full 3 days before any effect should be 

observed at the treatment 

Figure B3. Top: Total electricity generation load (MW), middle: Nitrates as N concentration 

in influent (Bio Inf) and effluent (Bio Eff) of the biological treatment system, and bottom: 

ORP in absorber reaction tanks 1 and 3 at Allen Station from 2/29/2012 to 9/7/2012.

0

200

400

600

800

1000

1200

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

Total Load (MW)

Loads Sum

(MW)

0

10

20

30

40

50

60

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

Nitrates as N (ppm)

Bio Inf

Bio Eff

0

100

200

300

400

500

600

700

800

900

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

ORP (mV)

Absorber 1

Absorber 3
Page 37 of 46

Figure B4. FGD Purge (Purge), biological treatment system influent (Bio Inf) and effluent 

(Bio Eff) at Allen Station from 2/29/2012 to 9/27/2012 (continuation of Figure B3). Top: 

manganese concentration, total (Mn, tot) and dissolved (Mn, diss). Middle: selenium 

0.01

0.1

1

10

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

Manganese Concentration (mg/L)

Purge: Mn, diss

Purge: Mn, tot

Bio Inf: Mn, tot

Bio Inf: Mn, diss

Bio Eff: Mn, tot

Bio Eff: Mn, diss

1

10

100

1000

10000

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

Selenium Concentration (ug/L)

Purge: Se, tot

Purge: Se, diss

Bio Inf: Se, tot

Bio Inf: Se, diss

Bio Eff: Se, tot

Bio Eff: Se, diss

0.1

1

10

100

1000

10000

2/29/123/20/124/9/124/29/125/19/126/8/126/28/127/18/128/7/128/27/12

Selenium Concentration (ug/L)

Purge: Se(IV)

Purge: Se(VI)

Bio Inf: Se(IV)

Bio Inf: Se(VI)

Bio Eff: Se(VI)

Bio Eff: Se(IV)
Page 38 of 46

concentration, total (Se, tot) and dissolved (Se, diss). Bottom: selenite, Se(IV)) and selenate 

(Se(VI)) concentration.

system effluent). That was followed by 23.2 ug/L on 1/25/13, which is still 1 day before any 

effects should be seen at the treatment system effluent. As such, the elevated effluent 

concentrations during this period could not have been initiated by the change in coal. Bioreactor 

influent ORP for late December 2012 and throughout January 2013 is markedly higher than 

before that period. ORP levels returned to normal levels in early March. No bioreactor influent 

nitrate data are available for mid-December through mid-January; however, the influent nitrates 

are below 10 mg/L on 1/17/13-1/24/13, then rise to near 30 mg/L by 1/27/13, and remain 

elevated into early March (including an observation of 50.6 mg/L on 2/14/13). Duke Energy 

began intermittent dosing of sodium bisulfite to mitigate free oxidants on 1/12/13 (9 days before 

the coal switch, and at least 14 days before the bioreactor effluent concentrations should have 

begun seeing any effect from the coal switch), which indicates that the treatment system 

operators were reacting to the presence of free oxidants. Daily addition of sodium bisulfite began 

1/26/13 and continued until 2/19/13 (dosed intermittently thereafter until 3/2/13). The daily 

addition sodium bisulfite did not begin until effluent concentrations were substantially elevated, 

indicating that the plant did not quickly initiate response actions as the characteristics of the 

untreated wastewater changed, resulting in some loss of control of treatment system 

performance. 

2.7 Increased Selenium Concentration Not Associated with a Coal Switch / Sodium Bisulfite 

Addition Not Noted in Sections 2.1-2.7

Although all values are below both the daily maximum and monthly average limitations, effluent 

concentrations increased in March/April 2012 (e.g., 3/7/12-4/4/12). This was not associated with 

a fuel switch. There was a dramatic change in bioreactor influent ORP beginning 3/6/12, with 

ORP dropping into negative values. In the preceding weeks, bioreactor influent ORP was over 

300 mV or 400 mV; however, it dropped to 87 mV on 3/6/12 and it was negative by 3/8/12 

(down to -279 mV on 3/9/12). ORP remained negative until 3/19/12 and rose quickly to over 400 

mV on 3/24/12, and then back below 200 mV by 3/30/12.

In addition to the time periods noted above in the discussions for specific fuel switches at Allen, 

Duke Energy added sodium bisulfite to the treatment system (presumably to mitigate free 

oxidants in the FGD purge) in the following periods: 3/19/12-5/15/12 (most days), 3/18/13-

3/28/13 (all but 1 day); 5/1/13-6/8/13 (most days); and 7/5/13-7/18/13 (approx. half of the days). 

The chemical was also added on 10/24/12, 10/28/12, 12/11/12, and 12/19/12.

3 Fuel Switches at Belews Creek Station

The FGD treatment system at Belews Creek has a 3.5-day hydraulic residence time, with the 

wastewater taking 2.5 days to get through the chem precip stage and another day through the 

bioreactor. Thus, there should be no observed effect on effluent concentrations for at least 3.5 

days after making a coal switch. Belews Creek switched to a non-Central Appalachian (non-
Page 39 of 46

CAPP) coal eight times during 2012 and 2013, using three Northern Appalachian (NAPP) coals 

and three Illinois Basin (ILB) coals. 

3.1 Coal Switch BC-1 (2/12/2012-2/17/2012; ILB Wild Boar) 

There was no effect on effluent selenium concentrations during or after this coal switch. All 

observations during this period and the following month were non-detect at <5 ug/L so clearly 

the facility could achieve the final limitations. There were no noticeable effects on bioreactor 

influent ORP and influent and effluent nitrates. FGD purge selenite concentrations appeared to 

increase during this time period (no change in selenate), but due to the variability of the data in 

the period preceding and following the fuel switch, it is not possible to conclusively determine 

whether this is due to the fuel switch. 

3.2 Coal Switch BC-2 (5/18/2012-5/25/2012; NAPP Enlow Fork)

The effluent concentrations for this time period and the following month were below the effluent 

limits. Effluent concentrations were observed to increase 5/28/12-6/1/12, following the period of 

the coal switch. However, the highest observed value was below the daily limit and the average 

of all 8 values for the month was below the monthly limit. Effluent selenate, selenite, and nitrates 

were not elevated during this time period. Also, there were no unusual observations for ORP, 

total selenium, selenate, selenite, or nitrate in the FGD purge or bioreactor influent.

3.3 Coal Switch BC-3 (6/5/2012-6/23/2012; NAPP Enlow Fork)

All data were below the ELG effluent limits. There were no elevated effluent selenium 

concentrations during the period of the coal switch, and all values in following month are below 

9 ug/L. Effluent selenate, selenite, and nitrates are not elevated during this time period. No 

unusual observations for ORP, total selenium, selenate, selenite, or nitrate in the FGD purge or 

bioreactor influent.

3.4 Coal Switch BC-4 (11/5/2012-12/21/2012; NAPP Federal #2 & Tunnel Ridge)

There was no significant increase in effluent selenium during November following the coal 

switch and all values for the month were below both the daily and monthly limits (highest 

observation in November was 9.27 ug/L; the average of all 11 values for the month was 6.2 

ug/L). The effluent selenium concentrations were slightly higher in December but also below the 

effluent limits (all observations were below the daily limit and the average of all 12 values for 

the month was 8.8 ug/L). Effluent selenium concentrations were lower in January 2013 with a 

monthly average of 6.1 ug/L, comparable to the concentrations observed in October 2012 (prior 

to the coal switch) and November 2012. Effluent selenate and selenite were low and no unusual 

nitrate values were observed. There was a shift to selenate in the FGD purge at end of December 

which is also reflected in the bioreactor influent. ORP values appear to be within typical 

variation.
Page 40 of 46

3.5 Coal Switch BC-5 (1/29/2013-2/4/2013; NAPP Tunnel Ridge)

All effluent values during this period were below the effluent limits and there was no observed

increase in effluent selenium concentrations during the period of the coal switch or in the months 

that followed.

Figure B5. Effluent selenium concentration at Belews Creek Steam Station. Colored vertical 

bars represent periods when the plant was not burning the standard coal (Central Appalachian 

coal (CAPP)). The legend shows the dates for those non-standard coal periods, along with the 

type of coal. At Belews Creek, the non-standard coal is NAPP (Northern Appalachian coal).
Page 41 of 46
Page 42 of 46

Figure B6. Close-up illustration of effluent selenium concentration at Belews Creek Steam 

Station for periods with fuel switching. Colored vertical bars represent periods when the plant 

was not burning the standard coal (Central Appalachian coal (CAPP)). The non-standard coal is 

either Northern Appalachian or Illinois Basin coal.
Page 43 of 46

3.6 Coal Switch BC-6 (5/6/2013-6/9/2013; NAPP Enlow Fork)

All effluent values during this period were below the effluent limits. There were no changes in 

effluent selenium concentrations during the period of the coal switch or in the months that 

followed (other than a single ND of <10 ug/L on 6/12/2013, the observations during and 

following the coal switch are either detected values near 5 ug/L or ND at <5 ug/L). Effluent 

selenate, selenite, and nitrates are not elevated during this time period. No unusual observations 

for ORP, total selenium, selenate, selenite, or nitrate in the FGD purge or bioreactor influent.

3.7 Coal Switch BC-7 (8/26/2013-9/22/2013; ILB Deer Run)

All effluent values during this period were below the effluent limits. There was no significant 

change in effluent selenium during the coal switch or in the following month. Selenite in the 

FGD purge increased, but no there was no significant change in selenate concentrations. The 

selenite was substantially removed by the chemical precipitation stage; as a result, there was no 

noticeable change in selenite for the bioreactor influent. No unusual ORP or nitrate values were 

observed.

3.8 Coal Switch BC-8 (11/6/2013-11/22/2013; ILB Warrior)

All effluent values during this period were below the effluent limits. No change in effluent 

selenium was observed during the coal switch. Selenite in the FGD purge increased 11/8-11/13 

(no significant change in total selenium and selenate in the FGD purge), and then returned to 

normal by 11/18. This was accompanied by an increase in total selenium and selenite in the 

bioreactor 11/11-11/18, but quickly returned to normal values by 11/20. No unusual ORP or 

nitrate values were observed.

3.9 Increases in Selenium Concentration Not Associated with a Coal Switch

Although there were some instances when increased effluent values are observed at or near times 

when the plant changed to a different coal, these increases were of lesser magnitude than 

increases observed at times when CAPP coal was burned. There were also instances when there 

was no increase in effluent concentrations even though the plant changed to a non-CAPP coal. 

In addition, there were times when effluent concentrations rose, but there was no change in coal. 

Examples of elevated effluent selenium concentrations that are not associated with a change in 

coal:

 Elevated concentrations in Aug-Oct 2008 were not associated with a change in coal. 

Note that these occurred soon after the end of the initial commissioning period for the 

treatment system and the operation may still have been affected by the plant’s limited 

experience with the treatment system.

 High value on 6/11/10 (25 ug/L) was not associated with a coal switch. Other than the 

elevated total selenium effluent concentration, no unusual influent or effluent 

concentrations observed for total selenium, selenate or selenite. Approximately 1 week 
Page 44 of 46

prior to this observation the bioreactor influent ORP shifted from mid-100s to 

approximately 300 mV and remained at that level for an extended period.

 High value (29.35 ppb) on 12/8/10 was not associated with coal switch. Effluent selenate 

was high, indicating the treatment system did not reduce the selenate species. No unusual 

ORP or influent or effluent nitrate values were observed. Influent concentrations of total 

selenium and selenate were elevated on that day, but the treatment system experienced 

higher levels of these pollutants at other times without affecting the effluent selenium 

concentration.

 High values in Oct 2011 (including 18.4 on 10/11/11) were not associated with coal 

switch. None of these values were higher than the daily limit and the average of all 10 

observations for the month are below the monthly limit. Effluent concentrations of 

nitrate, selenate and selenite were not elevated during this period. Bioreactor influent 

nitrates increased beginning Oct 5 and bioreactor influent ORP increased beginning Oct 

8 (continuing to be elevated until Oct 21).

4 Fuel Switches at Mountaineer Plant

For Mountaineer, EPA did not have information for specific dates when the coal type or source 

changed; therefore, the data for this plant was evaluated more broadly to identify whether there 

were any periods when effluent concentrations for selenium changed substantially. The data for 

this plant represented the use of coals from many sources and multiple coal basins. The coal data 

for Mountaineer are not presented here because it has been claimed confidential business 

information (CBI). The coal data is included as part of data submitted by AEP in March 2014 

and is included in the record for the final ELGs. 

The data for Mountaineer support the findings for Allen and Belews Creek, showing that 

although the plant obtained coal from multiple sources and changed the type of coal it was 

burning, there is no indication that changing coals would affect treatment system performance 

and prevent a plant from meeting the effluent limitations. In fact, effluent concentrations for all 

valid observations for the entire 2-year period at Mountaineer were substantially below both the 

daily maximum and monthly average effluent limitations for FGD wastewater.15 The effluent 

selenium data for Mountaineer are shown in Figure B7.

 

15 EPA excluded one observation for effluent selenium (out of a total of 64 observations for the 2-year period) because the data 

indicated either a sampling error or that the treatment system was in an upset condition. In response to EPA’s inquiry to AEP 

questioning the validity of the data, AEP reported that “after a limited engineering investigation, it was determined that the 

analytical results associated with the AEP Mountaineer Plant bioreactor effluent sample collected on 2/22/2012 at 0745 (Dolan 

Lab Report No. 120833) are invalid and not representative of the bioreactor effluent data collected before or after this event. A 

number of possible explanations exist, including incorrect analytical reporting, poor sampling technique, collection at the wrong 

sampling location, or breakthrough of pump seal flush water. In our review of the compiled analytical data for the system, the 

results for this sampling event are more representative of the CPS effluent sample location than the bioreactor effluent sampling 

location. Thus, based on this observation, it was likely that a sampling error occurred as the two sample locations are only a few 

feet apart.” 

AEP’s evaluation is consistent with EPA’s review. The bioreactor effluent selenium concentration for 2/22/2012 was reported at 

320 ug/L, which is 2-3 orders of magnitude higher than all other effluent selenium concentrations (the next highest reported value 

was 5.1 ug/L, and the concentrations for the preceding and following weeks were 1.6 ug/L and 2.0 ug/L, respectively). In 

addition, the effluent nitrate concentration for that day was reported at 16.9 mg/L, which is 1-2 orders of magnitude higher than 
Page 45 of 46

Figure B7. Effluent selenium concentration at Mountaineer Plant. Blue triangles represent 

detect data and red triangles represent non-detect data.

 

all other reported values for effluent nitrate or nitrate-nitrite (the next highest value was 2.21 mg/L). These effluent 

concentrations are comparable to the concentrations present in the bioreactor influent on that day (404 ug/L selenium; 14.2 mg/L 

nitrate). The similarity of the bioreactor effluent concentrations to influent concentrations for the bioreactor (both on that day, as 

well as generally observed for the entire dataset) suggests that the sample was misidentified during sample collection. 

Alternatively, comparison of the effluent and influent data demonstrates that the treatment system was in an upset condition, as 

indicated by the small reduction in selenium concentration (21%) and increase in nitrate concentration (19%) across the 

bioreactor. Regardless of the cause for the 2/22/2012 observations, they clearly do not represent BAT performance of the 

biological treatment technology (also see the biological treatment performance data for Allen and Belews Creek, as shown in this 

memorandum and Statistical Support Document: Effluent Limitations for FGD Wastewater, Gasification Wastewater, and 

Combustion Residual Leachate for the Final Steam Electric Power Generating Effluent Limitations Guidelines and Standards).
Page 46 of 46

5 Conclusion

In general, there are some cases when a change in coal is followed by increased effluent 

concentrations. However, the magnitude of the increases is not consistent. There are also 

instances of increased effluent concentrations at Allen and Belews Creek that are not associated 

with a change in coal. The hydraulic residence time for the treatment system provides ample time 

for operators to take steps to prepare for changes in characteristics of the influent wastewater 

(e.g., potential for increased nitrate or selenium concentrations, ORP changes, presence of free 

oxidants). Even assuming changes in FGD purge characteristics will be immediate (and they are 

not because it will take some time before absorber slurry characteristics change enough to be 

reflected in the FGD purge), it will take a minimum of 2.5 days before the effects will begin to 

reach the Belews Creek bioreactor and 4 days to reach the Allen bioreactor. In addition, effective 

communication between the coal pile operators, boiler operators, air pollution control system 

operators and wastewater treatment system operators would provide even more advance notice of 

any significant process/operational changes and allow operators to adjust the treatment system 

operations accordingly.