Page 1 of 46 

## 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 e ffluent limitations and g uidelines (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 a bility 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 cont acts 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 constitue nts 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

re cycled back to the spray/tray levels). The scrubber blow down 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 Technic al 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 biologi cal

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.Page 2 of 46

Generating Point Source Category ( Technical Development Document ) for additional

information.

The BAT FGD wastewater treatment system described in the pre amble 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 precipi tation 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 fo rms of nitrogen . In addition, the system is preceded by a n 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 w ith a stream of sorbent to remove SO 2 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 (CaCO 3 ) in water. The

lime/limestone serve s as a source of calcium and as a source of alkalinity . Scrubbers are typically operated under acid ic conditions around pH 5 . When SO 2 dissolves into the sorbent water it forms sulfurous acid (H 2 SO 3 ):

SO 2 (g) + H 2 O ⇌ H 2 SO 3 ( 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. T he 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 SO 2

.

Ca(OH) 2 ⇌ Ca 2+ + 2 OH

- ( 2 )

CaCO 3 (s) + H 2 O ⇌ Ca 2+ + HCO 3 - + OH

- ( 3 )

CaCO 3 (s) + H + ⇌ Ca 2+ + H 2 O + CO 2 (g) ( 4 )

The so

- called “scrubbing” reaction expresses the combination of dissolution of SO 2 and lime ( 5 )

or limestone ( 6 ) in acidic conditions resultin g in su l f ite (SO 3 2

- ) and calcium ions (Ca 2+ )

.

Ca(OH) 2 (s) + SO 2 (g) + 2H + ⇌ Ca 2+ + SO 3 2

- + H 2 O ( 5 )

CaCO 3 (s) + SO 2 (g) + 2H + ⇌ Ca 2+ + SO 3 2

- + H 2 O + CO 2 (g) ( 6 )

Scrubbers can operated with a forced, inhibi ted, or natural oxidation system . Forced oxidation scrubbers use powerful blowers to force air into the scrubber absorber tank slurry from thePage 3 of 46

bottom of the scrubber tower. This introduces oxygen (O 2 ) into the slurry where it can oxidize

sulfite into sulfate:

2SO 3 2

- + O 2 ⇌ 2SO 4 2

- ( 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:

Ca 2+ + SO 4 2

- + 2H 2 O ⇌ CaSO 4 ∙ 2H 2 O(s) ( 8 )

However , a mixed sulfite

- sulfate solid ( 9 ) and calcium sulfite ( 10 ) can also be produced.

2Ca 2+ + SO 3 2

- + SO 4 2

- + 1⁄2 H 2 O ⇌ (CaSO 3 ∙ CaSO 4 )

∙ 1⁄2 H 2 O(s) ( 9 )

Ca 2+ + SO 3 2

- + 1⁄2H 2 O ⇌ CaSO 3 ∙ 1⁄2H 2 O(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 a re sufficiently oxidizing chlorine gas (C l 2 ) 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, Cl

can be oxidized

to dissolved Cl 2 , which forms hypochlorous acid (HOCl) and can lower the pH:

Cl 2 + H 2 O ⇌ H OCl + H + ( 11 )

Similar reac tions would be expected for bromine . Bromine is naturally present in many coal s, 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 f orms such as mercuric chloride (HgCl 2 ) . Particulate mercury can be removed b y the ESPs. Oxidized mercury is a soluble ion (Hg 2+ ) in water and can dissolve in the absorber slurry. Elemental mercury is poorly captured by

these scrubbers and its v olatility can lead to mercury emission s 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 SO 2 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 t h e 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 b e mitigated. A s previously mentioned, the coal itself is the primary source of sulfur, chlorine, mercury, selenium, and arsenic in thePage 4 of 46

absorber slurry. Plant s taff 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 t he wastewater treatment system operators c an use this

infor mation 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 treatme nt. A brief description is

included here . F or further details see Section 7 of th e 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 . F erric ( 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 settl ing 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 prec ipitation can be designed to add all three chemicals to the same reaction tank or to
add each chemical in separate tank s

. By reducing the concentration of dissolved metals, adjusting the pH or alkalinity, and reducing the total suspended and dissolved soli ds 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 add ition 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 certa in 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 ca rbon (GAC) or other permane nt porous substrate that is inocul ated with specific naturally occurring microorganisms

. Power plants also add a nutrient feed such as a molasses

based feed to the wastewater before it en ters the bioreactor. The nutrient feed provides a source of carb on and other minerals to the microorganisms. Microorganisms require both an electron acceptor and a carbon source in order to grow. M icroorganisms 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 s odium 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.Page 5 of 46

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 microorgan isms 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 OR P in the bioreactor

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

be controlled by the amount of nutrient that i s fed to the system. Additionally, t he 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 micro organisms and potentially allow non desirable

micr oorganisms to propagate. Selenate and selenite is reduced to elemental selenium by the microorganism and forms so lid 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 me tal 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 p articles fixed within the activated carbon . T he 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 “bur p” out of the system. This type of flush is long enough to lift the biomatrix and release entrained gases, but short enough to avoid flushin g any water out of th e bioreactor.Page 6 of 46

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 bac kflush. 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, t oo 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 efflu ent . 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 Statio n and Belews Creek Steam Station.

5 Monitoring Capabilities

5.1 Coal C haracteristics

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 pr obably 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 de tailed 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 anticip ate 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 SO 2 Content

The loads and sulfur content information could be used to determine if the scrubber should expect changes in the SO 2 in the flue gas. Plants may also be using flue gas analyzers that can

detect SO 2 in the flue gas prior to the scrubber. The r elative amounts of SO 2 and O 2 in the

scrubber are important because O 2 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 SO 2 (Buecker, 2006) . If there is insufficien t O 2 some of the solids produced with be the mixed

sulfite

- sulfate solid ( 9 ) and calcium sulfite ( 10 ). The relative amounts of SO 2 and O 2 in the scrubber should also affect the ORP in the scrubber slurry because O 2 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 b lowers are designed to provide enough O 2 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).Page 7 of 46

sulfur coal being is burned, it follows that there will be too much SO 2 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 (O 2 ) 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 SO 2 than desi gned for the O 2 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 inc rease in ORP.

5.3 FGD Solids C omposition

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 in dicate s 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 a bsorber 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 bio reactors. An ORP sensor included downstream of the bioreac tors 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).Page 8 of 46

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 ce rium (IV) sulfate to determine the oxidant loading. (Babcock and Wilcox, 2010) Iodometric titration could also be an effective method. These are

tests tha t 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 bre aking through the bioreactor

, which would be one in dication that the backflushing operation needs to be modified (e.g., more frequent, longer duration)

. Plants could also use turbidi ty meters as an alternative method to monitor suspended solids . Conductivity meters could be used to measure total dissolve d 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 d rop can be diagnostic of the condition s 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

th e influent to the biological treatment system and the outdoor ambient temperature

. GE has

evaluated thei r 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 t o 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 C hlorides

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 est ablishing lower chlorides operational limits to better protect the scrubber metallurgy. GE has determined that the microorganisms in the bioreactor can functionPage 9 of 46

without any degraded performance in FGD wastewater with chlorides as high as 35,000 ppm (DCN S E04208) . 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

con centrations ma y dehydrate the microorganisms in the system. From a biological perspecti ve, 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. Ou tside 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 typi cally 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 . How ever, plants that monitor ORP in the absorber or in the FGD purge will have sufficiently advance d warning to respond to high ORP levels. Plants with anoxic/anaerobic biological treatment systems already monitor the ORP within the

bioreactor and use that da ta 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 possib ly 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

i s 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 wi ll react with oxidants present in the wastewater and lower the ORP into an acceptable level for biological treatment. Alt er natively, plants could add ferrous chloride as instead of ferric chloride in the chemical precipitation system. Ferrous

chloride wi ll 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.Page 10 of 46

Additionally, it is possible to respond to elevated ORP levels by increasing the feed rate of the

nutrient mix. As mentioned above, t he 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 managi ng 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 ma y 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 t hat 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. T his 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.Page 11 of 46

data in the record indicates that most power plants have influent nitrate concentrations substantially lower than 100 mg/L. For those plants with influen t nitrate concentrations higher than 100 mg/L, EPA included costs for a denitr ification 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 d osage 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

capac ity 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 c ontrolled by the bioreactor backflush cy cle. 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 contro l 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 chann eling 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 ha s 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 precipi tation 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 gas es 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

, n itrate and n itrite should be reduced to nitrogen gas (N 2 ) 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 m inimized 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 a lso decide to install additional treatment to remove excess ammonia

, which would not substantially increase the overall cost for the treatment sy s tem

. Note, however

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

6.6 P resence of Organics (BOD) in the Biological System Effluent

E levated 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 co ntained 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 chemic al 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

Significa nt 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, orga nosulfide, 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

pretre atment 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 co ncentrations and ORP in the bioreactor). To address an upward spike in

8 Plants experiencing this condition where t hey 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 remo ving selenium from the wastewater, but that the treatment s ystem 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 oper ated 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 adeq uately 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

wastew ater 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 exi ting 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 Shut down 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 season al industrial operations and can experience shutdowns for as long as 6 months. A relatively smal l amount of feed water ( either plant service water or FGD wastewater stored prior to the

shutdown 10 , along with some nutrient added) added weekly to the biologi cal reactor will keep the microorganisms alive, albeit at a lower rate of activity

. A constant re cycle thr ough 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 relative ly short time once wastewater is again available (D CN 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 duri ng 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 describ ed 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 purp ose. In addition to filling the treatment system equalization tank to its maximum operating level, plants typically have large maintenance tanks (normally empty) that are use d to hold the liquid drained from absorber vessels when the FGD system undergoes m aintenance 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 beg inning 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 concentration s 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 bioreacto r 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 v aries 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 wa rranted. 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 t o 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 speci fically 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 reprocess ed

.

6.10 Notificatio n 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. switch ing 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 (includ ing 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, nitr ates) or chemical precipitation stage indicate that it is necessary, treatment system operators can increase thePage 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 1 5 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 th ere 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 b iological treatment systems, EPA does not contend that every plant in the industry can simply take the design parameters from those two plants, install the biologi cal treatment system, and meet the effluent limitations. Each plant will need to work with engineering and design firms to assess the

wastewat er 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 b iological 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 limi tations 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 investigatin g 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 fl ow 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 sh utdowns, fuel switches ( preferably for all fuelPage 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” scena rio 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).

Plant s 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. E PA 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

c an 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 loa d

. Additionally there are reasonable actions th at 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 SO 2 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 t reatment 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 Titra tion 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 Treatmen t. 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, Mar ch 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 Son des: 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 a ttachment present s 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 t echnologies. Data for three coal

- fired steam electric power plants are presented: Duke Energy’s Allen and Belews Creek Stations and A merican Electric Power’s (A EP ) 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 dat a from these plants reflect operation during different seasons and th erefore 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 efflu ent 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. T here are some extended periods when electricity generation is substantially reduced due to one of the generating un its 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 A 1

– A 3 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 A 2

. 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 A 3

. Duration (days) of plant

- wide shutdowns of Belews Creek during the time period for which EPA has selenium effluent concentration data (S eptember 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 un its 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 c ycling /peaking plant with large swings in

electricit y 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 u nits 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 plant

wide 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 illustrate d in Figure A7.

Figure A 5

. 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 46Page 25 of 46

Mountaineer P lant

Mountaineer has a single 1,300 MW coal

- fired generating unit and is a baseload plant . Although the plant typically operates a t 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 shutdow n 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

Eval uation of the Belews Cr eek and Allen P lant s’ 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 cent ral 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 likeliho od of a selenium concentration above the ELG limits? The following discussion applies probit regression analysis to these hypotheses.

Effluent Selenium Conce ntration Greater than the ELG limits Probit regression analysis models the impact of several indep endent 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:

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 d istributed error term. Several alternatives are used for f(MW) to investigate differen t 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 per iod, but is active in the

current per iod. 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 indicato r variable that indicates if the percent change in generation between periods is in the 90 th 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 cont rol 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 day s. 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 va riable to control for non

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

concentrations.Page 28 of 46

Table A 1 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 plant s. 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 (P ost 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 shut downs, or large changes in generation, are statistically related to selenium

exceedances.

Table A 1

. Results of probit regression analysis comparing effluent s elenium concentration values greater than ELG limits at Belews and Allen with shifts in gener ation 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)

P ost Shut down

- 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. Tha t variable was not significant either. Additiona lly, we also explored lags of up to three periods for the capacity, shutdown, and 90 th 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

1 5 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 se 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 e ffluent concentrations for selenium changed substantially. The data for this plant represented the use of coals from many sources and multiple

coal basins. T his attachment examines the data for these three plants and demonstrate s that fuel

switc hes 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 exceed ed the level for the daily maximum limitation and the average of all 13 observations for that month was below the monthly avera ge 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 Illi nois 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 concentrati ons 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 w ere 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 indicat ion 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 an d monthly average effluent

13 Allen switched to a non

- standard coal 7 times, and after each of these events then switched b ack 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 standa rd 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 Switch es 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 concentr ations 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 B 1 and B 2.

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 inf luent ( which continued until

10/14 /2010) and the bioreactor influent nitrate concentrations appear ed 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, selen ate, selenite, nitrates, or ORP for the FGD purge, bioreactor influent, or bioreactor effluent.

2.3 Coal Sw itch 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 foll owing 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 b ioreactor influent.

Selenate and total selenium concentrations increased in the bioreactor influent and s elenate concentration (but not selenite or total selenium) increased in the FGD purge ; however, the selenium was removed by the treatment system as not ed by the lack of increase in effluent

selenium

.

2.4 Coal Switch A

- 4 (4/29/2011

- 5/25/2011; ILB Gibson County (100%))

A ll 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 d uring or after the coal switch. Increased selenate was present inPage 31 of 46

the FGD purge ( but no significant change in selenite or total selenium) , but n o unusual values

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

Figure B 1

. 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

- standa rd coal periods, along with the type of coal. At Allen, the non

- standard coal is always Illinois Basin coal: ILB

- G (Gibson County), ILB

S (Somerville), ILB

- Sh (Shay), or ILB

- C (Charger).Page 33 of 46

Figure B 2

. Close

- up illustration of effluent selenium c oncentration 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 (Gib son County), ILB

- S (Somerville), ILB

- Sh (Shay), or ILB

- C (Charger).Page 34 of 46

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

observation s 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

i n 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 tota l 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 effl uent 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 a lso 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 con centrations 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 influen t 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 generati on load ( Figure B 3 , top panel) at Allen Station increased during the summer months, peaking around mid

- July. A peak in nitrate as N loading to

the bio reactor trailed by approximately 1 month, peaking around mid

- August ( Figure B 3 , middle panel). The biological treatment system was able to remove the increased nitrate loading throughout the time period shown in Figure B 3 and the concentration of nitrate in the effluent

remained near or below the d etection 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 B 3 , 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 B 4 shows both solid and dissolved manganese concentration in the purge, biological system influent and effluent. In the reduced form ofPage 35 of 46

manganese (th e +2 oxidation state), manganese exists as a dissolved ion, Mn 2+ . In the oxidized form (the +3 and +4 oxidation states) manganese exists as precipitated solids, mostly MnO 2

. Prior to the shift when the absorber ORP was lower, the manganese in the pur ge was almost entirely dissolved which corresponds to Mn 2+ . However, after the increase in ORP in mid

- July, the manganese is almost entirely in the solid phase corresponding to MnO 2 . The shift in oxidation state of manganese from Mn 2+ to MnO 2 solids demon strates 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 o bserved for selenium. Prior to the ORP shift in mid

- July, the purge stream

co ntained 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 part iculate 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 occu rs 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 i nfluent 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 ar ound 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 concentration s 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 t he 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 observ ations actually begin rising before the coal switch, with 11.9 ug/L on 1/18/13. The concentration continued to rise toPage 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 B 3

. 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

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 Total Load (MW) Loads Sum

(MW)

0 10 20 30 40 50 60

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 Nitrates as N (ppm) Bio Inf

Bio Eff

0 100 200 300 400 500 600 700 800 900

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 ORP (mV) Absorber 1

Absorber 3Page 37 of 46

Figure B 4

. 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 B 3 ). Top: manganese concentration, total (Mn, tot) and dissolved (Mn, diss). Middle: selenium 0.01 0.1 1 10

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 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

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 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

2/29/12 3/20/12 4/9/12 4/29/12 5/19/12 6/8/12 6/28/12 7/18/12 8/7/12 8/27/12 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 fo r 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 oxida nts 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 o f 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,

indic ating 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 Increase d Selenium Concentration Not Associated with a Coal Swit ch / Sodium Bisulfite

Addition Not Noted in Sections 2.1

- 2.7

Although all values are below both the daily maximum and monthly average limitations, e ffluent

concentrations increased in Mar ch /Apr il 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 wa s over 300 mV or 400 mV; however, it drop ped to 87 mV on 3/6/12 and it wa s negative by 3/8/12 (down to

- 279 mV on 3/9/12). ORP remained negative until 3/19/12 and r ose 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 switche s 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

- 7/1 8/13 (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 Switch es 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 durin g 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 e ffluent 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 deter mine

whether this is due to the fuel switch.

3.2 Coal Switch BC

- 2 (5/1 8/2012

- 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 incr ease 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 elevat ed 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 efflu ent 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 ob servations for ORP, total selenium, selenate, selenite, or nitrate in the FGD purge or

bioreactor influent.

3.4 Coal Switch BC

- 4 (11/5/2012

- 12/2 1/2012; NAPP Federal #2 & Tunnel Ridge)

There was no significant increase in effluent selenium during November fol lowing 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 hi gher 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 B 5

. 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, alo ng with the type of coal. At Belews Creek, the non

- standard coal is NAPP (Northern Appalachian coal).Page 41 of 46Page 42 of 46

Figure B 6

. 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 Swi tch BC

- 6 (5/6/2013

- 6/9/2013; NAPP Enlow For k )

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 o bservations 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, seleni te, 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 switc h 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 i n selenite for the bioreactor influent. No unusual ORP or nitrate values were

observed.

3.8 Coal Switch BC

- 8 (1 1/6/2013

- 11/22/2013; ILB Warrior)

All effluent values during this period were below the effluent limits. No change in effluent

selenium was observe d 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 th e 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 concentra tions 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 limit ed 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, selen ate or selenite. Approximately 1 weekPage 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 swit ch. 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 hi gher 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 Oc t 5 and bioreactor influent ORP increased beginning Oct 8 (continuing to be elevated until Oct 21).

4 Fuel Switch es at Mountaineer Plant

For Mountaineer, EPA did not have information for specific dates when the coal type or source changed; therefore, the d ata 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 c oal 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 sys tem 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 B 7

.

15 EPA excluded one observation for effluent selenium (out of a total of 64 observations for the 2

- year period) because the data

indicate d either a sampli ng 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 associate d 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 even t. A number of possible explanati ons 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, th e results for this sampling e vent 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 ev aluation 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 thanPage 45 of 46

Figure B 7. 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 m g/L nitrate). The si milarity 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 nitra te 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 Bel ews 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 St andards ).Page 46 of 46

5 Conc lusion

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 Cr eek 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 nit rate 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

refle cted 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.