Draft Notes from Site Visit to GenOn Energy’s Conemaugh Generating Station on August 23, 2012

EPA conducted a site visit at GenOn Energy’s Conemaugh Generating Station (Conemaugh) 

located in New Florence, Pennsylvania on Thursday, August 23, 2012 to learn about operations at the 

plant and discuss recent upgrades to the FGD wastewater treatment system. These notes describe the 

information collected during the site visit. Table 2 identifies the site visit participants.

Plant Overview

• Conemaugh operates two electric generating units, both of which are baseload coal-fired units. 

The plant’s net generating capacity is 1,700 MW. The table below contains some general 

information on the current operation of each steam electric generating unit.

Table 1. Steam Electric Generating Unit Operations

SE Unit Fuel

Capacity 

(MW) Fly Ash Bottom Ash FGD

1 Coal: Bituminous 850

Dry Ash Handling –

Combined Vacuum 

and Pressure System Wet Sluiced Wet System

2 Coal: Bituminous 850

Dry Ash Handling –

Combined Vacuum 

and Pressure System Wet Sluiced Wet System

• In January 2012, Conemaugh added an ion exchange system and an anaerobic bioreactor to the 

existing FGD wastewater treatment system. These systems were added in response to more 

stringent limits on boron and selenium in their NPDES permit set by the State of Pennsylvania.

• Conemaugh also recently added a clarifier to treat cooling tower blowdown. The clarifier targets 

removals of iron, magnesium, and aluminum. 

• The plant is planning to install selective catalytic reduction (SCR) systems on Units 1 and 2 over 

the next two years. The use of the SCR systems will reduce the nitrogen oxide emissions from 

the plant, but will also oxidize the mercury and result in additional mercury removal from the flue 

gas in the FGD system. 

• Conemaugh is also planning to make upgrades to the FGD systems for both units. The upgrades 

include adding a tray to each of the absorbers and adding secondary hydrocyclones to the 

systems.

Fly Ash Handling

• Units 1 and 2 handle the fly ash collected from the electrostatic precipitators (ESPs) dry using 

combined vacuum and pressure systems. 

• The fly ash is transferred to two silos for each generating unit (four silos total). Each set of silos 

has the capacity to hold 4 to 5 days worth of fly ash.

• Conemaugh unloads fly ash from the silos five days per week using pugmills. Conemaugh uses

cooling tower blowdown to moisten the ash in the pugmills prior to loading it into trucks. 

• Most fly ash is sent to the on-site landfill with only a small percentage being reused or sold for 

concrete products, structural fill, or for mining applications.
Bottom Ash Handling

• Conemaugh wet sluices the bottom ash from Units 1 and 2 to dewatering bins. Overflow from 

the dewatering bins is transferred to a set of two ash settling ponds operated in parallel. The 

solids from the dewatering bins are sent to the plant’s on-site landfill. From the ash settling 

ponds, water is recycled and reused within the bottom ash sluice system. Solids from these ponds 

are periodically dredged and transferred to the landfill.

• During periods of heavy precipitation, excess water collecting in the ash settling pond is 

transferred to the desilting pond. Effluent from this desilting pond is used as FGD make-up or as 

FGD reagent and slurry preparation water. Because the bottom ash transport water may be reused 

in the FGD system, it can eventually be discharged after being treated in the FGD wastewater 

treatment system. Normally, no bottom ash transport water is discharged and the wastewater is 

completely recycled.

Flue Gas Desulfurization (FGD) System

• Conemaugh is currently operating a limestone forced oxidation FGD system on both Units 1 and 

2. The scrubbers achieve a sulfur dioxide (SO2) removal efficiency of between 97 and 99 

percent.

• DBA is currently used as an additive in the FGD system. Conemaugh personnel indicated that it 

may be possible to discontinue the use of DBA once planned upgrades have been made to the 

FGD system (i.e., installation of trays). The plant will evaluate this option once the trays and 

secondary hydrocyclones have been installed.

• Conemaugh operates the FGD system with a chlorides concentration of approximately 16,000 

ppm.

• Blowdown from the FGD system is transferred to the primary hydrocyclones for initial solids 

separation. Overflow from the hydrocyclones is sent to the lamella clarifier for additional solids 

separation before being sent to the FGD wastewater treatment system.

• Underflow from both the primary hydrocyclone and the lamella clarifier is sent to the vacuum 

belt filter. The solid gypsum from the filter is currently landfilled. The filtrate from the vacuum 

belt filter is recycled back to the lamella clarifier.

• Overflow from the lamella clarifier has a typical solids concentration of 100,000 to 200,000 ppm

(1 to 2 percent solids). This wastewater is sent to the FGD wastewater treatment system.

• Conemaugh has not done any manipulation of oxidation reduction potential (ORP) within the 

scrubber to determine what its effects are on FGD wastewater composition.

• Conemaugh personnel stated that they do not expect to see any significant increase in the mercury 

concentration in the FGD scrubber purge after the installation of the SCR systems because they 

expect that the additional mercury removed in the scrubber will partition to the gypsum solids.

• With the addition of the secondary hydrocyclones, the overflow from the primary hydrocyclones 

will be transferred to the secondary hydrocyclones instead of the lamella clarifier. The overflow 

from the secondary hydrocyclone will be transferred to the lamella clarifier. The underflow from 

both the secondary hydrocyclones will be recycled back to absorber. 

FGD Wastewater Treatment System

• Conemaugh completed the upgrade of the FGD wastewater treatment system in January 2012. 

The upgrades were in response to a consent decree between the Conemaugh Generating Station 

and Pennsylvania Department of Environmental Protection requiring compliance by February 1, 

2012. A block flow diagram of the new FGD wastewater treatment system is included as 

Attachment 1.
• As discussed previously, Conemaugh recently added an ion exchange system to target boron 

removal and an anaerobic biological reactor system to target selenium removal. Both systems 

were added to the plant’s existing FGD wastewater system which included primary and 

secondary clarification and a set of sequencing batch reactors (SBR).

• Overflow from the plant’s lamella clarifier is sent to the FGD surge tank, which is the beginning 

of the FGD wastewater treatment system.

• From the surge tank the wastewater is sent to the industrial wastewater treatment (IWT) portion 

of the system. The IWT system contains two pH adjustment sumps operated in series; however, 

Conemaugh only adds lime to the primary pH adjustment sump. Conemaugh adds lime to adjust 

the pH to 9.3 S.U. to target boron and manganese removal. The wastewater flows through the 

secondary sump and is then transferred to two primary clarifiers operated in parallel. 

• Conemaugh injects ferric chloride and trimercapto-s-triazine (TMT) into the wastewater upstream 

of the clarifiers. Conemaugh also adds a polymer directly to the clarifiers. Underflow from 

primary clarifiers is sent to a thickener and then to filter presses for dewatering. The solids are 

landfilled and the filtrate from the filter press is recycled back to the pH adjustment sump.

• The overflow from the clarifiers is sent to the boron pH adjustment tank where the plant adds 

hyrdrochloric acid to lower the pH to 8 S.U. in preparation for the boron ion exchange system.

• After adjusting the pH, Conemaugh transfers the wastewater to the boron equalization tank, 

which feeds the ion exchange system. The ion exchange system consists of two parallel trains, 

each capable of handling 75 percent of the flow. First, the wastewater is split and pumped through 

the two pressure sand filters operating in parallel. The wastewater is then sent through the ion 

exchange resin vessels. Each train contains two vessels. As designed the vessels operate in series, 

except when one of the vessels is being regenerated, which was designed to occur every six 

hours. However, because of scaling issues, Conemaugh has needed to operate the system such 

that one of the vessels in each train is always being regenerated. As the regeneration of one vessel 

is complete, the other vessel begins its regeneration process.

• Conemaugh uses sulfuric acid and caustic soda washes to regenerate the resins. The waste from 

the regeneration process is transferred to boron regeneration waste tank and then fed to the boron 

precipitation tank where the plant adds ferric chloride and lime. The plant then feeds the water to 

a clarifier where a polymer is added. The underflow from the clarifiers is pumped to a storage 

tank and then to filter presses. The filter cake is landfilled and the filtrate is transferred to the 

filtrate sump and recycled to the boron regeneration waste tank. The overflow from the clarifier is 

sent to a continuous backwash sand gravity filter and then to the boron effluent tank where it is 

commingled with the effluent from the ion exchange vessels. The backwash from the gravity 

filters is transferred to the filtrate sump. 

• As part of the ion exchange system, Conemaugh added several in-line boron analyzers to more 

closely monitor when the resins need to be regenerated.

• Conemaugh personnel reported seeing a roughly 50% reduction in boron concentrations across 

the ion exchange system. Conemaugh has measured boron concentrations between 400 and 500 

ppm in the purge stream and concentrations of 75 ppm in the wastewater transferred to the 

anaerobic reactor system, with 30% removal coming from the primary clarifier.

• From the ion exchange system, wastewater is transferred directly into the anaerobic reactor with 

phosphoric acid, micro-nutrients, a sugar solution, and a urea solution added inline prior to 

entering the reactor. Wastewater goes through a series of two reactors before a clarification and 

filtration step to remove solids. The wastewater is then transferred to the SBR feed tank while the 

liquid sludge is hauled offsite for disposal.

• The anaerobic reactors utilize a slow mix to mix the biological material with the wastewater but 

not create an aerobic environment. 

• The anaerobic biological reactor system does not include a heat exchanger to cool wastewater 

before entering the reactor. Conemaugh personnel indicated that the retention time in the system 
allows for sufficient heat loss before the wastewater reaches the anaerobic reactor system. The 

system is insulated for cold weather.

• With the addition of the anaerobic reactors targeting selenium removal, Conemaugh has achieved 

selenium concentrations of approximately 150 ppb in the final effluent. This is below the plant’s 

NPDES permit limit on selenium, which is 250 ppb.

• Conemaugh has seen the best selenium removals while maintaining the ORP of the anaerobic 

biological reactor system between -250 and -350 mV.

• Conemaugh worked with Infilco Degremont on the design of the upgrades to the FGD wastewater 

system, including the ion exchange system and anaerobic reactor system. The system is 

guaranteed to achieve effluent concentrations that are 20 percent less than the permit limits 

required by the State of Pennsylvania. The FGD wastewater system is designed to achieve a 

boron concentration of 80 ppm and a selenium concentration of ?? ppb. [QUESTION TO 

CONEMAUGH: Please provide the design concentration for selenium guaranteed by Infilco.]

• The anaerobic biological reactor system took approximately six months to start-up and reach 

equilibrium. During this period no plant wide shutdown was required. Conemaugh was able to 

by-pass individual reactors as they were initially brought online.

• Based on information from the Infilco Degremont, during a period of plant shutdown the 

anaerobic biological reactor system would only need to be fed nutrients. No wastewater 

recirculation would need to take place. The biomass would go dormant and a specific start-up 

process would need to be followed to bring the biomass back to its active state.

• A permit is required in order to dispose of the anaerobic reactor sludge in the on-site landfill.

• From the anaerobic biological reactors, the wastewater is transferred to a clarifier where ferric 

chloride and polymer are added. The majority of the underflow is recycled back to the first 

anaerobic biological reactor, while a small percentage is sent to the selenium waste tank and 

disposed of as a liquid sludge. Conemaugh needs a permit to dispose of this waste in its landfill, 

so currently the waste is sent off site as a liquid until they can get the permit to send it to the 

landfill. 

• The overflow from the clarifier is transferred to a continuous backwash sand gravity filter. The 

backwash from the filter is also recycled to the first anaerobic biological reactor. The effluent 

from the gravity filter is transferred to the pH adjustment sump, which acts as the sequencing 

batch reactor (SBR) feed tank and no longer receives any chemicals for pH adjustment.

• Wastewater from the SBR feed tank is sent to the SBR reactor tanks. With the addition of the ion 

exchange system and anaerobic reactors, the SBR reactors are currently used as an aeration and 

polishing step prior to discharge. 

• The plant adds urea and phosphoric acid to the SBRs. The SBRs reduce the BOD and iron 

present in the wastewater.

• The SBRs do not have an anaerobic stage and therefore, do not achieve denitrification of the 

wastewater after the nitrification of the wastewater is achieved through the aeration of the 

wastewater.

• The solids collected in the SBRs are transferred to a thickener and a portion is recycled to the 

SBR feed tank while the remainder is transferred to the landfill for disposal. The effluent from 

the SBRs is sent to a clearwell and then sent through three pressure sand filters operated in 

parallel. The backwash from the pressure filters is recycled to the SBR feed tank while the 

effluent from the pressure filters is commingled with several other waste streams and discharged 

to the Conemaugh River via Outfall 007. 

• The entire treatment system has a residence time of roughly 3 to 4 days, with each biological 

reactor having a residence time between 6 and 9 hours.

• Prior to installing both the ion exchange system and anaerobic biological reactor system,

Conemaugh evaluated several alternatives to each system. A magnesium hydroxide process and 

zero liquid discharge systems were evaluated as possible alternatives to the ion exchange system. 
The magnesium hydroxide process did not achieve comparable boron removal while the zero 

liquid discharge system was prohibitively expensive. A two-stage chemical precipitation system 

and a GE ABMet system were evaluated as alternatives to the anaerobic biological reactor 

system, with the ABMet system also being piloted at the plant. The chemical precipitation 

system alone was not able to achieve the required effluent concentrations. While the ABMet 

system uses similar chemistry to the anaerobic biological reactor system installed, Conemaugh 

stated that they were concerned about potential scaling and plugging in the ABMet system.

• A pilot-scale anaerobic biological reactor system was operated at Conemaugh prior to treatment 

system upgrades. Plant representatives stated that they observed similar removals and 

performance in the full-scale system with what was demonstrated in the pilot-scale system.

Landfills and Leachate Treatment Systems

• Conemaugh operates one active landfill and one retired landfill. The plant also has plans for a 

future landfill.

• The currently active landfill receives bottom ash, fly ash, calcium sulfate (gypsum), and other 

plant solid wastes. This landfill is lined and leachate is collected and commingled with coal pile 

runoff, stormwater from the landfill, and leachate from the retired landfill. This commingled 

wastewater is collected in a pond and transferred to a leachate treatment system. 

• The combined wastewater in the leachate collection pond is generally very acidic with a pH 

between 2 and 3 S.U. From the pond, the wastewater is transferred to a reactor where lime is 

added to bring the pH up to between 4 and 4.5 S.U. The wastewater is then sent to a second 

reactor where more lime is added to raise the pH to between 8 and 9 S.U. The effluent is then 

sent to a high density clarifier. Solids from the clarifier are pumped into a portion of the leachate 

collection pond where they can dewater prior to being trucked to the plant’s on-site landfill. 

Overflow from the clarifier is reused within the plant in the FGD system and as bottom ash sluice 

make-up water.

• Conemaugh is not currently adding new solids to the retired landfill but this landfill does hold fly 

ash, bottom ash, and calcium sulfate (gypsum) waste. Leachate collected from the retired landfill 

is combined with leachate from the active landfill and treated in the plant’s leachate treatment 

system.

• Plant representatives indicated that plans are in place to expand the current landfill. This 

expansion is not expected to open until 2016 and is known as ‘Stage 3’. This landfill will receive 

the same type of waste that is currently held in the active landfill, fly ash, bottom ash, calcium 

sulfate (gypsum), and other plant wastes.
Table 2. Site Visit Attendees

Name Organization (Representing) Telephone E-mail

Steve Frank GenOn (724) 597-8310 Stephen.frank@genon.com

Keith Schmidt GenOn (724) 597-8193 Keith.schmidt@genon.com

Jim Brunson GenOn (724) 235-4596 James.brunson@genon.com

John Balog GenOn-Conemaugh (724) 255-3352 John.baylog@ganon.com

Tony Garaventa GenOn-Conemaugh (724) 235-4597 Anthony.garaventa@ganon.com

Shelley 

Wjciechowski GenOn-Conemaugh (724) 235-4465 Shelley.wojciechowski@genon.com

Dave Benson Keystone-Conemaugh Projects (724) 357-1727 dbenson@kcpo.com

David Read GenOn (724) 354-5470 David.read@ganon.com

Donna Hill Southern Company (205) 257-5234 dbhill@southernco.com

Ron Jordan EPA/Office of Water (202) 566-1003 Jordan.ronald@epa.gov

Cuc Schroeder EPA/Office of Water (202) 566-2397 Schroeder.cuc@epa.gov

James Covington EPA/Office of Water (202) 566-1034 Covington.james@epa.gov

T.J. Finseth ERG (Contractor to EPA) (703) 633-1698 Thomas.finseth@erg.com

Danielle Lewis ERG (Contractor to EPA) (703) 633-1703 Danielle.lewis@erg.com