Final Notes from Site Visit at 

Louisville Gas & Electric Company’s Mill Creek Station 

on December 12, 2008 

EPA conducted a site visit at Louisville Gas & Electric Company’s Mill Creek Station (Mill 

Creek) located in Louisville, KY on Friday, December 12, 2008 to learn about the operations at 

the plant, specifically the FGD wastewater handling system. Table 1, located at the end of this 

memorandum, identifies the site visit participants. The presentation that the Mill Creek personnel 

gave during the site visit is provided as an attachment to this memorandum. Also included as 

attachments to this memorandum are a labeled aerial photo of the plant, an overall process flow 

diagram for the generating units, a process flow diagram for the FGD systems, and the 

specifications for the gypsum to be marketed for wallboard manufacturing. 

This memorandum describes the information collected during the site visit, as well as the pre-site 

visit phone call, which was held on November 17, 2008. 

Plant Overview 

 Mill Creek operates four coal-fired generating units at the power plant, which is a 

baseload plant with a net generating capacity of 1,470 MW. Mill Creek is 

Louisville Gas & Electric Company’s largest plant, based on generating capacity. 

The table below contains some general information for the four generating units. 

A process flow diagram for Units 1 through 4 is included as an attachment to this 

memorandum. 

Unit 

Installation 

Date 

Capacity 

(MW) 

Particulate 

Control NOx Control SO2 Control 

1 1972 356 ESP (cold-side) Low-NOx 

burners and 

overfire air 

Wet FGD installed (1980). 

Subsequently modified. 

2 1974 356 ESP (cold-side) Low-NOx 

burners and 

overfire air 

Wet FGD installed (1981). 

Subsequently modified. 

3 1978 443 ESP (cold-side) SCR, lowNOx burners Wet FGD in original 

design (1978) 

4 1982 544 ESP (cold-side) SCR, lowNOx burners Wet FGD in original 

design (1982) 

 Mill Creek burns eastern bituminous coal in all four units. The plant burns 

approximately 4.6 million tons of coal per year. The coal comes from the Illinois 

basin and is mostly mined in Kentucky, Indiana, southern Ohio, West Virginia, 

and Illinois. The coal is delivered to the plant by barge and by railroad. 

 The sulfur content of the coal used at Mill Creek is typically 3.3 to 3.4 percent 

sulfur (6.0 to 6.2 lbs SO2/mmBTU). Mill Creek stated that if the sulfur content is 

too low, it can cause issues with the ESP. Low sulfur content has a negative 

effect in the ESP on the flyash resistivity and thus reduces the ESP particulate 

removal efficiency. 
Page 2 

 The chlorine content of the coal typically averages <1,500 ppm. 

 The coal pile runoff is collected in a coal pile runoff pond that also receives 

limestone pile runoff. The overflow from the coal pile runoff pond is transferred 

to the ash treatment basin. 

 Prior to the site visit, Mill Creek personnel collected four samples from the coal 

pile runoff pond and measured the pH and temperature. The table below presents 

the results of the pH measurements. 

Date pH (S.U.) 

11/21/2008 7.41 

11/28/2008 7.21 

12/4/2008 7.01 

12/8/2008 7.11 

 Mill Creek operates an SCR system on Units 3 and 4. Mill Creek has typically 

operated the SCR systems only during the ozone season. Because of the 

uncertainty of CAIR, Mill Creek will begin operating the SCRs year round. 

 Mill Creek does not currently add hydrated lime or trona to the flue gas because 

the SCRs convert only a small amount of sulfur dioxide to sulfur trioxide. Mill 

Creek stated that hydrated lime will be injected into Units 3 and 4 by the end of 

2011 to meet compliance with the Best Available Retrofit Technology (BART) 

standards. 

 Mill Creek uses the Ohio River as the source of the power plant’s water. Mill 

Creek operates a once-through cooling system for Unit 1 and recirculated cooling 

tower systems for Units 2, 3, and 4. 

 The Unit 1 once-through cooling water is discharged directly to the Ohio River. 

The Unit 2 cooling tower blowdown is transferred to the ash treatment basin prior 

to discharge. Some of the Unit 3 and 4 cooling tower blowdown is transferred to 

the FGD clearwell and reused in the FGD process and the remainder is discharged 

directly to the Ohio River. 

 The Mill Creek plant is located on approximately 500 acres of land. 

 Mill Creek employs 213 full-time workers. 

FGD System 

 Mill Creek operates a limestone, forced oxidation wet FGD scrubber on each of 

the four generating units. A process flow diagram for the FGD systems is 

included as an attachment to this memorandum. 

 All four FGD systems were initially installed as lime, inhibited oxidation wet 

FGD scrubbers. Mill Creek mixed the calcium sulfite product with lime and fly 
Page 3 

ash and produced a poz-o-tec material, similar to concrete, that was landfilled. 

Between 1996 and 1998, Mill Creek converted each of the scrubbers to limestone, 

forced oxidation systems. 

 Mill Creek stated that the gypsum is cleaner and much easier to manage than the 

poz-o-tec material. Additionally, the plant is capable of selling most of the 

gypsum, whereas all of the poz-o-tec material had to be landfilled. 

 Mill Creek uses ball mills to crush the limestone to the correct size for use in the 

limestone slurry. FGD clearwell water or Unit 3 and 4 cooling tower blowdown 

is used to prepare the limestone slurry. 

 Mill Creek uses approximately 475,000 tons of limestone each year (dry basis). 

The limestone is delivered to the plant via barge. 

 The Unit 1 and 2 FGD systems each have two absorber modules and one reaction 

tank. The Unit 3 and 4 FGD systems each have four absorber modules and two 

reaction tanks. All of the absorber modules have four spray headers and one tray, 

with one of the spray headers located below the tray. 

 The Unit 3 reaction tanks were constructed partially below ground and have 

individual, separated cells within the reaction tanks. Mill Creek stated that the 

design of the Unit 3 reaction tank decreases the performance of the FGD system. 

Therefore, Mill Creek adds formic acid to the Unit 3 FGD system to increase the 

sulfur dioxide removal efficiency. Mill Creek used to add DBA, but they stated 

that DBA is no longer readily available to purchase. 

 The FGD systems remove approximately 88 to 93 percent of the sulfur dioxide 

from the flue gas. 

 Mill Creek continuously runs the blowdown pumps for each of the scrubber 

modules. Most of the time the blowdown is recycled back to the reaction tank in 

the module. The blowdown from the FGD systems is intermittently directed to 

the hydroclones based on solids control in the reaction tank. The solids in the 

reaction tanks are controlled to between 10 and 13 percent solids. Limestone 

slurry is added to the reaction tanks to control the pH between 5.5 and 5.9. To 

make up for the evaporative water loss in the scrubbers and to control the level in 

each reaction tank, Mill Creek adds water from the FGD clearwell to the reaction 

tanks. 

 The FGD systems are not constructed of high alloy materials, so the chloride limit 

for the scrubbers is relatively low. The Unit 1 and 2 FGDs are constructed of 

316L stainless steel and have a target chloride level of 1,500 ppm. The Unit 3 and 

4 FGDs are constructed of 317LM stainless steel and have a target chloride level 

of 2,000 ppm. 

 The blowdown from the reaction tanks is transferred to hydroclones to separate 

the solids from the aqueous phase. There are six sets of hydroclones at the plant, 

one for each reaction tank (Units 1 and 2 each have one set of hydroclones and 
Page 4 

Units 3 and 4 each have two sets of hydroclones). Each set of hydroclones are 

operated approximately 14 hours during the day. 

 Approximately 70 to 80 percent of the hydroclone overflow is recycled back to 

the reaction tanks; the remainder is purged to the ash treatment basin. Mill Creek 

estimates that the hydroclone overflow contains three to four percent solids. 

 Mill Creek monitors the chlorides in the reaction tanks daily to determine the 

amount of purge that is required to maintain the chlorides concentration. Mill 

Creek stated that the FGD scrubber purge flow rate is approximately 1,000 gpm 

for all four units combined. 

 The hydroclone underflow is transferred to the hydroclone underflow tanks. 

There is one hydroclone underflow tank for every two sets of hydroclones, so 

Units 1 and 2 share a hydroclone underflow tank and Units 3 and 4 each have a 

separate tank. Mill Creek estimates that the hydroclone underflow contains 40 to 

50 percent solids. 

 The hydroclone underflow is then transferred to two filter feed tanks, which are 

approximately 2,500 feet away at the gypsum processing plant. 

 From the filter feed tanks, the gypsum slurry is fed to the rotary drum vacuum 

filters. The gypsum is washed with hot-side cooling tower blowdown (cooling 

water downstream of the condensers and upstream of the cooling tower) to 

remove the chlorides. The gypsum produced has a moisture content of 

approximately 12 percent. 

 Mill Creek sells some of the gypsum that is produced in the FGD process to a 

wallboard manufacturer. Mill Creek produces 768,000 tons of gypsum per year 

and sells approximately 654,000 tons; however, the contract that the plant has 

with the wallboard manufacturer expires at the end of 2009. Therefore, Mill 

Creek is uncertain how much gypsum will be marketed after that time. 

Unmarketed gypsum is landfilled. 

 Mill Creek delivers the gypsum to the wallboard manufacturer via barge. For the 

wallboard plant to accept the gypsum, the gypsum must meet specifications 

agreed upon in the contract. The gypsum specifications are included as an 

attachment to this memorandum. 

 The filtrate from the rotary drum vacuum filters is transferred to the “E” settling 

pond, or emergency settling pond. The “E” settling pond covers approximately 1⁄2 

acre and also receives floor drain waters from the gypsum processing plant. 

 From the “E” settling pond, the FGD wastewater is transferred to the “Dead 

Storage” pond. The “Dead Storage” pond covers approximately one acre. The 

water in the “Dead Storage” pond can be transferred to the FGD clearwell, but it 

is not typically done. Most of the water in the “Dead Storage” pond is transferred 

to the gypsum processing and construction runoff pond. 
Page 5 

 The gypsum processing and construction runoff pond discharges directly to the 

Ohio River via Outfall 012. Mill Creek performs quarterly monitoring at Outfall 

012 for TSS, oil and grease, chlorides, and pH. Additionally, the plant tests for 

hardness, sulfate, and ammonia, but doesn’t have any limits on these parameters. 

 Mill Creek dredges the “E” settling pond, “Dead Storage” pond, and gypsum 

processing and construction runoff ponds a few times a year to maintain the pond 

in working condition. The solids removed from the ponds are placed in the onsite landfill. 

 Mill Creek operates a FGD clearwell, which is used as the source of make-up 

water for the FGD system. The FGD clearwell receives water from the Ohio 

River, some of the Unit 3 and 4 cooling tower blowdown, and occasionally 

receives water from the “Dead Storage” pond. 

Ash Handling and Wastewater Treatment 

 Mill Creek operates cold-side electrostatic precipitators (ESPs) on all four 

generating units. 

 Mill Creek estimates that approximately 425,000 tons of fly ash are generated 

each year from the operation of all four units. The plant has been able to market 

approximately 300,000 tons of the fly ash per year to a cement manufacturer. 

 There is a cement manufacturer (Cemex) located across the street from Mill 

Creek. Mill Creek stated that they have historically sold a large amount of the fly 

ash to Cemex, but recently have not been able to sell as much because Cemex 

reduced production due to the downturn in the economy. 

 Mill Creek has both a wet and a dry fly ash handling system; however, the wet fly 

ash handling system is only used for emergency purposes. Plant staff estimated 

that less than five percent of fly ash is handled wet. Mill Creek prefers to handle 

the fly ash dry because it is less expensive to do so. The plant initially sluiced all 

fly ash wet, but converted to dry fly ash handling when scrubbers were installed 

to enable it to operate a poz-o-tec process, whereby FGD solids (calcium sulfite) 

were mixed with lime and dry fly ash to create a cementitous product (poz-o-tec). 

 If the fly ash were handled wet, it would be transferred to the ash treatment basin. 

From the ash treatment basin, the plant would excavate the fly ash out of the pond 

and truck it to the on-site landfill; therefore, they would be double handling the 

fly ash, increasing operating costs. The source of ash sluice water is service water 

and/or Unit 2 cooling tower blowdown. 

 The dry fly ash is pneumatically conveyed to the two fly ash silos, which are 

located approximately 2,000 feet from the generating units. Mill Creek has two 

different methods for handling the fly ash from the silos. If the fly ash is being 

marketed, the fly ash is kept completely dry and transferred into closed top trucks. 

If the fly ash is not marketable, then it is sent through a pug mill where FGD 
Page 6 

clearwell water is added to the fly ash to a moisture content of 18 percent. The fly 

ash is then placed in a truck and transferred to the on-site landfill. 

 Mill Creek estimates that approximately 104,000 tons of bottom ash are generated 

each year from the operation of all four units. Mill Creek operates wet bottom ash 

handling systems for each unit. All of the bottom ash sluice is transferred to the 

ash treatment basin. 

 The bottom ash sluice enters the ash treatment basin in a channel. Most of the 

bottom ash settles out within the channel and Mill Creek excavates the bottom ash 

from the channel five days a week. 

 A vendor sorts the bottom ash by size and markets the bottom ash for use as 

“black sand”. Mill Creek estimates that approximately 71,000 tons of bottom ash 

is marketed per year. The remainder of the bottom ash is trucked to the on-site 

landfill for disposal. 

 As discussed previously, the ash treatment basin also receives some of the 

hydroclone overflow from the FGD process, which contains three to four percent 

gypsum. Like the bottom ash, the hydroclone overflow enters the ash treatment 

basin in a channel. The gypsum present in the overflow settles out of the water 

within the channel. Mill Creek excavates the gypsum from the channel weekly 

and trucks it to the on-site landfill. 

 The ash treatment basin also receives Unit 2 cooling tower blowdown and other 

low-volume wastewaters. The overflow from the ash treatment basin is 

commingled with the Unit 1 once-through cooling water and is discharged to the 

Ohio River. Mill Creek does not reuse any of the ash treatment basin water at the 

plant. 

 The ash treatment basin was built in the 1970’s and is not lined. Mill Creek stated 

that they will be able to use the basin for an extended period of time because the 

plant excavates almost all of the solids that are entering the pond. 

Landfills 

 Mill Creek has closed and capped one landfill at the plant. Mill Creek is currently 

operating another landfill. 

 The current landfill was originally designed to receive the poz-o-tec material that 

the FGD systems were originally generating, along with fly and bottom ash. 

After Mill Creek switched to forced oxidation scrubbers, it began sending the 

gypsum to the landfill. 

 Mill Creek estimates that the current landfill will only last for another two years 

before it reaches capacity and needs to be capped and closed. Therefore, the plant 

is already working on plans for a new landfill that will provide the plant with 12 

years worth of capacity. 
Page 7 

 Table 1. Site Visit Attendees 

Name Organization (Representing) Telephone E-Mike Buckner Louisville Gas & Electric – Mill Creek (502) 933-6515 mike.buckner@Phil Cook Louisville Gas & Electric – Mill Creek (502) 933-6878 phil.cook@eonOz Haeberlin Louisville Gas & Electric – Mill Creek (502) 933-6977 oz.haeberlin@eMike Kirkland Louisville Gas & Electric – Mill Creek (502) 231-5035 mike.kirkland@Roger Medina E.ON U.S. (502) 627-2997 roger.medina@Michael Winkler E.ON U.S. (502) 627-2338 michael.winkleDonna Hill UWAG (205) 257-5234 dbhill@southerJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.gRon Jordan EPA/Office of Water (202) 566-1003 jordan.ronald@T.J. Finseth ERG (Contractor to EPA) (703) 994-7130 thomas.finseth@