Site Visit Notes: Duke Energy Carolinas’ 

Marshall Steam Station (March 13, 2008) 

EPA conducted a site visit at Duke Energy Carolinas’ Marshall Steam Station located in 

Terrell, NC on Thursday, March 13, 2008 to learn about the operations at the plant, 

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

summary, identifies the site visit participants. 

This summary describes the information collected during the site visit and the pre-site 

visit phone call, which was held on February 1, 2008. 

Plant Overview 

• Marshall operates four generating units at the power plant. It is a baseload 

plant with a total generating capacity of 2,090 MW. The table below 

contains some general information for each unit. 

Unit 

Installation 

Date 

Capacity 

(MW) 

Particulate 

Control NOx Control SO2 Control 

1 1965 385 Cold-side 

ESP 

SNCR Wet FGD – 5/2007 

(shared with Unit 2) 

2 1966 385 Cold-side 

ESP 

SNCR Wet FGD – 5/2007 

(shared with Unit 1) 

3 1969 660 Cold-side 

ESP 

SNCR, but 

currently 

installing a 

new SCR 

Wet FGD – 3/2007 

4 1970 660 Cold-side 

ESP 

SNCR Wet FGD – 10/2006 

• All four units operate cold-side ESPs. Marshall recently installed new 

ESPs for Units 3 and 4 to increase the removal of fly ash before the FGD 

system so the gypsum would have a purity level sufficient for wallboard 

manufacturing. 

• All four of the units currently operate SNCRs; however, Marshall is 

currently installing a SCR on Unit 3. Marshall is converting to the SCR to 

achieve better removals of NOx. Marshall is also conducting a study to 

determine whether to install a SCR on Unit 4. 

• Marshall is operating three wet FGD scrubbers to scrub the flue gas for all 

four units. Marshall’s first FGD scrubber was installed on Unit 4 in 

October 2006. Marshall then installed a scrubber for Unit 3 in March 

2007, and installed a scrubber to scrub the flue gas from both Units 1 and 

2 in May 2007. 
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• Marshall burns eastern bituminous coal in all four units. The average 

percent sulfur of the coal is 1.25 percent. Marshall stated that before the 

FGD scrubbers were installed, the plant was burning coal with a sulfur 

content of 2.0 lbs SO2/MBTU. Marshall blends coal on site to achieve a 

sulfur content of 2.4 lbs SO2/MBTU. Marshall stated that the maximum 

design sulfur content for the FGD scrubbers is 2.8 lbs SO2/MBTU. 

• Marshall stated that the majority of the coal they use is from Central 

Appalachia, but some of the coal is from Northern Appalachia. Marshall 

stated that they receive coal from about 40 different sources. 

• Marshall burns approximately 5.5 million tons of coal per year. Marshall 

stores a 30-day supply of coal on site. 

• Marshall operates once through cooling systems for all four units using 

water from Lake Norman. The average intake flow rate for the plant is 

approximately 1 million gallons per minute for all four units combined. 

Ash Basin

• Marshall stated that they have the capability to wet sluice the fly ash; 

however, the plant primarily uses a dry fly ash handling system. 

• Marshall stated that the dry fly ash generated by the plant contains too 

much unburned carbon to sell for beneficial reuse. Therefore, Marshall 

disposes of the fly ash in an on-site structural fill. Marshall estimated that 

they generate 500,000 to 600,000 tons of fly ash per year. 

• Marshall stated that the wet fly ash handling system is used when there are 

equipment issues or after maintenance outages. Marshall stated that the 

duration of the wet fly ash sluicing can be up to a couple of days, 

depending on the situation, but it is used very minimally. 

• Marshall operates a wet bottom ash sluice system. The wet bottom ash 

sluice, as well as any wet fly ash sluice are sent to the ash basin. 

• Marshall stated that the ash basin is approximately 5-6 meters deep at the 

deepest point. 

• Marshall stated that they recently dredged some of the ash in the basin by 

moving the ash to the more inactive portions of the basin. 

 

• Marshall has two concrete basins that are adjacent to the confines of the 

ash basin. Marshall directs the bottom ash sluice to one of the concrete 

basins during the day. The solids in the sluice water settle to the bottom of 

the concrete basin and the wastewater overflows to the ash basin. The 

next day, Marshall directs the bottom ash sluice to the other concrete basin 

and allows the solids in the first concrete basin to dry. Later in the day, 
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Marshall removes the solids from the first concrete basin. Marshall 

repeats this process every day. 

• Marshall has an on-site contractor that manages the sale of the bottom ash. 

The contractor takes the dried bottom ash and sends it through a sieve that 

sorts the bottom ash by size. The contractor then sells the bottom ash for 

beneficial reuse (cinder block manufacturing). 

 

• The pyrites generated from the pulverizing process are sluiced to different 

basins (not the concrete basins) adjacent to the confines of the ash basin, 

and the plant treats the pyrites solids in the same manner as the bottom 

ash: allowing the solids to settle, decanting the overflow water into the ash 

basin, and removing the solids. Marshall stated that they are working to 

set up a coal/pyrite separation process that will separate the unburned 

carbon in the pyrites and allow the plant to reuse it in the boiler. 

• The ash basin discharges though NPDES permitted outfall 002 to Lake 

Norman (i.e., the Catawba River). 

FGD Scrubber

• Marshall is currently operating three FGD scrubbers that scrub the flue gas 

for all four units (one of the FGD scrubbers scrubs the flue gas for Units 1 

and 2). 

• Marshall started operation of the first scrubber in October 2006; the last 

scrubber started operation in May 2007. 

• Each of the FGD scrubbers has a spray tower design and has four spray 

levels; therefore, each scrubber has four recirculation pumps. 

• Marshall operates ball mills to crush limestone to make the limestone 

slurry that is used as the sorbent in the scrubber. 

• Marshall stated that they are not currently using DBA in the FGD 

scrubbers, but they have the capability and plan on using it in the future 

(note: its use has been delayed). Marshall did add DBA during the 

performance testing of the scrubbers, but stopped using it immediately 

after the test. Marshall stated that they want to achieve a 97% removal of 

SO2, but currently they are achieving 95% removal. 

• Marshall stated that the design maximum chloride level in the scrubber is 

12,000 ppm. Marshall stated that they plan to operate the system at 

<10,000 ppm chlorides. 

• Marshall has two sets of hydroclones for each of the FGD scrubbers. In 

addition, there are two vacuum belt filters for the FGD operation. Marshall 

stated that the blowdown from the scrubbers to the primary hydroclones is 
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intermittent, about four hours of every six hours. The overflow from the 

primary hydroclones is sent to a holding tank. The primary hydroclone 

underflow is sent to a feed tank, then to the vacuum filter belts to dewater 

the gypsum. 

• The primary hydroclone overflow in the holding tank is transferred to the 

secondary hydroclone for additional solids removal. The underflow from 

the secondary hydroclones is returned to the FGD scrubbers. The 

overflow from the secondary hydroclones is sent to another holding tank 

(purge tank) and then sent to the FGD wastewater treatment system. The 

purge to the FGD wastewater treatment system is intermittent at a flow 

rate of approximately 440 gpm (maximum design purge flow rate is 538 

gpm). Marshall stated that the TSS levels in the purge stream are usually 

between 6,000 and 8,000 mg/L. The TSS design level is 16,000 mg/L. 

• Marshall washes the gypsum with service water as it is applied to the 

vacuum filter belts. The filter belts dry the gypsum to less than 10 percent 

moisture. The wash water and filtrate from the vacuum filter belts are sent 

back to the absorbers. 

• Marshall estimates that they produce 1,200 to 1,400 tons of gypsum per 

day. The gypsum is transferred to an outdoor storage area and is 

eventually hauled to a National Gypsum wallboard plant located near 

Charlotte. Any gypsum that is considered off-specification is sent to a 

new, lined on-site landfill. 

FGD Wastewater Treatment System 

• The Marshall FGD wastewater treatment system consists of a solids 

removal process followed by constructed wetlands. The targeted effluent 

quality for the wetlands effluent (outfall 004) is 0.5 ug/L mercury and 200 

ug/L selenium. The wetlands effluent is sent to the ash basin; the ash 

basin effluent is outfall 002. 

• Figure 1, at the end of this memorandum, presents a process flow diagram 

of the solids removal process. 

• An equalization tank is the first component of the FGD wastewater 

treatment system. The equalization tank receives the FGD purge 

wastewater from the secondary hydroclones and return water from the 

filtrate sump (which is mainly the clarifier filter press filtrate). 

• Marshall stated that the return flow from the filtrate sump is intermittent. 

Marshall operates 2-3 filter press cycles daily. The contribution from the 

filtrate sump is 39,600 gpd. 
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• The FGD wastewater flows continuously from the equalization tank to the 

clarifier. Marshall stated that if the equalization tank level is low, the flow 

rate will automatically decrease so it can flow continuously. 

 

• Marshall stated that a cationic polymer is added to the wastewater as it is 

flowing to the clarifier. Marshall stated that they changed the polymer 

from the anionic polymer previously used because a bench test showed 

that a cationic polymer was more effective. In addition, Marshall stated 

that the anionic polymer was in the dry form and had to be blended on site, 

whereas the cationic polymer is in liquid form and already preblended. 

• The solids removed from the clarifier, containing approximately 30 

percent solids, are sent to a sludge holding tank. From the holding tank, 

the sludge is sent to one of three filter presses. The solid cake formed by 

the filter press is transferred to the lined on-site landfill that also receives 

the off-specification gypsum. The filtrate from the filter press is sent to a 

sump and then transferred to the equalization tank at the beginning of the 

FGD wastewater treatment system. 

• The overflow from the clarifier is transferred to the wetlands supply tank, 

where Marshall adds service water to reduce the temperature and the 

chlorides level. Marshall stated that the TSS design specification for the 

clarifier overflow was <1,000 ppm; however the results are typically less 

than 100 ppm. 

• Marshall stated that they were initially adding service water to the FGD 

wastewater at a ratio of 0.85:1 (service water: FGD wastewater), but 

increased the ratio to 1:1 or 1.2:1 because the high concentrations of boron 

in the wastewater were impacting the plants. Marshall stated that the 

boron levels need to be less than 50 mg/L. Also, the plants need the 

wastewater to have less than 4,000 ppm chlorides and temperature less 

than 95F. 

• The wastewater from the wetlands supply tank is pumped 1.5 miles to the 

constructed wetlands treatment system. Figure 2, at the end of this 

memorandum, presents a process flow diagram of the constructed wetland 

treatment system. Marshall stated that they were initially interested in the 

constructed wetlands as a treatment option for the FGD wastewater 

because other technologies did not appear to be as effective. 

• The first stage of the constructed wetlands consists of two equalization 

tanks which allow further cooling of the wastewater prior to entering the 

wetlands. The wastewater is then split to enter three identical constructed 

wetland treatment trains in parallel. The first and second cell of each train 

contain bulrush, then the wastewater flows over cascading rocks (Cell 3) 

and enters the fourth cell containing cattails. The effluent from the 

constructed wetlands system discharges to the ash basin. 
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• Marshall stated that they began transferring wastewater to the wetlands in 

December 2006. Marshall stated that initially they had issues with the 

levels of solids in the system because the polymer feed was not operating 

properly causing more solids to enter the constructed wetlands than they 

had expected. 

• Marshall stated that the wetlands system is not yet optimized. The system 

is reportedly achieving good removal of mercury, but is not treating the 

selenium as well as they had planned. Plant staff stated that selenium 

reduction across the wetlands is approximately 10-20 percent. Marshall is 

working with the vendors to improve the selenium removal. Marshall 

stated that they are considering making adjustments to the amount of 

organic material in the system and increasing the water level in the 

wetlands, which creates an anoxic environment. 

• Marshall stated that the system is probably about 2 years away from 

maturity. Marshall stated that some of the cells still have open space 

available and they are considering adding more plants. Marshall stated 

that the bulrush in the first two cells have not been producing as much 

organic material as they had expected and they may switch to cattails 

because they are expected to produce more organic material. Marshall is 

also considering adding additional organic material to the cells, such as 

mushroom compost. In July 2008, a pilot test will be conducted using 

nutrient addition to one of the trains. The study will last for several 

months. 

• Marshall is also working with EPRI on a pilot test for a vertical flow 

constructed wetland system. The vertical flow system is similar to the 

current constructed wetland system, except the plant would need to install 

leachate piping underneath the current system to allow water to be 

removed from the bottom of the constructed wetland. Marshall stated that 

the study is also looking into using a different source of organic material 

(mushroom compost). 

• Marshall stated that the constructed wetlands are designed for a 30-year 

life span. Marshall stated that the wetlands are lined at the bottom with a 

geosynthetic clay liner. The soil is on top of the clay. At the end of the 30 

year life, Marshall plans to seal off the wetlands. 

• EPA asked if there is any potential to generate methyl mercury in the 

system. Marshall stated that as long as the oxidation-reduction potential in 

the system is above a certain level, then the mercury will not convert to 

methyl mercury. Marshall also stated that most of the mercury in the FGD 

purge stream is being removed in the clarifier; therefore, the level of 

mercury in the wetlands is low. At the ash basin discharge, mercury is 

typically <5 ppt throughout the year. 
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• Marshall stated that they are not achieving nitrification/denitrification in 

the wetlands. The amount of nitrate/nitrite and ammonia entering the 

system are typically equal to the amount leaving the system. 

• Marshall stated that they are considering adding ferric chloride to the 

purge tank or equalization tank of the solids removal process (or add a 

reaction tank to the treatment system) to help increase the selenium 

removals across the FGD wastewater treatment system. 

FGD Wastewater Treatment Data 

• EPA asked if the plant has 2007 data for the wastewater treatment system. 

Marshall stated that they have collected samples of the ash basin effluent 

and the constructed wetlands effluent. In addition, Marshall has collected 

samples of the FGD purge stream. Marshall stated that most of the metals 

data are for total metals, but they do have some results for dissolved 

analyses. 

• EPA stated that they would like to see the data that Marshall has available. 
Table 1. Site Visit Attendees 

Name Organization (Representing) Telephone E-maWilliam Bare Duke Energy (704) 622-0907 wjbare@duke-enDonna Burrell Duke Energy (828) 478-7820 dlburrel@duke-eBobbie Chambers Duke Energy (828) 478-7647 bpchambers@duBill Kennedy Duke Energy (980) 721-7626 wkennedy@dukRon Lewis Duke Energy (980) 373-5710 relewis1@duke-Debbie Nispel Duke Energy (317) 838-1957 dnispel@duke-eJay Perkins Duke Energy (704) 875-5348 jperkins@duke-Dave Styer Duke Energy (828) 478-7506 dwstyer@duke-eRobert Wylie Duke Energy (704) 382-4669 rrwylie@duke-eDonna Hill Southern Company (205) 257-5234 dbhill@southernKarrie-Jo Shell EPA/Region 4 (404) 562-9308 shell.karrie-jo@Josh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.gRon Jordan EPA/Office of Water (202) 566-1003 jordan.ronald@eT.J. Finseth ERG (Contractor to EPA) (703) 994-7130 thomas.finseth@
Figure 1. Process Flow Diagram of Solids Removal Portion of FGD Wastewater Treatment Sys
Figure 2. Process Flow Diagram of Constructed Wetland Portion of FGD Wastewater Treatment