Final Notes from Site Visit at Mirant Mid-Atlantic, LLC’s Dickerson Generating Station 

on March 9, 2010 

EPA conducted a site visit at Mirant Mid-Atlantic, LLC’s Dickerson Generating Station 

(Dickerson) located in Dickerson, MD on March 9, 2010 to learn about the operations at the 

plant, specifically the flue gas desulfurization (FGD) wastewater treatment system. These 

notes describe the information collected during the site visit. Table 2, located at the end of 

these notes, identifies the site visit participants. The presentation that the Mirant Mid-Atlantic, 

LLC personnel gave during the site visit is provided as Attachment 1 to these notes. 

Additionally, a process flow diagram for the FGD system (Attachment 2), a process flow 

diagram for the FGD wastewater treatment system (Attachment 3), and a process description of 

the FGD wastewater treatment system (Attachment 4) are provided as attachments to the notes. 

Plant Overview 

 Dickerson operates three coal-fired generating units, two combustion turbines, and one 

black start combustion turbine at the power plant. Units 1, 2, and 3, which began 

operation in 1959, 1960, and 1962, respectively, each have a gross generating capacity 

of 191 megawatts (MW). The total net generating capacity for the three coal-fired and 

two combustion turbines is 871 MW. The coal-fired units are operated as intermediate 

units (i.e., less frequent than baseload, but more frequent than cycling). The table 

below contains some general information for the three coal-fired generating units. 

Table 1. Steam Electric Generating Unit Operations 

Unit 

Installation 

Date 

Gross 

Capacity 

(MW) Particulate Control 

NOx 

Control 

SO2

Control 

1 1959 191 Electrostatic 

precipitator (ESP) & 

common baghouse 

Selective 

NonCatalytic 

Reduction 

(SNCR) 

Common 

Wet FGD 

installed in 

December 

2009 

2 1960 191 ESP & common 

baghouse 

SNCR 

3 1962 191 ESP & common 

baghouse 

SNCR 

 Dickerson recently retrofitted the three coal-fired units with a single common FGD 

scrubber system to control the sulfur dioxide (SO2) releases from the plant. The FGD 

system was installed in December 2009. The wet FGD scrubber is designed to achieve 

greater than 98% removal of SO2 from the flue gas. With the recent addition of the 

FGD scrubber system, Dickerson also installed an FGD wastewater treatment system to 

control the discharges of the wastewater generated during the operation of the FGD 

scrubber. 
 Each coal-fired unit has a net generating capacity of 179 MW. Prior to the installation 

of the FGD system, the net generating capacity for each unit was 182 MW, but the 

FGD system and FGD wastewater treatment system have a parasitic load of 9 MW (3 

MW from each unit). 

 Dickerson operates ESPs and a baghouse to control fly ash and particulate emissions 

from the plant. For each coal-fired unit, 40% of the flue gas passes through a unitspecific ESP, and the remaining 60% of the flue gas is sent to a common baghouse used 

for all three units. 

 The Potomac River is the water source for the plant. Dickerson also uses treated 

wastewater from the industrial wastewater treatment plant on-site. 

 Coal is delivered by train. The sulfur content of the coal used at the plant typically 

varies between 2.0 and 3.0%. Half of the coal contains 0.08% chlorides and the other 

half contains 0.1 to 0.28% chlorides. 

 Dickerson operates selective SNCRs on all three units. The SNCR systems were 

installed in July 2009 to remove NOx as needed during the ozone season (i.e., May 1 to 

September 30). Dickerson uses urea as the source for the ammonia in the system. 

 Dickerson has one FGD system to service all three coal-fired units. The FGD system 

came online on Dec 21, 2009. The system contains three levels of flooding headers and 

one tray. The system is designed to remove 98% of the SO2 in the flue gas. 

 Together the FGD and SNCRs reduce ionic Hg by more than 80%. 

 Dickerson has a no net increase limit for nitrogen in its NPDES permit because the 

wastewater is discharged to a tributary of the Chesapeake Bay. To offset the nitrogen 

load, Dickerson treats its make up water to reduce nitrogen. Dickerson measures the 

nitrogen concentration and flow volume monthly. 

 An onsite industrial wastewater treatment plant (IWWTP) is used to treat boiler 

blowdown, treated sewage effluent, coal pile runoff, low volume waste sources, storm 

water runoff, metal cleaning waste and decanted water from the bottom ash hydrobins. 

Dickerson typically uses 100% of the industrial wastewater treatment plant effluent as 

make up to the plant, largely as make up for the FGD system; however, sometimes the 

plant does not need all the water from the system and in these cases, the additional 

water is discharged to the river. 

 The IWWTP includes flocculation, polymer addition, and clarification (no filters). 

Ash Handling 

 Dickerson uses dry fly ash handling systems to convey the fly ash from the hoppers 

under the ESPs and baghouse to the storage silos. Fly ash from the Unit 1 and 2 ESP 

hoppers are conveyed to one silo, while the fly ash from the Unit 3 ESP hoppers are 
conveyed to another silo. The fly ash collected in the baghouse hoppers can be directed 

to either of the silos. 

 The ESPs are equipped with a vacuum conveyance system while the baghouse is 

equipped with a pressure conveyance system. 

 Bottom ash is handled by a wet system. The bottom ash sluice is pumped to one of two 

hydrobins for dewatering. One hydrobin receives ash while the other is decanted. The 

decant and overflow water is transferred to a surge tank and is then reused without 

treatment (i.e., Dickerson does not adjust the pH prior to reuse). Excess water from the 

system is transferred to the onsite IWWTP. 

 Both fly ash and bottom ash are transported via dump truck to a landfill (Westland Ash 

Storage Site) located across the street from the plant. The landfill is operated by Mirant 

Ash Management, which is a separate part of Mirant Mid-Atlantic, LLC. Dickerson 

personnel noted that the bottom ash has only been sent to the landfill for approximately 

one year. Prior to sending the bottom ash to the landfill, Dickerson marketed the ash.

 The landfill is lined with an earthen clay liner, but Dickerson is in the process of 

installing an HDPE liner. 

 Landfill Cell “C” is inactive. The active cell “B” has received ash for 30 years and is 

not yet closed. 

 Dickerson built a leachate collection pond to manage the leachate from the landfill. 

According to Dickerson personnel, leachate has recently begun entering the pond via 

hard piping. The pond does not receive water continuously. The plant uses some of the 

water from the leachate pond as dust control for the landfill. Additionally, the plant 

discharges some of the water via the NPDES outfall. The plant also has an emergency 

overflow permitted outfall. Water is discharged using pumps or overflow (in case of 

emergency). Dickerson monitors the pond discharges for TSS, pH, iron and the full 

suite of metals. 

FGD System 

 Dickerson operates one forced oxidation wet FGD system, which services all three 

units. The FGD system has three flooding headers and one tray. A diagram of the 

FGD system is provided as Attachment 2 to these notes. 

 Dickerson uses a limestone slurry as the sorbent for the sulfur dioxide absorption. The 

limestone is delivered to the plant via rail. Dickerson uses ball mills onsite to pulverize 

the limestone. From the ball mills, the slurry is transferred to hydrocyclones to collect 

limestone particles that are too large for use in the FGD system. The underflow from 

the hydrocyclones is returned to the ball mill for additional processing. The overflow is 

from the hydrocyclones, which is approximately 28% solids (by weight), is transferred 

to a storage tank prior to being fed to the FGD system. 
 The fresh limestone slurry is fed to the reaction tank of the FGD system and the recycle 

pumps pump the slurry to the various levels of the FGD absorber. The FGD absorber 

has three levels of flooding headers. As the slurry falls downward and the flue gas 

flows counter-current to the slurry, the spray droplets of the limestone slurry contact the 

flue gas and absorb the sulfur dioxide, which reacts with the limestone particles and 

produces calcium sulfite. Additionally, the dual flow tray generates a layer of sorbent 

slurry in the absorber that the flue gas must pass through, which provides additional 

contact time for the reactions to occur. The sorbent slurry passes through the tray and 

flows downward in the absorber until it reaches the reaction tank. Dickerson bubbles air 

through the reaction tank to oxidize the calcium sulfite to calcium sulfate (i.e., 

gypsum). 

 Dickerson does not use any additives in the FGD system. 

 Dickerson intermittently blows down the gypsum slurry from the reaction tank to 

control the chlorides concentration in the absorbers. According to plant personnel, the 

FGD blowdown frequency depends on the stability of the chlorides concentration, 

temperature, and the gypsum density, but typically the blowdown occurs for three to 

four hours and then the system builds up for a 6 hour period. The chlorides 

concentration in the absorbers is typically around 15,000 ppm, with a design maximum 

chlorides concentration of 20,000 ppm. The typical percent solids within the absorber is 

18 to 22 percent. 

 Dickerson transfers the blowdown from the absorber to the gypsum dewatering 

operations. The blowdown is first transferred to the primary hydrocyclones where the 

solid gypsum is separated from the water by centrifugal force. The solid gypsum is 

forced outward to the walls of the hydrocyclones and falls downward, while the water 

exits the top of the hydrocyclones. The gypsum from the primary hydrocyclones is 

transferred to the vacuum belt filters. Dickerson washes the gypsum with service water 

as it is applied to the vacuum belt filter to reduce the chlorides in the gypsum. The 

filtrate from the vacuum belt filter is transferred to the FGD filtrate tank and returned to 

the FGD absorber. 

 Dickerson has a 20-year contract to sell the gypsum to LaFarge. The gypsum must meet 

the following specifications to be sold to LaFarge: 94-99% purity, 9% moisture, and 

<100 ppm chlorides. Dickerson generated two to three days of off- specification 

gypsum during the start up of the system. The off- specification gypsum was sent to a 

waste management site in Virginia. 

 The overflow from the primary hyrdrocyclones is pumped to the secondary 

hydrocylones. The underflow from the secondary hydrocyclones is transferred to the 

FGD filtrate tank and returned to the FGD absorber. The overflow from the secondary 

hydrocyclones is transferred to the purge tank and then pumped to the FGD wastewater 

treatment system. 
FGD Wastewater Treatment System 

 Dickerson’s FGD wastewater treatment system includes physical/chemical treatment 

(which removes TSS and metals), biological treatment in sequencing batch reactors 

(which removes 85-95% nitrogen), and sludge dewatering. A flow diagram and process 

description of the FGD wastewater treatment system are provided as Attachments 3 and 

4 to these notes. 

 At the time of the site visit, Dickerson was still fine-tuning the system for metals 

removal. Dickerson personnel expected that it would take one to two more months to 

finish tuning the system. 

Physical/Chemical Treatment 

 The wastewater from the purge tank, which contains secondary hydrocylcone overflow, 

is transferred to the equalization tank of the FGD wastewater treatment system at a 

typical flow rate of 75 gpm and a maximum flow rate of 110 gpm Flow from the filtrate 

sump pumps, which includes filter press filtrate, backwash reject from the continuous 

backwash gravity filters, building trench drains, and tank overflow, is also directed to 

the equalization tank. 

 Wastewater from the equalization tank is pumped to reaction tank 1 via wastewater 

forwarding pumps. Dickerson adds lime and organosulfide into reaction tank 1. The 

plant receives the lime slurry directly from a vendor (i.e., the plant does not slake the 

lime on site). The lime is added to increase the pH to 8.3 S.U. to precipitate the soluble 

metals as insoluble hydroxides and oxyhydroxides. The organosulfide (i.e., 

trimercapto-s-trianzine (TMT-15)) is added to form sulfide complexes with very low 

solubility products. 

 Dickerson is evaluating the use of GE’s MetClear and Nalco’s Nalmet 1689 for the 

organosulfide addition. According to plant personnel, the TMT-15 can adversely affect 

the clarifier operation and sludge bacteria. 

 Water from reaction tank 1 flows via gravity to reaction tank 2, where the plant adds 

ferric chloride to form a dense flocculant and enhance the settling characteristics of the 

precipitate. The ferric chloride also promotes iron co-precipitation. 

 The wastewater then flows to a solids contact clarifier. The plant adds a polymer to the 

wastewater at the inlet to the clarifier. Sludge from the clarifier is either pumped to the 

sludge holding tank and then dewatered in a filter press or recycled back to reaction 

tank 1 to act as seed crystals for gypsum nucleation. 

 The plant adds hydrochloric acid to the clarifier overflow to reduce the pH to 7 S.U.to 

prevent the wastewater from damaging the bacteria present in the biological system.

 The plant has the capability of diluting the wastewater with make-up water if the 

chlorides concentration is greater than 15,000 ppm, which could harm the bacteria. 
Sequencing Batch Reactors 

 The SBRs at Dickerson are currently open to the atmosphere, but they have a large tarp 

covering them. The plant plans to install a building to house the SBRs by the winter of 

2011. 

 The wastewater from the clarifier is transferred to the SBR influent equalization tank. 

Dickerson then pumps the wastewater to a non-contact heat exchanger designed to 

reduce the temperature from between 112 and 135°F (typically 118°F) to 95°F. 

Temperature and pH adjustment are required to create favorable operating conditions 

for the SBRs. The optimal temperature for the SBRs is 85°F. 

 From the heat exchangers, Dickerson then returns the wastewater to the SBR influent 

equalization tank. The influent typically contains 80 ppm total nitrogen, including 

approximately 70 ppm nitrates, 5 ppm organic nitrogen, and 5 ppm ammonia. Organic 

nitrogen can vary significantly based on storms and has been recorded as high as 40 

ppm. 

 Hypochloric acid and a carbon source (i.e., glycerin) are added in the SBR influent 

equalization tank. 

 Dickerson uses activated sludge in the sequencing batch reactors to reduce nitrogen to 

less than 4 ppm in the discharge. The system does not remove sulfur nitrogen organics. 

The SBR does achieve phosphate removal. According to plant personnel, the system 

could potentially achieve selenium removal, if the system was operated to reach the 

ORP needed for selenium reduction. The plant didn’t have any data to determine 

whether selenium was reduced by the system. 

 Typical activated sludge biological systems are not designed to operate in high TDS 

waters. A typical municipal wastewater treatment plant receives water with 2,000 to 

3,000 ppm TDS, while FGD water may have between 5,000 and 70,000 ppm TDS. 

Dickerson obtained seed bacteria from a paint pigment facility’s wastewater treatment 

plant (at the Eastern Shore). 

 From the SBR influent equalization tank, Dickerson pumps the wastewater, in batches, 

to one of the two sequencing batch reactors. The SBR has five different stages, which 

occur in following order for each batch: fill (anoxic and aerated), react, settle, decant, 

idle. Each of these stages is described below: 

 Fill 

 Anoxic: Food is loaded into the basin and soluble BOD is absorbed and stored 

by the facultative biomass until air is received to metabolize the food. Good 

settling, facultative organisms predominate. 

 Aerated: Air is pumped in and mixed to initiate metabolism. During the initial 

stages of Aerated Fill, both nitrification and denitrification occur. The ammonia 

is converted to nitrates within the highly aerated plume of the jet. The nitrates are 

converted to nitrogen gas in the low dissolved oxygen areas of the basin. The 
denitrifying organisms use the BOD as a food source and the oxygen off the 

nitrates converting the nitrates to nitrogen gas. A secondary denitrification phase 

can be inserted at any point during the aerated cycle (Aerated Fill & React 

cycles). During this step, the blowers are turned off and the jet pump continues to 

mix, providing anoxic mixing to drive denitrification. 

 React: No additional flow or food is added forcing the organisms to scour for any 

remaining BOD. Aeration continues until complete biodegradation is achieved. The 

cycle continues until the biomass enters its “famine” state at which time the residual 

dissolved oxygen levels begin to rise quickly. 

 Settle: No influent is introduced and no effluent is decanted allowing the biomass to 

settle in quiescent conditions. 

 Decant: An automatic valve is opened and the upper one-third of the basin is 

discharged. 

 Idle: While waiting to receive flow settled sludge is wasted. 

 The plant pumps the decant from the SBRs to the SBR effluent equalization tank, 

where the plant adds hypochlorite to the wastewater. The effluent is then pumped to the 

tertiary continuous backwash gravity filters. Backwash reject is returned to the SBR 

influent equalization tank for reprocessing. The filtered effluent is conveyed via gravity 

to the treated water storage tank, where bi-sulfite is added. 

 According to plant personnel, the SBRs also have a standby mode where the sludge is 

aerated, glycerin is fed to the system, and the temperature is kept at the necessary level. 

The biomass can live up to ten years in standby mode. Before restarting the system, the 

biomass will require blowdown due to growth. It is also necessary to match up the 

chloride levels because, according to plant staff, increasing the TDS level by greater 

than 10% over a short period could kill the bacteria. 

Sampling 

 Dickerson currently measures the performance across the physical/chemical 

precipitation portion and the biological portion of the treatment system. 

 Samples are taken in various locations for TSS, dissolved chlorides, nitrates, ammonia, 

organic nitrogen, and pH. 

 EPA identified the following as potential sample points for the EPA sampling program: 

o Secondary hydroclone overflow before reaching equalization tank; 

o Gravity filter effluent; and 

o Solids contact clarifier overflow, prior to HCl addition (directly from the 

overflow trough).
Table 2. Site Visit Attendees 

Name Organization (Representing) Telephone E-maClark Phebus Mirant Mid-Atlantic, LLC (301) 601-6605 clark.phebus@miranNeha Rustagi Mirant Mid-Atlantic, LLC (301) 601-6515 neha.rustagi@miranRay D’alesandro Mirant Mid-Atlantic, LLC (404) 917-3987 raymond.dalesandroAnn Wearmouth Mirant Mid-Atlantic, LLC (240) 299-3378 ann.wearmouth@miMike Parr Mirant Mid-Atlantic, LLC (301) 601-6520 mike.parr@mirant.cKevin McLeod Mirant Mid-Atlantic, LLC (301) 601-6581 kevin.mcleod@miraJay Bellingham Mirant Mid-Atlantic, LLC (301) 601-6521 jay.bellingham@mirDonna Hill Southern Company/UWAG (205) 587-4846 dbhill@southernco.cJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.gov Ron Jordan EPA/Office of Water (202) 566-1003 jordan.ronald@epa.gBill Swietlik EPA/Office of Water (202) 566-1129 swietlik.william@epCuc Schroeder EPA/Office of Water (202) 566-2397 schroeder.cuc@epa.gT.J. Finseth ERG (Contractor to EPA) (703) 633-1698 thomas.finseth@erg.Kavya Kasturi ERG (Contractor to EPA) (703) 633-1701 kavya.kasturi@erg.c