1 

Draft 

Sampling Plan 

Mirant Mid-Atlantic, LLC’s 

Dickerson Generating Station 

Dickerson, NC 

Sampling Episode 6562 

Prepared for:

U.S. Environmental Protection Agency

Engineering and Analysis Division 

Office of Water 

1200 Pennsylvania Avenue, NW 

Washington, D.C. 20460 

Prepared by: 

Eastern Research Group, Inc.

14555 Avion Parkway 

Suite 200 

Chantilly, VA 20151 

July 29, 2010 

EPA Contract No. 68-C-02-095 
2 

Work Assignment 7-29 

 The fresh limestone slurry is fed to the reaction tank of the FGD system where the slurry 

is pumped to the various levels of the FGD scrubber with the recycle pumps. The FGD system 

has three levels of flooding headers. [QUESTION FOR DICKERSON: How is the operation of 

flooding headers different from spray headers? How often does each level operate? What 

determines when each level operates?] Flooding headers function is to supply liquid to the dual 

flow tray. The amount of flow to these headers (number of recycle pumps) dictates removal 

efficiency, not the number of headers in service. As the slurry falls downward in the scrubber 

vessel, the flue gas flows countercurrent to the limestone slurry. The droplets of the slurry 

contact the flue gas and absorb the SO2, which reacts with the limestone particles and produces 

calcium sulfite. The droplets continue to flow downward in the scrubber until they settle on the 

tray, forming a layer of slurry that the flue gas must pass through to exit the FGD system. 

[QUESTION FOR DICKERSON: Does the plant have a dual flow tray in the FGD system? If 

not, what type of tray is used in the FGD system?] The tray is a B&W dual flow tray. The spray 

droplets continue to flow downward in the scrubber until they reach the reaction tank. Dickerson 

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

gypsum). 

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

the chlorides concentration in the scrubber to less than 20,000 parts per million (ppm). Typically, 

the chlorides concentration in the scrubber is about 15,000 ppm. The solids concentration in the 

scrubber ranges from 18 to 22 percent solids. Dickerson transfers the blowdown from the 

scrubber to the gypsum dewatering operations. 

 The scrubber blowdown is pumped 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 filter belts. The 

overflow from the primary hydrocyclones is pumped to the secondary hydrocyclones. 

[QUESTION FOR DICKERSON: The mass balance diagram provided at the site visit shows 

that primary hydrocyclone overflow and underflow can both go to the FGD filtrate tank. How 

often and under what circumstances does this occur?] Normal operation is for primary 

hydroclone overflow to be discharge to the Filtrate Tank. Underflow would only be during start 

up and periods of recirculation of flow. The underflow from the secondary hydrocyclones is sent 

to the FGD filtrate tank for recycle back to the absorber. The overflow from the secondary 

hydrocyclones is sent to the purge tank and then pumped to the FGD wastewater treatment 

system. 

 Dickerson washes the gypsum sent to the vacuum filter belts with service water, as the 

gypsum is poured onto the belts. The vacuum filter belts dry the gypsum to a moisture content of 

approximately 10 percent. Dickerson has a 20 year contract to sell gypsum that is 94 to 99 

percent pure, has a moisture content of less than 9 percent, and has less than 100 ppm chlorides. 

Off-spec gypsum and other solids are sent to a waste management site in Virginia. Dickerson 

generated 2 or 3 days of off-specification gypsum at the beginning of the FGD start up, but has 

always met the specifications since that time. Dickerson expects to generate approximately XXX 

tons of gypsum per year. [QUESTION FOR DICKERSON: How much gypsum is generated per 

year?] 150,000 tons per year.
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1.1 FGD Wastewater Treatment System

 The FGD wastewater treatment system is designed to receive intermittent transfers of 

FGD wastewater from the purge tank, which contains secondary hydrocyclone overflow, at a 

typical flow rate of 75 gallons per minute (gpm) and a design maximum flow rate of 110 gpm. 

[QUESTION FOR DICKERSON: What is the frequency and volume of purge tank flow to the 

wastewater treatment system?] Water is typically discharged to the WWTS 4-6 times per day 

dependant on unit load. The total daily flow rate is approximately XXX 84,000 gpd average

gallons per day (gpd). [QUESTION FOR DICKERSON: What is the total daily flow rate 

through the FGD wastewater treatment system?] 84,000 is average. Maximum is 158,000.

 The plant operates the FGD wastewater treatment system 24 hours per day. Dickerson 

installed the wastewater treatment system to treat the FGD scrubber purge wastewater to meet its 

permit limits, which include limits for total suspended solids (TSS), nitrogen, and fecal coliform. 

The Dickerson FGD wastewater treatment system contains the following treatment operations: 

 Physical/Chemical Separation; 

— Equalization, 

— pH adjustment (lime addition), 

— Organosulfide addition, 

— Ferric chloride addition, 

— Polymer addition, 

— Clarification; 

— Hydrochloric acid addition; and 

 Biological Reactors; 

— Influent equalization, 

— Cooling (heat extraction), 

— Carbon source addition, 

— Hypochloric acid addition, 

— Sequencing batch reactor (SBR) process, and 

— Effluent equalization, 

— Hypochlorite addition, 

— Filtration, and 

— Bi-sulfite addition. 

 Figures 2-1 and 2-2, located at the end of this section, present process flow diagrams of 

the FGD wastewater treatment system, as well as the planned sampling locations for EPA’s 

sampling episode. Figure 2-1 is a simplified diagram of the FGD wastewater treatment system, 

showing different portions of the system. Figure 2-2 is a diagram of the chemical precipitation 

and biological reactor portions of the treatment system. 

 The secondary hydrocyclone overflow from the scrubbers is transferred to the purge tank 

and then sent to the equalization tank at the start of the wastewater treatment system. This 

wastewater is expected to contain approximately 1 percent solids and have a pH of 5.5 to 6.5 

S.U. The equalization tank also receives water from the filtrate sump (which includes filter press 

filtrate, backwash reject from the continuous backwash gravity filters, building trench drains, and 

tank overflow). 
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 From the equalization tank, the wastewater enters reaction tank 1 in which lime slurry is 

added to increase the pH to 8.3 S.U. to precipitate the soluble metals as insoluble hydroxides and 

oxyhydroxides. The plant also adds organosulfide (i.e., TMT-15) to reaction tank 1 to form 

sulfide complexes which have very low solubility products. Wastewater then cascades by gravity 

into reaction tank 2 in which ferric chloride is added to form a dense flocculant, enhance the 

settling characteristics of the precipitate, and promote iron coprecipitation. [QUESTION FOR 

DICKERSON: Please confirm that organosulfide is added to reaction tank 1 and ferric chloride 

is added to reaction tank 2.] confirmed 

 The plant adds a polymer as the wastewater flows from reaction tank 2 to the solids 

contact clarifier. [QUESTION FOR DICKERSON: What polymer is used? How and where is the 

polymer added (e.g., through inline mixing at the exit of reaction tank 2)?] Anionic polymer 

introduced to the inlet of the clarifier.The solids contact clarifier contains a sludge bed that 

attracts the solids in the wastewater. As the clarifier overflow is transferred to the sequencing 

batch reactor (SBR) influent equalization tank, the plant injects hydrochloric acid into the 

wastewater to adjust the pH to 7 prior to the biological treatment system. After the hydrochloric 

acid addition, the wastewater flows to the SBR influent equalization tank. [QUESTION FOR 

DICKERSON: What is the HRT of the physical/chemical portion of the treatment system (i.e., 

equalization through clarification)?] HRT is approximately 16 hours utilizing operating level 

conditions. 

 The majority of the sludge from the clarifier is transferred to a sludge holding tank; 

however, XXX percent of the clarifier sludge is recycled to reaction tank 1 to act as seed crystals 

for gypsum nucleation. [QUESTION FOR DICKERSON: What percentage of the sludge is 

recycled to reaction tank 1? Is there an operating characteristic which determines how much is 

recycled?] Clarifier sludge is continually recirculated to reaction tank 1 at 50-60 gpm to 

maintain a 5% solids concentration of TSS in the flow out of reaction tank #1.. The sludge in the 

sludge holding tank is pumped to a filter press for dewatering. The filter press cake is transferred 

to a landfill and the filtrate is sent to the filtrate sump. [QUESTION FOR DICKERSON: Is the 

filter press cake transferred to the fly ash landfill (operated by Mirant and located near the plant), 

the waste management site in Virginia (where FGD solids and off-spec gypsum are sent), or 

elsewhere?] Dickerson performs approximately 5 filter press operations per day. 

 Water from the clarifier as well as the SBR decant flush and backwash reject from the 

tertiary continuous backwash filters is transferred to the SBR influent equalization tank where it 

is then pumped through heat exchangers. After the heat exchangers have reduced the temperature 

of the water to approximately 95 degrees Fahrenheit, the wastewater is recycled back to the SBR 

influent equalization tank. Hypochloric acid (to reduce the pH of the water) and a carbon source 

(i.e., glycerin, to promote cell growth in the biological treatment process) are also added to the 

SBR influent equalization tank. [QUESTION FOR DICKERSON: At what food:mass ratio do 

you operate?] (This varies from 4:1 to 8:1 dependant on desired MLVSS concentrations) Water 

is then batch transferred to two 178,705 gallon SBR tanks operating in parallel. 

 Dickerson uses an SBR to biologically treat the wastewater to remove nitrogen. 

Currently, the SBR tanks are open to the atmosphere, but the plant has erected tents to cover the 

tanks. Dickerson plans to construct a building that will enclose the SBR by the winter of 

2010/2011. [Why is the plant constructing the building around the SBR tanks?] 
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 Each batch of the SBR consists of the following five cycles [QUESTION FOR 

DICKERSON: What is the HRT of the biological portion of the treatment system (i.e., SBR 

influent equalization tank through treated wastewater storage tank)? Also, what is the hydraulic 

retention time of the SBR itself and the duration of each treatment cycle?]: HRT is 

approximately 16 hours utilizing operating level conditions. SBR minimum HRT is 4 hours. 

 

 Fill 

o 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. 

o Aerated: Air is pumped in and mixed to initiate metabolism. Nitrification 

and denitrification occur. 

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

remaining BOD. The cycle continues until the biomass enters its “famine” state. 

Dissolved oxygen levels begin to rise. 

 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. 

 During the decant cycle, the water is transferred from the SBR to the SBR effluent 

equalization tank where hypochlorite is added. [QUESTION FOR DICKERSON: Is this sodium 

or calcium hypochlorite?] this has been eliminated. The wastewater is then pumped to the 

tertiary continuous backwash gravity filters. Backwash reject returns 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. 

 The SBR system at Dickerson reduces total nitrogen in the waste stream from XXXX 81 

PPM is design average to less than 4 ppm in the discharge. [QUESTION FOR DICKERSON: 

What is the total nitrogen concentration entering the SBR? Is the effluent TN concentration of 

<4ppm before or after the gravity filters?] TN is after filters. The plant also states the SBR 

system achieves phosphate removal and may also achieve selenium removal. [QUESTION FOR 

DICKERSON: What are the effluent concentrations of phosphate and selenium?] Typically, 

SBR systems are not designed to treat wastewaters high in TDS. A typical municipal wastewater 

treatment plant receives water with 2,000 to 3,000 ppm TDS. Dickerson’s FGD wastewater 

contains approximately 35,000 to 40,000 ppm TDS. Dickerson, therefore, obtained seed bacteria 

for the system from a paint pigment facility’s wastewater treatment plant (at the Eastern Shore), 

rather than from municipal sludge. 

 Water in the treated water storage tank is monitored for NPDES compliance prior to 

commingling with the main cooling water discharge. [QUESTION FOR DICKERSON: Where is 
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treated water monitored and where are effluent limits applied?] The commingled water then 

discharges through a canal that carries it approximately 1⁄4 mile downstream where it is 

discharged through Outfall 001 to the Potomac River. 

 Figure 2-2 shows the planned sampling points for the sampling episode at Dickerson. The 

influent sampling point at Dickerson’s FGD wastewater treatment system is located downstream 

of the purge tank and upstream of the equalization tank. Thus, the influent sampling point is 

upstream of any chemical addition or treatment system recirculation flow streams. The inprocess sampling point within Dickerson’s FGD wastewater treatment system is located at the 

heat exchanger tank pump discharge sample tap (after HCl addition). The biological reactor 

effluent sampling point at Dickerson’s FGD wastewater treatment system is located downstream 

of the tertiary continuous backwash gravity filters and upstream of treated water storage tank. 

[QUESTION FOR DICKERSON: Is this diagram accurate? Please provide a final process flow 

diagram of the treatment system.] 

The backwash of the tertiary filter now goes to the Equalization tank via the Sump. 

Hypochlorite/Bi Sulfite are not used