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# 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-02-0952 Work Assignment 7-29

The fresh limestone slurry is fed to the r eaction tank of the FGD system where the slurry is pumped to the various levels of the FGD sc rubber with the recycle pumps. The FGD system has three levels of flooding headers. [QUEST ION FOR DICKERSON: Ho w 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 head ers (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 counter current to the limestone slurry. The droplets of the slurry contact the flue gas and absorb the SO 2 , 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 scrubb er until they reach the reaction tank. Dickerson bubbles air through the reaction tank to oxidize th e 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 le ss 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 tran sfers the blowdown from the scrubber to the gypsum dewatering operations.

The scrubber blowdown is pumped to the pr imary hydrocyclones where the solid gypsum is separated from the water by centrifugal force. Th e 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 tr ansferred to the vac uum filter belts. The overflow from the primary hydrocyclones is pumped to the sec ondary 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 Filt rate Tank. Underflow would only be during start up and periods of recirculation of flow. The underflow from the sec ondary 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 th en pumped to the FGD wastewater treatment
system.

Dickerson washes the gypsum sent to the v acuum 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 ye ar 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 DICK ERSON: How much gypsum is generated per year?] 150,000 tons per year.3 1.1 FGD Wastewater Treatment System

The FGD wastewater treatment system is de signed 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 ra te is approximately XXX 84,000 gpd average gallons per day (gpd). [QUESTI ON 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 scrubbe r 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 th is section, present pro cess 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 diag ram of the FGD wastewat er 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 transfe rred to the purge tank and then sent to the equalizati on tank at the start of the wast ewater treatment system. This wastewater is expected to cont ain approximately 1 percent soli ds and have a pH of 5.5 to 6.5 S.U. The equalization tank also receives water fro m the filtrate sump (which includes filter press filtrate, backwash reject from the continuous back wash gravity filters, building trench drains, and tank overflow).4 From the equalization tank, the wastewater en ters reaction tank 1 in which lime slurry is added to increase the pH to 8.3 S. U. to precipitate the soluble me tals as insoluble hydroxides and oxyhydroxides. The plant also a dds organosulfide (i.e., TMT-15) to reaction tank 1 to form sulfide complexes which have very low solubilit y products. Wastewater th en 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 react ion 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 FO R 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 clar ifier 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 S BR 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 clarif ier is transferred to a sludge holding tank; however, XXX percent of the clarifie r sludge is recycled to reaction tank 1 to act as seed crystals for gypsum nucleation. [QUESTION FOR DICKERSO N: 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 recircul ated 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 fo r 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 landfil l (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 filte rs is transferred to the SBR in fluent 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. Hypoc hloric acid (to reduce the pH of the water) and a carbon source (i.e., glycerin, to promote cell growth in the biol ogical treatment process) are also added to the SBR influent equalization tank. [QUESTION FO R DICKERSON: At wh at food:mass ratio do you operate?] (This varies from 4:1 to 8:1 depend ant 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 erec ted tents to cover the tanks. Dickerson plans to construct a buildi ng that will enclose the SBR by the winter of 2010/2011. [Why is the plant constructing the building around the SBR tanks?]5 Each batch of the SBR consists of th e following five cycles [QUESTION FOR
DICKERSON: What is the HRT of the biological portion of the treatment system (i.e., SBR influent equalization tank thr ough treated wastewater storage ta nk)? Also, what is the hydraulic
retention time of the SBR itself and th e duration of each treatment cycle?]: HRT is approximately 16 hours utilizing operating leve l conditions. SBR mi nimum HRT is 4 hours.

 Fill

o Anoxic: Food is loaded into the basi n 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 e ffluent is decanted allowing the biomass to settle in quies cent 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 tran sferred 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. B ackwash reject returns to the SBR influent equalization tank for reprocessing. The filtered effl uent is conveyed via gravity to the treated water storage tank where bi-sulfite is added.

The SBR system at Dickerson reduces tota l nitrogen in the waste stream from XXXX 81 PPM is design average to less than 4 ppm in the disc harge. [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 efflue nt concentrations of phosphate and selenium?] Typically, SBR systems are not designed to treat wastewaters high in TDS. A typical municipal wastewater treatment plant receives wate r with 2,000 to 3,000 ppm TDS. Dickerson’s FGD wastewater contains approximately 35,000 to 40,000 ppm TDS. Di ckerson, therefore, obt ained 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 disc harge. [QUESTION FOR DICKERSON: Where is6 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 Dicker son’s FGD wastewater treatment system is located downstream of the purge tank and upstream of the equalizat ion tank. Thus, the influe nt sampling point is upstream of any chemical additi on or treatment system recircul ation flow streams. The inprocess sampling point within Di ckerson’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 Dicker son’s FGD wastewater treatment system is located downstream of the tertiary continuous backwa sh gravity filters and upstream of treated water storage tank.

[QUESTION FOR DICKERSON: Is this diagram accu rate? 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 Sulf ite are not used