Draft 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 desulfuriza tion (FGD) wastewater treatment system. These notes describe the information collected during the site visit. Table 1, 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 genera ting 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.

Unit Installation
Date Gross Capacity

(MW) Particulate Control NOx
Control SO 2 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 (SO 2 ) releases from the plant. The FGD system was installed in December 2009. The wet FGD scrubber is designed to achieve greater than 98 percent removal of SO 2 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 ca pacity of 179 MW. Prior to the installation of the FGD system, the net generating capacity for each unit was 182 MW, but theFGD 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 percent of the flue gas passes through a unit-specific ESP, and the remaining 60 percen t 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 coal used at the plant typically varies
between 2.0 and 3.0% contains 2.35% sulfur

. Half of the coal contains 0.08% chlorides
and the other half contains 0.1 to 0.28% chlorides.

 Dickerson operates selective SNCRs on a ll three units. The SNCR systems were installed in July 2009 to and remove 85

- 90% of NOx as needed during the May 1 to September 30 ozone season . Dickerson uses urea as the source for the ammonia in the
system. Dickerson operates the systems as needed between fr om May 1 to September 30 (ozone season).

 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 SO 2 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 treatm ent plant (IWWTP) is used to treat boiler blowdown, treated sewage wastewater effluent , coal pile runoff, low volume waste

sources, parking lot storm water runoff , metal cleaning waste and decanted water from the bottom ash hydrobins. [QUESTION FOR DICKERSON: Please confirm the wastewater streams transferred to the industrial wastewater treatment plant.] Dickerson uses 100% of the industrial wastewater treatment plant effluent as make up to the plant, largely as make up for the FGD system. [this is not always the case. Sometimes we don’t need all the water from the system and it discharges to the river]

 The IWWTP includes flocculation, polymer addition, and clarification (no filters).
Chlorides are not an issue for the IWWTP.

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 Formatted: Not Highlight

Formatted: Not Highlight

Formatted: Not Highlight

Comment [RRI1]: Don’t know what this meanshoppers 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. Dickerson is planning to convert the vacuum systems to pressure systems in 2011. [QUESTION FOR DICKERSON: Has this conversion been completed? No, not done. ]

 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 [where was it sent prior?]

. it was marketed.

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

 Landfill Cell “C” is The old/ inactive landfill cell was filled after 10 years . 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 isfrom the hydrocyclones, which is approximately 28 percent 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 hydroc yclones 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. Dick erson 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 clar ifier 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. Thedenitrifying 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 th e 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 1. Site Visit AttendeesNameOrganization (Representing)TelephoneE-mailClark Phebus Mirant Mid-Atlantic, LLC (301) 601-6605clark.phebus@mirant.comNeha Rustagi Mirant Mid-Atlantic, LLC (301) 601-6515neha.rustagi@mirant.comRay D’alesaondroMirant Mid-Atlantic, LLC (404) 917-3987raymond.dalesondro@mirant.comAnn Wearmouth Mirant Mid-Atlantic, LLC (240) 299-3378ann.wearmouth@mirant.comMike Parr Mirant Mid-Atlantic, LLC (301) 601-6520mike.parr@mirant.comKevin McLeod Mirant Mid-Atlantic, LLC (301) 601-6581kevin.mcleod@mirant.comJay Bellingham Mirant Mid-Atlantic, LLC (301) 601-6521jay.bellingham@mirant.comDonna HillSouthern Company/UWAG(205) 587-4846 dbhill@southernco.comJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.govRon Jordan EPA/Office of Water (202) 566-1003 jordan.ronald@epa.govBill Swietlik EPA/Office of Water (202)566-1129 swietlik.william@epa.govCuc Schroeder EPA/Office of Water (202) 566-2397 schroeder.cuc@epa.govT.J. Finseth ERG (Contractor to EPA)(703) 633-1698 thomas.finseth@erg.comKavya Kasturi ERG (Contractor to EPA) (703) 633-1701 kavya.kasturi@erg.com