Shaw Stone & Webster, Inc. - Stoughton, MA 1 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

SYSTEM DESCRIPTION 

 The treatment system(s) for all three (3) stations utilize the same treatment processes. 

 General 

The FGD purge stream is unsuitable for reuse by other power plant facilities and therefore 

must be treated separately and discharged. The characteristics of this wastewater stream 

require extensive treatment. A dedicated FGD wastewater treatment system offered for this 

application consists of three (3) major sub-systems: 

 Physical/Chemical Treatment 

 Biological Treatment System 

 Sludge Dewatering System 

The FGD purge stream is initially directed to the Purchaser-supplied equalization tank to 

partially attenuate any chemical or hydraulic fluctuations resulting from the FGD operations. 

In addition, the equalization tank shall receive flow from the filtrate sump pumps, which 

consists of filtrate from the filter presses, backwash reject from the continuous backwash 

gravity filters, building trench drains and tank overflow. 

At a fairly constant rate, flow from the equalization tank shall be pumped to reaction tank No. 

1. The wastewater will cascade by gravity to reaction tank No. 2, then on to the solids 

contact clarifier and on to the heat exchanger feed tank. Reaction tank No. 1 and No. 2 are 

elevated chemical conditioning tanks where heavy metal precipitation, reaction and 

coagulation are carried out prior to clarification. 

In reaction tank No. 1, pH adjustment/gypsum “desaturation” is conducted. In the pH 

adjustment/gypsum “desaturation” phase, hydrated lime is added to elevate the pH to 

between 8.5 and 9.2 to aid in the precipitation of soluble metals as insoluble hydroxides and 

oxyhydroxides. This operating pH range has been selected to achieve optimal metals 

reduction while minimizing the formation of magnesium hydroxide which can occur at higher 

pH. The other function of the reaction tank is to provide “desaturation” of gypsum from the 

wastewater which has a tendency to be supersaturated when received from the FGD 

process. If not brought to equilibrium, this supersaturation can result in gypsum scale 

formation in the downstream wastewater treatment plant equipment. In order to achieve the 

“desupersaturation” operation in a controlled fashion via a crystal growth mechanism, sludge 

is recycled from the downstream solids contact clarifier to provide seed crystals for gypsum 

nucleation. In addition, an organosulfide reagent is added to this tank to form organosulfide 

heavy metal complexes which have very low solubility products thus resulting in maximum 

heavy metal removal. Of particular importance for this project is the removal of mercury. 

In reaction tank No. 3, ferric chloride is added to form a dense flocculant and enhance the 

settling characteristics of the precipitate. Additionally, the hydrolyzed form of this coagulant 

provides precipitation sites for co-precipitation of other metals. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 2 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

One (1) 100 percent capacity solids contact clarifier will be provided for clarifying the 

chemically conditioned waste stream. Polymer shall be injected into each clarifier influent 

line in order to form a denser floc which in turn will enhance the settling characteristics. 

Overflow from the solids contact clarifier will flow by gravity to the heat exchanger feed tank 

for subsequent transfer to the heat exchangers and on to the SBR influent equalization tank 

for subsequent batch transfer to the biological treatment system. Upstream of the heat 

exchanger feed tank, HCl is dosed into the wastewater to reduce the pH to approximately 

7.5 to 8.0. A static type mixer is provided to enhance the pH adjustment process. 

In order to discharge the treated effluent into the receiving body of water, the 

physical/chemical treated effluent must undergo further treatment in order to reduce the total 

nitrogen concentration. 

The reduction of total nitrogen (ammonia nitrogen and nitrates) is achieved by the use of a 

biological treatment process. In order to be in compliance with the discharge limitations for 

total nitrogen, the biological treatment system must first convert ammonia nitrogen to 

nitrates. This process is termed nitrification and is achieved in the presence of oxygen thus 

the nitrification process is aerobic. After nitrification, nitrates are converted to elemental 

nitrogen (a gas) and this process is termed denitrification and is achieved in the absence of 

oxygen thus the denitrification process is anoxic. 

The nitrification, denitrification and subsequent clarification processes could be carried out 

in a continuous flow system with separate tankage for each process step. However, if a 

continuous flow process is utilized, there is very little operational flexibility as the various 

process steps (nitrification, denitrification and clarification) are fixed based on the tank 

volumes. 

Recognizing on FGD applications that design values versus that which may actually occur 

can vary, Siemens highly recommends the use of sequencing batch reactor (SBR) 

technology which greatly enhances the operational flexibility of the system. 

The proposed biological treatment system shall consist of a SBR influent equalization tank 

which is used to store the pH adjusted physical/chemical treatment effluent for subsequent 

batch transfer to the two (2) sequencing batch reactors which carry out the biological 

treatment process in adjustable batch cycles and a SBR effluent equalization tank which is 

used to store the batch biologically treated SBR effluent for subsequent polishing and 

transfer to the continuous backwash gravity filters for further suspended solids reduction. 

The filtered effluent will flow by gravity to the treated water storage tank and shall be 

subsequently conveyed to the point of discharge by the treated wastewater discharge 

pumps. During low flow conditions and/or if the pH/TSS of the treated wastewater is out of 

compliance, the discharge pumps shall be used to recycle flow back to the equalization tank 

for reprocessing. 

Sludge from the solids contact clarifier is pumped via the sludge recycle/transfer pumps to 

the sludge holding tank. Dewatering is achieved by recessed plate and frame filter presses. 

Filtrate from the dewatering operation, as well as any drain water, is directed to the filtrate 

sump and pumped to the equalization tank for subsequent treatment. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 3 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Equalization Tank 

The equalization tank (100 percent capacity) will receive flow from the FGD purge pumps 

and the various waste streams from the filtrate sump. 

The purpose of the equalization tank is to store the fluctuating flow/solids loads generated 

by the FGD system so that a fairly constant flow rate/solids load can be conveyed to the 

downstream treatment system. By obtaining-maintaining a fairly constant flow rate/solids 

load to the downstream treatment system, a fairly constant treatment performance can be 

achieved/maintained with less operator attention. 

As a means of blending the waste streams and in order to prevent the settling out of solids, 

the equalization tank will be equipped with a bridge-mounted propeller type mixer. 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Horsepower 20 15 15 

Mixing Intensity (HP/million gallons) 279 363.7 363.7 

Pumping Rate (gpm) 106,904 67,545 67,545 

Turnover Rate (once every) 40 seconds 36.6 seconds 36.6 seconds 

The equalization tank will also be equipped with anti-vortexing baffles, equally spaced 

around the perimeter of the tank. 

Wastewater forwarding pumps will be used to supply a fairly constant flow rate to reaction 

tank No. 1. The pumps will be controlled by an ultrasonic level control provided by Siemens. 

The equalization tank will be of field erected carbon steel construction provided by Shaw 

Stone & Webster, with an operating capacity based on a 1 ft-6 in. freeboard and as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 29 22 22 

Height (ft) 16 16 16 

Operating Capacity (gallons) 71,645 41,232 41,232 

Hydraulic Retention Time (hours) 5.97 6.25 6.25 

Transfer Rate (gpm) 263 145 145 

 Wastewater Forwarding Pumps A/B [P&ID D-603 - Sheet 02]

Two (2) 100 percent capacity, skid-mounted wastewater forwarding pumps will be provided. 

The wastewater forwarding pumps will be horizontal, centrifugal, rubber lined, belt-driven 

type. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 4 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Each wastewater forwarding pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm at ft TDH) 263 at 70 145 at 70 145 at 70 

Horsepower 15 7.5 7.5 

The wastewater forwarding pump set shall be provided with pneumatically actuated suction 

and discharge valves which will allow for automatic forward/back flushing of either the 

suction or discharge lines. 

The common discharge line of the wastewater forwarding pumps to reaction tank No. 1 will 

be equipped with a Siemens-supplied flow meter for monitoring/ controlling the forward flow 

rate to the reaction tank via the VFD. 

Each pump discharge will be provided with a pressure indicator. 

 Reaction Tank No. 1 

The wastewater forwarding pumps will discharge into the 100 percent capacity reaction tank 

No. 1. From reaction tank No. 1, the tank contents will be blended with the solids contact 

clarifier recycled solids along with lime, where pH adjustment/gypsum “desaturation” is 

conducted. 

The pH adjustment/gypsum “desaturation” process is achieved by the addition of hydrated 

lime, which elevates the pH to a range of 8.5 to 9.2. The addition of the hydrated lime is 

controlled by the influent flow meter and trimmed pH monitor/controller. In addition, an 

organosulfide reagent is added/blended with the influent stream to form organosulfide heavy 

metal complexes which have very low solubility products, thus resulting in maximum heavy 

metal reduction. 

Reaction tank No. 1 will be equipped with anti-vortex baffles and a bridge-mounted agitator 

to maintain solids suspension and to blend the liquid stream with the ferric chloride. The 

effluent from reaction tank No. 1 will flow to the reaction tank No. 2. The agitator will provide 

a mixing energy intensity as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Horsepower 7.5 7.5 7.5 

Mixing Intensity (HP/million gallons) 537.3 773.5 773.5 

Pump Rate (gpm) 19,761 19,761 19,761 

Turnover Rate (once every) 42.3 seconds 29.4 seconds 29.4 seconds 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 5 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Reaction tank No. 1 will be of steel construction and will provide the operating conditions as 

follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 12 10 10 

Height (ft) 18 18 18 

Operating Capacity (gallons) 13,958 9,693 9,693 

Hydraulic Retention Time (minutes) 48 60 60 

 Reaction Tank No. 2 

The wastewater forwarding pumps will discharge into the 100 percent capacity reaction tank 

No. 2. From reaction tank No. 2, the tank contents will be blended with the solids contact 

clarifier recycled solids along with lime, where pH adjustment/gypsum “desaturation” is 

conducted. 

The pH adjustment/gypsum “desaturation” process is achieved by the addition of hydrated 

lime, which elevates the pH to a range of 8.5 to 9.2. The addition of the hydrated lime is 

controlled by the influent flow meter and trimmed pH monitor/controller. In addition, an 

organosulfide reagent is added/blended with the influent stream to form organosulfide heavy 

metal complexes which have very low solubility products, thus resulting in maximum heavy 

metal reduction. 

A ferric chloride reagent is added/blended with the stream to form a dense flocculant which 

in turn enhances the downstream settling characteristics. In addition, the hydrolyzed form of 

this coagulant provides precipitation sites for co-precipitation of other metals. 

Reaction tank No. 2 will be equipped with anti-vortex baffles and a bridge-mounted agitator 

to maintain solids suspension and to blend the liquid stream with the ferric chloride. The 

effluent from reaction tank No. 2 will flow to the solids contact clarifier. The agitator will 

provide a mixing energy intensity as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Horsepower 7.5 7.5 7.5 

Mixing Intensity (HP/million gallons) 537.3 773.5 773.5 

Pump Rate (gpm) 19,761 19,761 19,761 

Turnover Rate (once every) 42.3 seconds 29.4 seconds 29.4 seconds 

Reaction tank No. 1 will be of steel construction and will provide the operating conditions as 

follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 12 10 10 

Height (ft) 18 18 18 

Operating Capacity (gallons) 13,958 9,693 9,693 

Hydraulic Retention Time (minutes) 48 60 60 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 6 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Solids Contact Clarifier 

The solids contact clarifier will be of the torque cage type with a sloped bottom influent/ 

recirculation column, a reaction cone and sludge scraper arms installed by the Purchaser 

within a coated circular carbon steel tank furnished by Shaw Stone & Webster. 

Polymer is added/blended with the influent stream to further enhance the settling 

characteristics. 

The solids contact clarifier will have an effective surface area, resulting surface settling rate, 

effective capacity, and hydraulic retention time as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 30 23 23 

Height (ft) 24 24 24 

Effective Surface Area (sq ft) 678.56 387.19 387.19 

Surface Settling Rate (gpm/sq ft) 0.30 0.29 0.29 

Capacity (gallons) 104,865 62,541 62,541 

Hydraulic Retention Time (hours) 6 6.47 6.47 

The reaction cone will effectively divide the solids contact clarifier into an inner flocculatingreaction zone and an outer settling zone. The flocculating-reaction zone will be equipped 

with a variable-speed up-draft style turbine mixer to mix the chemically conditioned influent 

stream and recirculated precipitate. The turbine will be installed within a draft tube extended 

to the bottom of the clarifier to ensure the recirculation of the precipitated solids. The design 

recirculation rate of the clarifier is approximately 11 times the forward flow rate. The draft 

tube will be equipped with recirculation inlet ports near the bottom and outlet ports near the 

liquid surface. 

The chemically conditioned influent will enter the draft tube and be blended with recirculated 

precipitate, which enters the draft tube near the bottom. The blended stream will exit near 

the liquid surface, discharge into the reaction cone (which provides solids contact), and flow 

downward. 

After the initial downward flow pattern, the influent will begin to flow horizontally and upward. 

The quiescent condition outside of the reaction cone is ideally suited for liquid/solids 

separation. 

Heavier suspended solids settle toward the bottom of the clarifier and clarified liquids flow 

upward toward the effluent collection troughs. 

The solids that settle out to the bottom of the clarifier are gently conveyed to the centrally 

located sludge hopper by the rotating sludge collector mechanism. The sludge collector 

mechanism will consist of a drive unit and two rake arms with scraper blades which will 

sweep the clarifier bottom twice each complete revolution. The output speed of the drive 

unit will be such that one revolution will occur approximately every 15 minutes. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 7 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

The clarifier will be equipped with an automatic rake lift mechanism which will be controlled 

by the torque monitor which will raise the rake mechanism up to 2 feet if a high torque load 

is encountered. 

The clarifier rake arm drive unit will be equipped with a torque overload device, drive and lift 

motors. 

Clarified liquid will be collected in effluent collection troughs located radially around the tank 

and will flow by gravity to the heat exchanger feed tank. The effluent collection troughs will 

be “L” shaped and will be equipped with V-notch weirs on both sides. 

The clarifier effluent to the heat exchanger feed tank will be injected with hydrochloric acid to 

reduce the pH to approximately 7.0. The discharge line downstream of the injection point 

will be equipped with a pH monitor/controller. 

 Heat Exchanger Feed Tank and Feed Pumps 

Clarified/pH adjusted effluent from the physical/chemical treatment portion of the wastewater 

treatment system shall be conveyed by gravity to the shop fabricated, FRP constructed heat 

exchanger feed tank for subsequent transfer by the heat exchanger feed pumps to/through 

the heat exchangers and on to the SBR influent equalization tank. 

The heat exchanger feed tank shall be based on the following: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 8 8 8 

Height (ft) 15 15 15 

Capacity (gallons) 6,016 6,016 6,016 

Hydraulic Retention Time (minutes) 28 50 50 

Two (2) 100 percent capacity, skid-mounted heat exchanger feed pumps will be provided. 

The heat exchanger feed pumps will be horizontal, centrifugal, vinyl ester type. 

Each heat exchanger feed pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm at ft TDH) 210 at 50 120 at 50 120 at 50 

Horsepower 7.5 5 5 

The common discharge line of the heat exchanger feed pumps to the heat exchangers will 

be equipped with a Siemens-supplied flow meter for monitoring/ controlling the forward flow 

rate to the reaction tank via the VFD. 

Each pump discharge will be provided with a pressure indicator. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 8 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Temperature Reduction; Heat Exchangers 

The downstream biological treatment system utilizes mesophilic bacteria. Therefore, the 

temperature of the waste stream needs to be reduced to approximately 95 degrees F in 

order to optimize the biological treatment process. 

Cooling of the waste stream will be achieved by the use of two (2) 100 percent capacity 

plate and frame heat exchangers. 

The waste stream (hot side) will be pumped to the heat exchangers by the heat exchanger 

feed pumps and following cooling will discharge to the SBR influent equalization tank. 

Plant service water will be used for cooling. The cooling water supply pressure shall be 75 

psig or greater and shall be used for flushing/plant utility water after passing through the 

heat exchangers. 

Each heat exchanger will be equipped with pressure and temperature indicators on hot/cool 

sides of the waste and cooling water lines. 

 

 SBR Influent Equalization Tank 

The downstream biological treatment system for the reduction of ammonia nitrogen and 

nitrates is a batch process. Therefore, the SBR influent equalization tank is used to store 

clarifier effluent for subsequent transfer to the sequencing batch reactors during a fill cycle. 

In addition, the SBR influent equalization tank shall receive backwash reject from the tertiary 

continuous backwash filters for reprocessing. 

The SBR influent equalization tank shall be equipped with a jet mixing system which will be 

utilized to blend the tank contents and to help keep the solids in suspension. The jet motive 

source shall be the contents of the SBR influent equalization tank and pumping source shall 

be the SBR feed pump (A or B) after the completion of a fill cycle. The jet mixing header 

shall be equipped with entrainment ports which result in entraining approximately 4 times 

the jet motive pump capacity resulting in a high turnover rate of the tank contents. 

The SBR influent equalization tank shall be field erect carbon steel construction by Shaw 

Stone & Webster and be based on the following: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 34 26 26 

Height (ft) 16.5 16.5 16.5 

Capacity (gallons) 101,865 55,597 55,597 

Volumetric Capacity 

(Equal to number of SBR batch 

cycle volumes) 

2.75 2.7 2.7 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 9 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 SBR Feed Pumps A/B [P&ID D-603 - Sheet 08]

Two (2) 100 percent capacity skid-mounted SBR feed pumps will be provided. The SBR 

feed pumps will be horizontal, centrifugal type of vinyl ester construction. 

Each SBR feed pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm) 310 170 170 

Head (ft TDH) 18 18 18 

Horsepower 5 3 3 

Each pump discharge will be provided with a pressure indicator. 

Each pump shall be equipped with a suction isolation valve, drain valve, discharge isolation 

valve and discharge check valve. 

 Sludge Recycle/Sludge Transfer Pumps A/B 

A portion of the clarifier settled solids will be periodically removed from the solids contact 

clarifier by the sludge recycle/sludge transfer pumps. They will recycle the settled solids 

back to the equalization tank to maintain the proper solids concentration within the reaction 

tank. 

Solids contact clarifier settled solids will be periodically removed from the solids contact 

clarifier by the sludge recycle/sludge transfer pumps. They will pump the solids during 

normal operation to the sludge holding tank. 

The pumps will be horizontal, belt-driven, rubber lined, centrifugal type. 

Each solids recycle/sludge transfer pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm) 160 70 70 

Head (ft) 70 70 70 

Horsepower 7.5 5 5 

The solids recycle/sludge transfer pump operation shall be periodically adjusted (cycled) by 

the operator based on the process needs (solids concentration in reaction tank and actual 

sludge yield). Based on the design conditions, the estimated solids recycle and sludge 

transfer rates are as follows: 

 Solids Recycle Rate 54% 

 Sludge Transfer Rate 46% 

The solids recycle/sludge transfer pump set shall be provided with pneumatically actuated 

suction and discharge valves which will allow for automatic forward/back flushing of either 

the suction or discharge lines. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 10 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Each recycle and transfer discharge lines will be equipped with a flow meter for monitoring 

the recycle and/or sludge transfer rate. The recycle line on each pump set shall be provided 

with a flow control valve for controlling the recycle rate. 

Each pump discharge will be provided with a pressure indicator. 

 Sequencing Batch Reactor 

Siemens’ Omniflo® Sequencing Batch Reactor process offers clear advantages not available in 

any other “periodic process”. Patented mechanical features including the Omniflo® process 

piping, ID/SC Manifold and decanter, combine with the Omniflo® batch proportional operating 

strategy to offer process flexibility and reliability unmatched by any other process. 

 Periodic Processes

 The Omniflo® Sequencing Batch Reactor process is a unique combination of equipment 

and controls for operating a complete secondary wastewater treatment facility. Because 

there are currently less than six manufacturers offering “periodic” processes, one must 

understand how each manufacturer addresses the treatment goals, in order to appreciate 

the unique advantages of the Omniflo® process. Periodic processes are simply batch 

activated sludge processes that accomplish, in discrete time periods within one tank, what 

conventional continuous flow plants accomplish in multiple tanks. 

 There is a widespread belief that periodic processes are a recent development, and still in 

the developmental stage. In truth, most early activated sludge plants were designed to 

operate in a batch treatment mode. Recent improvements in microprocessor control 

systems, automatic valves, and aeration equipment have made batch, or periodic, 

processes not only practical, but for most applications, very desirable. 

 The Sequencing Batch Reactor Process

 The sequencing batch reactor process is an activated sludge process, as are all periodic 

processes, but bears no similarity to the extended aeration process. A sequencing batch 

reactor operates in a true batch mode under non-steady state conditions. It provides for 

inherent flow equalization and flow blending, and greatly reduces operator skill and 

attention requirements. Controls range from a simplified float and timer based system to a 

PC based SCADA system with color graphics. 

 Siemens’ Omniflo® D.O. controlled aeration to reduce energy consumption and enhance 

selective pressures for BOD, nutrient removal and control of filaments. The Omniflo®

sequencing batch reactor has improved the technology in four major areas: 

 Control strategy 

 Mechanical equipment requirements 

 Decanter design 

 Sludge wasting 

 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 11 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 In a true batch sequencing batch reactor, there are five basic cycles in each batch. All 

cycles and variations of cycles are described as follows: 

 Fill 

 React 

 Settle 

 Decant 

 Idle 

 

 Anoxic Fill

 During Anoxic Fill, the basin is loaded with food in the influent through Siemens’ patented 

Influent Distribution/Sludge Collection Manifold (ID/SC) creating a high F:M ratio with zero 

D.O. conditions. The Fill period is primarily anoxic, or without aeration. Aeration is usually 

initiated late in the Fill period, and continues in the React period, after the influent flow has 

been diverted to another basin. The Influent Distribution Manifold (ID/SC) allows intimate 

contact of the influent (food) with the settled biomass in the sludge blanket throughout the 

length of the basin (please refer to the “Equipment” section of the manual for a complete 

description of the ID/SC Manifold). During this time, the soluble BOD is absorbed and 

stored by the facultative biomass until air is received to metabolize the food. The selective 

pressures exhibited on the biomass of zero D.O. in Anoxic Fill allows good settling, 

facultative organisms to predominate. 

 Aerated Fill

 Subject to the ammonia nitrogen concentration prior to the end on the fill cycle, the blowers 

and pumps are automatically turned on to provide air and complete mixing which initiates 

the “feast” environment for the biomass. The biomass begins to metabolize the food they 

have absorbed. They utilize the oxygen provided very rapidly with high D.O. uptake rates 

and low residual D.O. values. It is during the initial stages of Aerated Fill that 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 D.O. 

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. 

 Denitrification

 Siemens offers an exclusive secondary denitrification phase in which the blowers are 

turned off and the jet mixing pump continues to mix. This provides anoxic mixing to drive 

denitrification. Denitrification can be inserted at any point during the aerated cycle 

(Aerated Fill, React) through the control system. Typically, the denitrification step falls 

somewhere in the middle of the aerated cycle. The control system allows movement of the 

denitrification cycle by the operator for maximum flexibility. This can also be easily 

controlled via a Siemens D.O. Control System. 

 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 12 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 React

 React begins after the basin has completed filling, and the influent flow has been diverted 

to another sequencing batch reactor tank. No more food (influent) enters the reactor basin 

during React. This forces the organisms to scour for any remaining BOD. Aeration 

continues in the full reactor until complete biodegradation is achieved; mixed liquor is 

drawn through the ID/SC and used as motive liquid for the jet aerator. React continues 

until the food is consumed and the biomass enters its “famine” state. True React (no 

influent, no effluent) is critical to achieve intense famine conditions for optimum organism 

selection. Tests have shown that the food is consumed when the residual D.O. in the 

basin exceeds 2 mg/L, at which time the residual D.O. begins to rise quickly. Siemens 

uses self-cleaning D.O. probes to monitor the D.O. level and signal the blower and pump 

to cease operation, allowing energy conservation. 

 Decant

 Following the Settle cycle the effluent withdrawal, or Decant cycle, begins. Decant is 

initiated by opening an automatic valve. Treated effluent is discharged through the 

decanter from approximately 18 inches below the surface, avoiding discharge of any 

surface contaminates. Decant continues until approximately the upper one-third of the 

basin is discharged. 

 The treated effluent shall discharge into the sequencing batch reactor effluent equalization 

tank. 

 Perfect Quiescent Settle

 The biomass is allowed to settle in perfect quiescent conditions; no influent is introduced 

during settle and no effluent is decanted. Since the reactor is under true quiescent 

conditions during settle, 100 percent of the reactor capacity is available for liquid/solids 

separation. The relative loading rate during clarification is zero for the batch reactor. 

A conventional plant has loading rates ranging from 300 to 1,200 gpd per sq ft. 

 The control system allows Perfect Quiescent Settling in the cycle at flows up to 3.5 times 

average design flow, maintaining consistent treatment during heavy rain infiltration. 

 Idle/Waste Sludge

 While the reactor waits to receive flow, the automatic waste sludge valve shall open 

drawing settled sludge through the Influent Distribution Sludge/Collection Manifold (ID/SC) 

and static pressure shall convey the sludge to the sludge chest transfer wetwell. The jet 

motive liquid pump is utilized as a waste sludge pump. The settled sludge is withdrawn 

through the sludge collection manifold which runs the length of the basin. The multi-point 

sludge withdrawal yields the thickest sludge possible, reducing side stream sludge 

treatment operation and maintenance. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 13 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Sequencing Batch Reactor Tanks

The biological treatment system shall consist of two (2) field erect coated carbon steel tanks 

provided by Shaw Stone & Webster and as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 38 26 26 

Height (ft) 24.5 24.5 24.5 

Capacity, Total (gallons) 381,732 182,676 182,676 

Number of Batch Cycles/Day 8 8 8 

Cycle Duration (hours) 6 6 6 

MLSS concentration (mg/L) 3,000 3,000 3,000 

 SBR Jet Motive Pumps A/B/C 

Three (3) 50 percent capacity SBR jet motive pumps shall be provided to supply liquid 

motive to the SBR jet aeration/mixing headers. 

SBR jet motive pump A shall draw suction from SBR A and shall discharge to SBR jet 

aeration/mixing header A. SBR jet motive pump B shall serve SBR jet aeration/mixing 

header B. SBR jet motive pump C shall be a common standby. 

Each pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm) 1,465 732 732 

Head (ft TDH) 21 21 21 

Horsepower 20 10 10 

 SBR Effluent Equalization Tank 

At the end of each SBR batch cycle, roughly one-eighth of the total daily flow rate will 

discharge from the SBR during the decant cycle which has a duration of approximately 30 

minutes. In order to level off the flow to the downstream polishing steps, the system design 

incorporates a SBR effluent equalization tank. 

The SBR effluent equalization tank shall be as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 34 26 26 

Height (ft) 16.5 16.5 16.5 

Capacity (gallons) 101,865 55,597 55,597 

Volumetric Capacity 

(Equal to number of SBR batch 

cycle volumes) 

2.7 2.7 2.7 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 14 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

The SBR effluent equalization tank shall be equipped with a jet aeration/mixing header to 

add dissolved oxygen to the biologically treated effluent. 

 Tertiary Filter Feed Pumps A/B 

Two (2) 100 percent capacity tertiary filter feed pumps shall be provided to transfer 

biologically treated effluent from the SBR effluent equalization tank to the tertiary continuous 

backwash gravity filters for polishing. 

The tertiary filter feed pumps shall be of the horizontal, centrifugal type, of vinyl ester 

construction, and shall be provided with variable frequency drives for flow control. 

The tertiary filter feed pumps shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm) 210 113 113 

Head (ft TDH) 25 25 25 

Motor Horsepower 3 2 2 

 Tertiary Continuous Backwash Gravity Filters A/B 

The FGD wastewater treatment system will include a filtration step to reduce the TSS and 

enhance the reduction of TSS metals in the biologically treated effluent. 

Siemens’ design makes use of continuous backwash gravity filters because of their 

compactness, simple operation and low operating cost. 

The wastewater to be processed is transported into the filter by means of the feed line. The 

wastewater enters the filter bed through the supply pipe and the distributors. The 

wastewater is filtered as it flows through the media filter in an upward direction. The filtrate 

is discharged in the upper part of the filter. 

The filter media bed is continuously moving downward as the water flows up. The dirty sand 

is removed from the sand bed and washed, after which it is released back on the top of the 

sand bed. This sand cleaning operation is automatic and continuous. 

The sand circulation is based on the airlift principle, forcing a mixture of dirty sand and water 

upward through a central pipeline. The intensive scouring movements separate the 

impurities from the sand particles. At the top of the pipeline the air is released, the dirty 

water is discharged and drained to the filtrate sump for reprocessing. The sand settles in 

the washer. 

The washer with its well-designed hydraulic features is positioned around the airlift pipeline. 

The sand particles fall through the washer, where they are finally washed by a small amount 

of clean filtrate, flowing through the washer counter-currently. The filtrate flow is generated 

by a difference in discharge level between the filtrate and the wash water. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 15 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

The filter performance can be optimized in function of the feed water quality and the filtrate 

targets. The air flow through the airlift is adjustable, in order to vary the sand circulation 

speed. 

The filtered effluent shall be conveyed by gravity to the treated water storage tank. 

Backwash reject shall be conveyed to the SBR influent equalization tank for reprocessing. 

The design data for the gravity filter is as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Number of Units (100 Percent Capacity) Two (2) Two (2) Two (2) 

Total Surface Area (sq ft/filter) 53.8 32.28 32.28 

Filtration Rate: 

 gpm/sq ft with two (2) filters in service 

 gpm/sq ft with one (1) filter in service 

1.95 

3.9 

1.75 

3.5 

1.75 

3.5 

 Treated Water Storage Tank 

The filtered effluent will gravity flow to the treated wastewater storage tank. 

The 100 percent capacity tank will have a closed top and be of FRP construction. 

The design data for the tank is as follows: 

 Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Nominal Diameter (ft) 10 8 8 

Nominal Height (ft) 20 18 18 

Operating Capacity (gallons) 11,162 6,203 6,203 

Hydraulic Retention Time at Design 

Discharge Pumping Rate (minutes) 53 54 54 

The tank will be equipped with a level control for controlling the operation of the pumps. 

 Treated Wastewater Discharge Pumps A/B 

Two (2) 100 percent capacity, horizontal, centrifugal type treated wastewater discharge 

pumps will be provided, which will draw suction from the treated wastewater storage tank 

and convey the treated effluent to the point of discharge which it has been assumed to 

not require more than 65 ft TDH. 

The common discharge line of the treated wastewater discharge pumps will be equipped 

with a flow meter for monitoring/controlling the discharge flow rate via the variable frequency 

drive. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 16 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Each treated wastewater discharge pump shall be rated as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (gpm) 210 115 115 

Head (ft TDH) 65 65 65 

Horsepower 10 5 5 

 SBR Waste Sludge Pumps A/B 

A portion of the settled solids will be periodically removed from the SBRs by the SBR waste 

sludge pumps. They will convey the waste sludge to the sludge holding tank for subsequent 

dewatering. 

The pumps will be horizontal, vinyl ester, centrifugal type. 

Each SBR waste sludge pump shall be rated for 70 gpm at 50 ft TDH and shall be equipped 

with a 5 HP, 1800 rpm, 460-Volt, 3-Phase, 60-Hertz, TEFC motor. 

The discharge line will be equipped with a flow meter for monitoring the waste sludge 

transfer area. 

Each pump discharge will be provided with a pressure indicator. 

 Process Blowers 

A total of five (5) 25 percent capacity process blowers shall be provided to supply air 

(oxygen) to the various process users within the treatment system. 

The capacity of each process blower shall be as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Capacity (scfm) 240 112 112 

Discharge Pressure (psig) 11 11 11 

Horsepower 25 15 15 

Each blower shall be complete with a motor/blower base, V-belt drive, inlet filter, inlet 

silencer, discharge silencer, pressure indicator, check valve and pressure relief valve. 

 Sludge Holding Tank 

Solids removed by the sludge recycle/transfer pumps and SBR waste sludge pumps will be 

pumped to the sludge holding tank for subsequent dewatering. 

The 100 percent capacity sludge holding tank will be FRP construction, open top and 

equipped with anti-vortex baffles and shall be provided by Siemens. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 17 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

As a means of blending the solids and in order to prevent the settling out of solids, the 

sludge holding tank will be equipped with a bridge-mounted propeller type mixer. 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Horsepower 7.5 7.5 7.5 

Mixing Intensity (HP/million gallons) 334 454.6 454.6 

Pump Rate (gpm) 29,707 29,707 29,707 

Turnover Rate (once every) at H.W.L 45 seconds 33.3 seconds 33.3 seconds 

The estimated average sludge volume is as follows: 

Sludge 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

lbs/day (dry) 53,950 29,666 29,666 

Solids Concentration (%) 9.8 9.8 9.8 

gallons/day 66,028 36,204 36,204 

The design data for the sludge holding tank is as follows: 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Diameter (ft) 14 12 12 

Height (ft) 21 21 21 

Operating Capacity (gallons) 

(Based on 1 ft-6 in. Freeboard at 

H.W.L.) 

22,452 16,495 16,495 

Hydraulic Retention Time (hours) 8.16 10.9 10.9 

 Filter Press Feed Pumps 

On a batch basis, solids removed from the solids contact clarifier and SBRs shall be 

dewatered by recessed plate and frame filter presses. Based on the estimated sludge 

production, the proposed dewatering system will process the solids in the following number 

of batch cycles per day: 

Morgantown Station 7 

Dickerson Station 5 

Chalk Point Station 5 

The filter press feed pumps shall be used to transfer approximately two-thirds of the batch 

volume within approximately one-third of the batch cycle time and then pressurization shall 

occur transferring the balance of the batch volume and to pressurize the cake to enhance 

the solids concentration. 

The proposed dewatering system has been designed to allow simultaneous operation of 

both filter presses except for core air blow which shall be sequencing at approximately 

10-minute intervals. The core air blow requires a substantial amount of service air. By 

sequencing the core blows, the size of the air compressor system is minimized. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 18 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Two (2) 100 percent capacity, horizontal, centrifugal Discflo pumps shall be provided. Each 

pump shall be equipped with a 50 HP, 460-Volt, 3-Phase, 60-Hertz, TEFC, VFD controlled 

motor. 

Each pump discharge shall be equipped with a 0 to 120 psig pressure indicator. 

 Filter Presses A/B 

Two (2) 100 percent capacity recessed plate and frame filter presses shall be provided for 

dewatering the solids removed from the waste stream. The filter presses shall be located on 

an elevated mezzanine to simplify solids handling. 

Based on a solids density of 100 lbs/cu ft and a predicted dewatered cake dry solids 

concentration of 50 percent, the design solids volume will be as follows: 

Morgantown Station 1,079 cu ft 

Dickerson Station 594 cu ft 

Chalk Point Station 594 cu ft 

The filter presses proposed shall be semi-automatic and equipped with 1500 mm plates 

based on a 1.25-inch cake thickness will have rated capacity as follows: 

Morgantown Station 175 cu ft 

Dickerson Station 125 cu ft 

Chalk Point Station 125 cu ft 

A typical press cycle shall consist of the following steps and/or cycles: 

 PLC shall confirm press closure. 

 VFD controlled fast fill pump shall draw suction from the sludge holding tank and convey 

the solids to the filter press. 

 Filter press pump speed shall ramp up as pressure differential increases. 

 At the end of the pressurization cycle, compressed air shall be used to blowdown the 

press core (fill lines). 

 Following the core blow cycle, the press shall open and the plates shall be shifted to 

dislodge the cake from the plates which shall fall into roll-off boxes located on the 

ground level. 

 During the fill cycle, filtrate and core blow shall be conveyed to the filtrate sump to 

subsequent reprocessing. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 19 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Filtrate Sump/Return Pumps 

A below grade concrete sump shall be provided to collect filtrate from the filter presses, 

press area drains, secondary containment drain, and backwash from the continuous gravity 

filter. 

The filtrate sump shall be designed with inside dimensions as follows: 

Morgantown Station 16 ft wide by 16 ft long by 13.25 ft deep 

Dickerson Station 12 ft wide by 12 ft long by 13.25 ft deep 

Chalk Point Station 12 ft wide by 12 ft long by 13.25 ft deep 

Based on a 1-1/2 ft freeboard, a 3 ft pump submergence, and an 8 ft-3 in. high water level, 

the filtrate sump shall have an effective capacity as listed below which provides a volumetric 

capacity equal to approximately 2 filtrate volumes at design. 

Morgantown Station 15,798 gallons 

Dickerson Station 8,886 gallons 

Chalk Point Station 8,886 gallons 

Two (2) 100 percent capacity submersible type filtrate return pumps shall be provided. Each 

pump shall be rated at 50 ft TDH with a 20 HP, 460-Volt, 3-Phase, 60-Hertz motor and the 

following capacity: 

Morgantown Station 600 gpm 

Dickerson Station 400 gpm 

Chalk Point Station 400 gpm 

Each pump shall be equipped with a 0 to 50 psig pressure indicator and automatic flow 

valve. 

The liquid level within the filtrate sump shall be monitored by an ultrasonic level indicator/ 

transmitter. 

One (1) 100 percent capacity bridge-mounted propeller type agitator shall be provided to 

prevent the settling out of solids. The filtrate sump shall be equipped with four (4) antivortexing baffles. 

Morgantown 

Station 

Dickerson 

Station 

Chalk Point 

Station 

Horsepower 3 3 3 

Mixing Intensity (HP/million gallons) 189.9 337.6 337.6 

Pump Rate (gpm) 21,143 21,143 21,143 

Turnover Rate (once every) at H.W.L. 44.8 seconds 25.2 seconds 25.2 seconds 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 20 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Chemical Feeds 

 Lime [ 

The purpose of the lime is for pH adjustment. 

Lime slurry shall be delivered to the site in 10 percent lime slurry form and off loaded into 

the lime slurry storage tank. 

The lime feed system shall consist of a lime slurry storage tank (two for Morgantown) 

with agitator and duplex lime slurry feed pumps. Each lime storage tank shall be of FRP 

construction with a domed top, 14 ft diameter by 21 ft high with an effective capacity of 

22,452 gallons. 

The chemical is added to the process based on a predetermined dose and the flow rate 

to the system. 

 Sulfide 

The purpose of the sulfide is to form organosulfide heavy metal complexes to maximize 

heavy metal reduction. 

The sulfide is from Purchaser-supplied totes. The prepared chemical is added to the 

process based on a predetermined dose and the flow rate to the system. Two (2) 100 

percent chemical feed pumps will be provided on the sulfide feed skid. 

 Ferric Chloride 

The purpose of the ferric chloride is to promote the formation of dense flocculent which 

in turn enhances the settling characteristics and co-precipitation of metals. 

Ferric chloride is from Purchaser-supplied totes. The prepared chemical is added to the 

process based on a predetermined dose and the flow rate to the system. Two (2) 100 

percent chemical feed pumps will be provided on the ferric chloride feed skid. 

 Polymer (Clarification Process) 

The purpose of the polymer is to agglomerate the finer particles of flocculent and to 

enhance the coagulation process in the solids contact clarifier. 

The polyblend unit is used to mix the polymer from Purchaser-supplied totes with plant 

water. The prepared chemical is added to the process based on a predetermined dose 

and the flow rate to the system. Two (2) 100 percent chemical feed skids will be 

provided. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 21 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

 Hydrochloric Acid 

The purpose of the acid is to reduce the pH of the treated water. 

Hydrochloric acid is from Purchaser-supplied totes. The chemical is added to the 

process based on the pH of the water. One (1) chemical feed pump and one (1) spare 

are provided on the hydrochloric acid feed skid. 

The hydrochloric acid feed system shall include a HCl fume (caustic) scrubber. 

 Carbon Source Feed 

The purpose of the carbon feed is to promote cell growth within the biological treatment 

process. 

Carbon is from Purchaser-supplied totes. The chemical is added to the process based 

on the pH of the water. One (1) chemical feed pump and one (1) spare are provided on 

the carbon source feed skid. 

 Hypochlorite Acid 

The purpose of the hypochlorite is to breakpoint chlorinate the treated effluent to ensure 

low concentration of ammonia nitrogen; thus low total nitrogen in the final effluent. 

Hypochlorite acid is from Purchaser-supplied totes. The chemical is added to the 

process based on the pH of the water. One (1) chemical feed pump and one (1) spare 

are provided on the hypochlorite acid feed skid. 

 Bisulfite Feed 

The purpose of the bisulfite is to reduce the chlorine residual prior to discharge. 

Bisulfite is from Purchaser-supplied totes. The chemical is added to the process based 

on the pH of the water. One (1) chemical feed pump and one (1) spare are provided on 

the bisulfite feed skid. 

 Plant and Instrument Air System 

The treatment system will be provided with an air compressor package which will produce 

instrument quality compressed air to use within the system. 

Two (2) 100 percent capacity rotary screw type air compressors will be provided. Each air 

compressor shall be rated for 333 scfm at 100 psig and shall be equipped with a 75 HP, 

460-Volt, 3-Phase, 60-Hertz motor. 

Instrument quality air will be achieved by conveying wet air through a 100 percent capacity 

desiccant type dryer. 
 

Shaw Stone & Webster, Inc. - Stoughton, MA 22 

[Mirant Mid-Atlantic FGD Retrofit - Morgantown, Dickerson, and Chalk Point Generating Stations] 

Instrument quality air will be conveyed to the air receiver. The vertical air receiver will have 

a capacity of 1,500 gallons and will be equipped with a pressure relief valve, pressure 

indicator, pressure switch and automatic condensate drain. 

Condensate from the air compressor system will be collected in the building drain trench 

and conveyed to the filtrate sump for subsequent processing within the treatment system.