Page 1 

Wastewater Treatment for FGD Purge Streams 

Paper # 33 

Michael R. Riffe 

Technical Director 

Siemens Water Technologies Corp. 

2000 Ericsson Drive 

Warrendale, PA 15086 

michael.riffe@siemens.com

Phone: 724-772-1316 

Brian E. Heimbigner 

Business Development Manager 

Siemens Water Technologies Corp. 

2000 Ericsson Drive 

Warrendale, PA 15086 

brian.heimbigner@siemens.com

Phone: 678-446-5273 

Peter G. Kutzora, P.E. 

Principal Engineer - Chemistry 

We Energies 

231 W. Michigan Street – P145 

Milwaukee, WI 53203 

pete.kutzora@we-energies.com

Phone: 414-221-3926 

Kenneth A. Braunstein, P.E. 

Consulting Process Engineer 

URS-Washington Division 

510 Carnegie Center 

Princeton, NJ 08543-5287 

ken.braunstein@wgint.com

Phone: 609-720-3341 

Presented at: 

MEGA Symposium 2008 

Baltimore, MD 

August 25 – 28, 2008 
Page 2 

ABSTRACT 

In developing a wet limestone flue gas desulfurization (FGD) retrofit program, utilities 

must also develop a strategy for the associated wastewater treatment system (WWTS) to 

treat the scrubber chloride purge stream. This purge stream is required to control 

chloride concentrations and solids fines build-up in the scrubber. 

In this paper, characteristics of FGD purge streams will be identified and items affecting 

those characteristics will be discussed, including the coal source, limestone quality, 

scrubber design, and makeup water quality. Key elements affecting the design and sizing 

of FGD wastewater treatment systems will also be discussed. A number of case studies 

will be presented to illustrate different wastewater treatment systems that resulted from 

the combination of the FGD purge characteristics and the final treated effluent 

requirements. Operational procedures and system flexibility for future changes will be 

covered. Lessons learned will be presented on systems currently in operation, as well as 

some under construction or design. 

INTRODUCTION

Flue gas desulfurization continues to be a topic of current interest as many utilities are 

engaged or have completed retrofit projects to meet emissions standards from Phase 2 of 

the Clean Air Act. A large number of the projects are underway and will be completed in 

the time frame of 2006–2011; however it appears retrofits will continue to be made as 

late as 2015–2020. A significant number of the FGD projects are wet limestone forced 

oxidation (LSFO) scrubbers. 

To maintain the required operating conditions in the scrubber, a purge stream is 

discharged from the scrubber system primarily for chloride control (for compatibility 

with the scrubber’s materials of construction and to achieve SO2 removal efficiency). 

Sometimes the purge stream is necessary for fines control in the absorber. The FGD 

purge stream contains pollutants from coal, limestone, and make-up water. It is acidic, 

supersaturated with gypsum, and contains high dissolved solids and suspended solids, 

comprised of, in addition to gypsum, heavy metals, chlorides, magnesium, and dissolved 

organic compounds. “Heavy metals” is a term that refers to a broadly defined group of 

elements that exhibit metallic properties, including transition metals, metalloids, 

lanthanides and actinides.1

 A subset of the heavy metals is usually the focus of NPDES 

permits and of the performance requirements of wastewater treatment systems for FGD 

purge streams. 

For some plants located on large rivers or water bodies, it is permissible to direct the 

FGD purge to the ash pond for suspended solids settling, pH adjustment, and dilution of 

the heavy metals concentrations simply by mixing the smaller FGD purge with ash pond 

water. For these plants, the ash pond discharge stream meets the NPDES permit 

requirements for discharge. 
Page 3 

More commonly, treatment of the purge is required prior to discharge to the receiving 

water body. This is often the case even for plants that have once-through cooling water 

systems available for co-mixing and which discharge to large rivers. A number of 

options may be considered: 

• Physical/ chemical treatment to reduce total suspended solids, adjust the pH, desupersaturate the stream, and reduce heavy metals. This is the most commonly 

used system in current FGD retrofit programs with about 15 systems installed and 

operating since 2004 and over 25 under construction in the United States. 

• Biological treatment to reduce selected heavy metals, and/or COD/BOD5, and/or 

total nitrogen. Used in selected plants to reduce selenium to low levels,2 to reduce 

COD/BOD5 resulting from use of organic acids, or to reduce total nitrogen3

(usually due to ammonia slip from an SCR unit). A biological system is usually 

be preceded by a physical chemical system. About eight biological systems have 

been installed or planned for installation since 2004. 

• Zero liquid discharge with thermal units (evaporator, crystallizer, spray dryer). 

Only one operated in the U.S. briefly in 1990s with multiple challenges of scaling, 

corrosion, and high costs.4 Two more have been ordered in recent years, with one 

in construction and one cancelled during construction.5

 Some installations 

overseas are installed or under construction – no report on performance yet. 

• Zero liquid discharge with deep well injection. Only one in operation in the U.S. 

and one in construction.6

 This option has a fairly high capital cost ($5–6 million 

per well, with two to three wells per site.) Deep well injection still requires a 

physical chemical treatment system preceding it to de-supersaturate the 

wastewater to prevent scaling in the high temperature, high pressure deep well 

environment. 

• Sludge stabilization by mixing FGD purge with fly ash for land filling. None 

currently being built, however about 15 were constructed in the 1970/80s. This 

approach eliminates the ability to sell fly ash used for commercial products and 

requires extra landfill volume. At one power plant, this option was considered but 

availability of sufficient fly ash quantities could not be assured. 

• Stacking the gypsum for either final disposal or future reclaim will absorb some 

of the purge in the stack. Runoff of excess liquid is collected in holding basins or 

mixed with ash water. A couple of these designs are under construction. 

• Constructed wetlands – biological process. Three have been completed with 

mixed results.7,8

Of all of these treatment processes, the most commonly used is the physical/ chemical 

system, which will be the primary subject of this paper. 
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CHARACTERIZATION OF FGD WASTEWATER 

The FGD wastewater to be treated comes from the hydroclones of the gypsum 

dewatering system, as depicted in Figure 1. 

Figure 1: Simplified Limestone Forced Oxidation (LSFO) FGD Water Balance 

FGD wastewater composition can vary from plant to plant, as depicted in Figure 2. The 

wastewater flowrate and characterization are affected by the coal burn rate, scrubber 

equilibrium chloride concentrations, efficiency of fly ash removal, type and efficiency of 

the gypsum dewatering system, type of FGD process used, and composition of the coal, 

limestone and make-up water. 

The purge rate required to maintain a target equilibrium chloride concentration is directly 

dependent on the coal chloride content and coal burn rate. Powder River Basin (PRB) 

coals generally have much lower chloride content than Eastern Bituminous coals. 

Therefore, for a given target equilibrium chloride concentration, the concentrations of 

metals in the FGD wastewater will tend to be higher for systems burning PRB coals 

versus Eastern Bituminous Coals. Also, for a specific power plant, if the coal burned has 

a lower chloride content than the design basis coal, the scrubber might be operated at a 

chloride purge rate lower than the design value, which could result in higher 
Page 5 

concentrations of other constituents. The increased concentrations can be significant for 

some metals, such as selenium, whose concentration in the treated wastewater is 

dependent on the influent concentration and not just dependent on solubility of metal 

compounds. This impact of coal chloride content on predicted concentrations of 

constituents of the chloride purge (for a target equilibrium chloride concentration) and on 

changes in purge flow rate and characterization, due to changes in the actual coal chlorine 

content, are nearly independent of the coal sulfur concentration.

In addition to the variations in untreated wastewater characterization, the discharge limits 

can also vary significantly from plant to plant, dependent upon the state in which the 

plant is located, the availability of an existing high flow rate discharge stream (e.g., oncethrough cooling water), the nature of the receiving body of water, and the year in which 

the FGD system goes into service. Plants retrofitting later appear to have more stringent 

NPDES permit requirements for the FGD wastewater compared to earlier plants within 

the same regulatory region. Systems have varied in size from 20 gpm to 1,200 gpm. The 

table below depicts the purge characteristics (as specified) and discharge limits for three 

plants. This indicates the variety of both purge and discharge characteristics and the fact 

that some specified purge constituents are not reported (NR) for some plants and many 

constituents are not applicable (NA) for the discharge limits at some plants. Thus, each 

plant will have their unique specified purge and discharge limits. 

Table 1: FGD Wastewater Characteristics and Discharge Requirements for Three 

Representative Plants (different than the case study plants later in paper) 

 Plant 1 Plant 2 Plant 3 

Parameter Specified 

Purge 

Discharge 

Limits 

 Specified 

Purge 

Discharge 

Limits 

Specified 

Purge 

Discharge 

Limits 

pH (S.U.) 5.0 to 6.0 6.5 – 9.0 5.5 – 6.5 6.5 – 8.0 5.5 – 65 6 - 9 

Temp (oF) 125 NA 125 NA 130 

 

Constituents ppm ppm ppm Ppm ppm ppm 

TSS <18,000 <10 <15,000 30 20,000 15 

TDS* 40,000 40,000 30,000 NA NR NA 

Chloride* 15,000 15,000 15,000 NA 15,000 NA 

Total Nitrogen NR NA 81 10 NR NA 

Aluminum 10 1.5 12 2 14 2 

Antimony* 0.55 0.55 0.55 NA 0.2 0.2 

Arsenic 1.5 0.1 3 NA 3 0.1 

Barium* 4 4 NR NA 0.004 0.004 

Beryllium* 0.1 0.1 0.1 NA 

Boron* 300 NA NR NA 600 600 

Cadmium 0.45 0.1 0.5 0.03 0.5 0.1 

Chromium, total 0.3 (Cr3+) 0.1 0.5 (Cr3+) 0.1 1 0.1 

Cobalt 0.2 0.1 0.2 0.2 0.1 

Copper 0.85 0.1 0.2 0.1 0.8 0.1 

Iron 20 

(dissolved) 

0.5 20 1 NR NA 

Lead 0.5 0.1 4 0.1 4 0.1 

Magnesium 1,500 NA 1,500 NA 7,000 7,000 

Manganese NR NA NR NA 300 3 
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Mercury 0.5 0.002 0.8 0.001 0.8 0.001 

Molybdenum 0.25 0.25 NR NA NR NA 

Nickel 2 1.0 5 (90% 

particulate) 

0.3 6 1 

Selenium 4.6 3 5 5 2.835 

Silver 0.3 0.02 0.3 0.05 0.03 0.05 

Thallium* 0.2 0.2 0.1 NA 0.1 0.1 

Vanadium* 1.425 1.425 1 NA 1 1 

Zinc 5 0.1 5 0.1 8 0.1 

* Parameters not impacted by physical chemical treatment, however some residual 

treatment can occur. 

TREATING THE FGD PURGE STREAM 

The scrubber purge stream is treated in a dedicated wastewater facility, rather than an 

existing treatment system, for the following reasons: 

• The power plant’s existing wastewater treatment facility may not have adequate 

capacity for the additional FGD purge flow. 

• The materials of construction of the existing treatment facility most likely are 

unsuitable for treating a high chloride stream common to FGD wastewaters. 

• The high TDS and supersaturated conditions of the FGD wastewater may result 

in scaling if the existing system does not have the correct chemical regime 

throughout. 

• The treatment facility’s process design may not be adequate for the very strict 

wastewater discharge limits likely to be enforced for the FGD wastewater. 

• The quantity of additional sludge generated by treatment of the FGD wastewater 

may exceed the sludge and solids handling capacity of the existing system. 

The specific FGD wastewater treatment system must be designed to meet the current 

wastewater composition and discharge requirements, but also have some flexibility for 

meeting future requirements either as designed and constructed or with hopefully minor 

modifications or add-ons to the system. 

A basic building block in the WWTS has been an integrated physical/ chemical treatment 

system, as represented in Figures 2, 3, and 4. From a two stage hydroclone system, the 

stream will contain about 0.5–2% suspended solids (including gypsum and fines of unreacted or inert materials) and be directed to an equalization tank, which provides a 

means to attenuate the flow and chemistry of the purge. 
Page 7 

Figure 2: Process Flow Diagram of Typical Physical Chemical Treatment System 

with Three Stage Reaction 

The wastewater is pumped to reaction tank #1 for addition of alkali (usually hydrated 

lime) for pH adjustment to about 8.5–9.2 and gypsum de-supersaturation of the 

wastewater. Proper control of the pH is essential to optimize the effectiveness of the 

other chemicals added for precipitating heavy metals. Also, it is important to not allow 

the pH to go higher than 9.2 to avoid magnesium precipitation, which would greatly 

increase the total amount of solids to be dewatered, impact dewaterability, and greatly 

increase scaling potential in the system. Recycle of sludge from the downstream clarifier 

provides seed crystals for gypsum growth to aid in the de-supersaturation process, which 

is critical to prevent scale from forming on the downstream equipment. Addition of the 

alkali also causes precipitation of abundant metals, such as aluminum, iron and 

manganese as metal hydroxides. 

Some heavy metals will precipitate as hydroxides in this reaction step; however metal 

sulfides have much lower solubility than metal hydroxides. Thus, in order to meet the 

low effluent requirements for heavy metals, an organo sulfide is dosed into the stream in 

reaction tank #2 to further precipitate many of the heavy metals. 

 
Page 8 

For enhanced coagulation, an iron salt such as ferric chloride is added in reaction tank #3. 

The iron salt helps denser flocs to form, thus improving clarifier performance. In 

addition, the iron salt will co-precipitate other metals, some non-metals and some organic 

matter. 

After the coagulation step, polymer is added in a mixing zone in the clarifier to aid in 

coagulation and solids settling in the clarifier. The wastewater undergoes clarification 

using a solids contact clarifier, in this particular process design. After clarification, the 

pH is adjusted to neutral using hydrochloric acid (HCl), which is used rather than less 

expensive sulfuric acid (H2SO4). Sulfuric acid would add to the saturation of the treated 

effluent and could cause scaling in downstream equipment. The treated effluent is 

polished by gravity filtration to enhance TSS and metals reduction prior to discharge. 

The backwash wastes from the gravity filter are ultimately returned to the equalization 

tank for reprocessing. The treated effluent is directed to the plant’s discharge. In some 

plants, the treated effluent goes to the ash pond or is mixed with the discharge from a 

cooling system prior to discharging to the final receiving body of water. 

The clarifier sludge is usually in the range of 10–15% solids by weight. It is pumped to 

an agitated sludge holding tank prior to being dewatered in filter presses on a batch basis. 

The sludge tank volume is sized consistent with the utility’s plans for filter press 

operation. Some utilities only operate the filter presses on day shift and weekdays. In 

that case, the sludge tank is sized to accumulate sludge for an entire weekend or holiday 

periods. The most common filter press being used for FGD wastewater sludge is the 

recessed chamber type press (commonly referred to as the plate and frame type). For 

very large sludge production (>200,000 lb/day dry solids), belt filter presses are used. 

The dewatered solids (commonly called “filter cake”) are discharged from the filter 

presses at 40–50 wt% solids and are disposed of in non-hazardous lined landfills, either 

onsite or offsite. The dewatered solids are generally required to pass the EPA “Paint 

Filter Liquids Test” and their leachate should meet Federal regulations regarding toxicity, 

in order to be considered non-hazardous. The dewatered solids are discharged either to 

portable containers (“roll off boxes”) or can be discharged directly into a truck. 

Considerations for the method used include: frequency of filter press operation, cost of 

truck operation, and noise (the sound of roll off boxes may be objectionable if the plant is 

located close to a residential area). 

Dependent upon the discharge permit for the power plant, additional treatment may be 

required downstream of the physical/ chemical treatment system. Some state or regional 

requirements (such as the Chesapeake Bay Initiative) limit total nitrogen in the discharge. 

The source of the nitrogen in FGD purge is usually ammonia slip from an SCR unit. 

Some states limit selenium on selected streams (North Carolina), and others limit the 

organic load (COD/BOD) created by use of dibasic acid or other organic acids in the 

scrubber operation. To reduce these contaminants to acceptable levels, a biological 

treatment system is incorporated after the physical/ chemical system. These biological 

systems include sequencing batch reactors for nitrogen and COD/BOD reduction at 

power plants located on the Chesapeake Bay watershed or fixed film biological treatment 

systems for selenium reduction at plants in North Carolina. The physical/ chemical 
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system is still needed to perform the tasks of pH adjustment, de-supersaturation, 

suspended solids reduction, and removal of heavy metals. If these tasks were not 

accomplished, the biological systems would not operate properly and in many cases 

would scale from saturated conditions or be overloaded by the suspended solids. 

Variations of the physical/ chemical treatment system may be used, including: 

Figure 3: Process Flow Diagram of Typical Physical Chemical Treatment System 

with Two Stage Reaction 

This design is very similar to the system in Figure 2, except that the alkali and organo 

sulfide are added to the same reaction tank sized with adequate retention time in the 

reaction tank for both chemicals to react with the wastewater for their intended functions. 

The two stage reaction design treatment configuration, depicted in Figure 3, has become 

the preferred arrangement due to its efficiency, cost effectiveness, and reduced 

maintenance requirements. 
Page 10 

Figure 4: Process Flow Diagram of Typical Physical Chemical Treatment Process 

with Primary and Secondary Clarifiers 

In this system, an initial reaction tank for alkali addition and de-supersaturation and a 

reaction tank for polymer addition are located prior to a primary clarifier, which is 

followed by two stage reaction tanks and a secondary clarifier. This arrangement is used 

when the total suspended solids in the FGD purge stream could be greater than 2 wt% or 

when there is a desire to separate the metal hydroxide from the metal sulfide precipitated 

solids or when additional periodic blowdown for fines control occurs. However, for the 

case in which suspended solids may be higher than 2 wt% it usually would be more cost 

effective to ensure a second stage of hydroclones was included in the gypsum dewatering 

system, than to have a primary clarifier. If more than 2 wt% solids are forecasted in the 

flow to the WWTS, this likely indicates some gypsum product is unnecessarily being lost 

with the wastewater and the gypsum dewatering system should be tightened up. This 

design was used in European power plants in the 1970s-1980s (ref 9) and is more 

complex to operate and expensive than the configuration shown in Figure 3. 

Another application of this arrangement would be if other plant streams, such as coal pile 

runoff and general drainage, are directed to the FGD WWTS. This situation may occur 

in a new power plant with FGD, where the wastewater treatment system is not dedicated 

to only FGD purge. 
Page 11 

Case Study 1: We Energies Pleasant Prairie Power Plant (PPPP) 

We Energies began operation of its Unit 1 wet limestone forced oxidation FGD and SCR 

systems in November 2006 at the Pleasant Prairie Power Plant (PPPP) in Wisconsin. Unit 

2 FGD operation began in April 2007. A SCR unit had previously been installed on Unit 

2. PPPP burns PRB coal and has been selling commercial grade gypsum produced by the 

FGD system since January 2007. FGD blowdown (to remove fines and chlorides) from 

two 600 MW scrubbers is treated using the alkali-sulfide process to achieve very low 

levels of mercury and other heavy metals. More detailed information on PPPP has been 

presented earlier (ref. 10). 

URS-Washington Division provided overall Engineering / Procurement / Construction 

(EPC) services and Siemens Environmental Systems and Services (formerly 

Wheelabrator) supplied the FGD scrubber equipment. FGD chemistry mass balance 

modeling used factors such as coal, flue gas, limestone, and makeup water 

characterization, scrubber performance, materials of construction, and gypsum quality to 

determine the design requirements of the FGD wastewater treatment system (WWTS). 

The WWTS design was based on a blowdown composition of 13,000 ppm chloride, 1.5% 

solids, and a flowrate of 75 gpm to provide operational and maintenance flexibility. 

Total mercury concentration in the PPPP FGD absorber blowdown was predicted to be as 

high as 2,000 ppb. The concentration of mercury in the FGD wastewater effluent would 

need to be significantly reduced to meet expected discharge limits, so mercury removal 

became the focus for the WWTS design. 

In addition to the usual permit limits on total suspended solids, oil and grease, and pH, 

new limits were set by the Wisconsin Department of Natural Resources, as follows: 

Table 2: New Discharge Limits for Pleasant Prairie Power Plant 

Parameter Location Limit Frequency 

Mercury WWTS effluent 1.5 μg/L (ppb) 2/Week grab 

Mercury WWTS effluent 0.00135 lbs/day 2/Week calculated 

Mercury Discharge to lake 80 ng/L (ppt) 2/Week grab 

Arsenic Discharge to lake Monitoring only Monthly composite 

Beryllium Discharge to lake 0.084 μg/L (ppb) Monthly composite 

Chloride Discharge to lake 1514 mg/L (ppm) Monthly composite 

Copper Discharge to lake 117 μg/L (ppb) Monthly composite 

Selenium Discharge to lake 550 μg/L (ppb) Monthly composite 

 Note: The selenium limit was added in early 2008 as a result of startup sampling 

performed in November and December 2007. 

The discharge permit allows FGD wastewater only to be discharged when it is combined 

with cooling tower blowdown, coal pile runoff or other treated wastewater streams to 

ensure that mercury, chloride, and other parameters remain below the discharge permit 
Page 12 

limits. An operational limit has been established that requires a minimum of 1,500 gpm 

of cooling tower blowdown or other treated wastewater discharges to allow up to 50 gpm 

of FGD wastewater discharge. 

Table 3: Characteristics for Pleasant Prairie Power Plant WWTS 

Parameter Units Specified Purge Discharge 

Requirements 

Total Suspended Solids ppm 15,000 (1.5%) 15

pH std. units 5.5 – 6.5 6 – 9 

Oil and Grease ppm ND 10

 

Alkalinity ppm 100 as CaCO3 NA

Arsenic ppm 3.0 0.010 

Antimony ppm 0.200 0.100 

Beryllium ppm 0.004 0.0004 

Cadmium ppm 0.500 0.100 

Calcium ppm 10,000 n/a

Chlorides ppm 15,000 15,000

Chromium ppm 5.0 0.100 

Copper ppm 0.850 0.100

Lead ppm 4.0 0.100 

Magnesium ppm 7,000 n/a

Mercury, Total ppm < 0.100

≥ 0.100 < 0.800

≥ 0.800 < 2.0 

0.0002

0.0005

0.0015

Nickel ppm 6.0 1.0 

Selenium ppm 3.0 

(> 80% as selenite - 

SeIV) 

2.5 

Silver ppm 0.300 0.300 

Sulfate ppm 2,200 n/a

Zinc ppm 8.0 0.100 

FGD Wastewater Treatment System Design 

Siemens Water Technologies Corp. (formerly USFilter) was selected to design and 

provide the engineered equipment for the PPPP alkali-sulfide FGD WWTS. URSWashington Division installed the system and provided system startup activities. Below 

is the process flow of the FGD WWTS. 
Page 13 

Figure 5: Pleasant Prairie Power Plant WWTS 

This design includes two stages of clarification and three chemical addition/reaction 

tanks to accomplish the wastewater treatment. The WWTS is design to process up to 75 

gpm of FGD purge at a maximum suspended solids concentration of 1.5%. Accounting 

for filtrate recycle volumes, the design flowrate into Reaction Tank 1 is 19 to 94 gpm. 

Sizing of the individual tanks and clarifiers was based on a nominal 50 gpm system flow. 

FGD WWTS Startup, Operating Experience, and Lessons Learned 

Upon startup of the Unit 1 FGD scrubber in November 2006, the WWTS was operated 

with all treated FGD wastewater and filtrate returned to the absorbers (closed loop 

operation) until system performance met discharge permit requirements. Dewatered 

sludge from the primary and secondary clarifiers was confirmed as non-hazardous 

material and placed in on-site landfill storage. Lessons learned from the startup and 

performance testing periods include: 

• To better control foaming in the scrubber, the WWTS was operated in early 2007 

to remove smaller fines (<10 microns) in addition to adding a silicon-based 

antifoam agent to the scrubber and installing water sprays in the stilling well and 

absorber overflow. 

Polymer

Feed

TMT-15

Feed

Ferric 

Chloride 

Feed

HCL Acid 

Feed

0-1 

Effluent 

Tank

12,000 Gal. 

0-2 

Effluent 

Tank

12,000 Gal. 

Sand 

Filter 

“A”

Sand 

Filter 

“B”

Secondary 

Clarifier

(Lamella)

Floc 

Tank

Sludge 

Holding 

Tank

Primary 

Clarifier

Reaction 

Tank #2

Reaction 

Tank #3

Reaction 

Tank #1

7,300 gal.

33,000 gal.

1,100 gal. 1,100 gal.

17,300 gal.

Discharge 

to Lake

Recirc. To 

Absorber 

Sump / Eq 

Tank

Flash

 Mixer

Unit #1 

Absorber

Unit #2 

Absorber

Wastewater 

Equalization 

Tank

108,000 gal.

Primary and 

Secondary 

Hydroclones

Absorber 

Sump

Recirc. 

From FGD 

WWT

Maintenance 

Tank

250,000 gal.

Hydrated 

Lime Silo

600,000 gal. 600,000 gal.

Blowdown

LEGEND

Wastewater Flow Path

FGD Wastewater Building

PLEASANT PRAIRIE FGD WASTEWATER FLOW DIAGRAM

To Filter 

Press (2)
Page 14 

• Absorber chloride levels increased to 12,000 ppm by late August 2007, because 

the absorbers were operated with no chloride purge by returning all treated FGD 

wastewater and filtrate to the absorbers. Semi-continuous FGD blowdown began 

in November 2007, and chlorides were control between 8,000 to 12,000 ppm in 

early 2008. Regular FGD blowdown and discharge at 40 gpm in May and June 

2008 reduced chlorides to about 5,000 ppm. 

• During the chloride buildup period, the WWTS removed about 99.5% of mercury 

in the untreated wastewater, but final mercury concentrations were about 3 to 5 

ppb. Mercury needed to be 1.5 ppb out of the WWTS. Evaluation of the system 

determined better suspended solids removal was needed for particles in the 0.45 

to 5 micron size range. Multiple tests were performed. Installation of absolute 

rated cartridge filters down to 0.45 micron provided consistent performance with 

typical final mercury concentrations of 0.2 to 1.0 ppb from the WWTS. 

• Process chemistry changes were made in November 2007 to improve the removal 

of mercury in the secondary clarifier. Jar testing determined the best mercury 

removal by TMT-15 organo sulfide precipitation was achieved with pH in the 

range of 6.0 to 6.5 and higher concentrations of ferric chloride than originally 

used. The HCl injection system and instrumentation were modified to meet this 

pH range. Control of pH in Reaction Tank 3 was set for 6.4 to assure some 

margin, as the anionic polymer loses its effectiveness below a pH of 6.0. Because 

the PPPP WWTS has an arrangement similar to that of Figure 5, this process 

chemistry was relatively easy to implement. After confirming secondary clarifier 

performance, the absolute rated cartridge filters were removed from service in 

February 2008. 

• For the primary clarifier, it was determined the sludge was denser at pH 8.6, 

rather than allowing to operate as high as 9.2. This lower pH reduced the 

consumption of both hydrated lime and hydrochloric acid. Also, a more 

continuous transfer of sludge from the clarifier to reaction tank no. 1 was 

considered to be better, in order to eliminate the interruption of flow from the 

clarifier during sludge transfer in low WWTS flow conditions such as when only 

one absorber is online during initial phased startup or during periodic scrubber 

maintenance or power block downtime. 

• Scaling, due to gypsum and ferric-sulfate, occurred in the sand filters, secondary 

clarifier, and associated piping. The cause of the excessive gypsum scaling in the 

WWTS is the much higher concentration of sulfate (20,000 to 30,000 ppm) in the 

absorber slurry and wastewater than originally specified. The cause of this high 

sulfate concentration is still being investigated, although reactive magnesium in 

the limestone is one suspected cause. The FGD system was designed for 

limestone containing up to about 2% MgCO3 of non-reactive magnesium. We 

Energies is evaluating limestone quality and is considering modifications to 

Reaction Tank 1 chemistry to reduce the scaling potential of the wastewater and 

considering an anti-scalant prior to the sand filters. The sand filters were taken 
Page 15 

out of service in February 2008 because the secondary clarifier effluent was 

meeting performance requirements for TSS and mercury concentrations. Scale in 

piping and the secondary clarifier is currently being removed through periodic 

water flushes and mechanical cleaning. 

• Care must be taken for the polymer to be adequately mixed to avoid stratification 

in the day tank and plugage in the polymer tubing. Regular operator checks and 

cleaning of the chemical feed equipment is also important, as well as having spare 

parts immediately on hand. 

• The HCl skid was rebuilt with CPVC when the corrosion resistant Hastelloy 

tubing developed chronic leaks at tube fittings and threaded connections. 

• Mercury analysis (EPA Method 245.7 with a minimum detection limit of 1.1 ppt 

mercury) is performed using cold vapor atomic fluorescence spectroscopy on grab 

samples collected in specially cleaned 250 ml bottles. Turbidity is used as an 

indirect indication of mercury, since it was found to correlate well with the formal 

mercury analysis. Turbidity can more easily be checked and used for control 

settings. The formal analysis is still performed on the schedule required to report 

to the Wisconsin Department of Natural Resources. 

• One horizontal section of pipe between Reaction Tank 3 and the clarifier flash 

mix tank was shortened, sloped, and a flush connection added to reduce flow 

restrictions caused by suspended solids and scaling , which prevented operation at 

higher flowrates. 

• The PPPP FGD alkali-sulfide WWTS has been discharging treated wastewater 

since November 2007. The system mercury removal performance has been 

verified up to the design flow rate into Reaction Tank 1 of 94 gpm with mercury 

concentrations consistently less than 1.0 ppb. Additional tuning of the alkalisulfide process and evaluation of other enhanced mercury removal technologies 

will continue at PPPP to minimize future discharges of mercury from the FGD 

system. 
Page 16 

Case Study 2: Wastewater Treatment System for Heavy Metals and 

Total Nitrogen Reduction 

An eastern U.S. 1360 MW (gross) power plant is upgrading to add a LSFO FGD system 

for operation by 2010 and to produce commercial grade gypsum. Presently, it burns low 

sulfur compliance coal, typically from Eastern USA. In the future, the power plant will 

fire a wider range of coals, including low sulfur international coal, mid-high sulfur 

Eastern coal and possibly other coal types after retrofit of the FGD system. The FGD 

system will require a chloride purge stream to be discharged from the FGD absorber, via 

the gypsum dewatering system hydroclones, to maintain absorber chemistry limits. This 

wastewater will have high concentrations of heavy metals, chloride, biochemical oxygen 

demand (BOD), phosphate, total suspended solids (TSS) and nitrogen. In order to 

comply with state standards for water quality, the FGD purge will require treatment prior 

to discharge. 

Additionally, it was determined that an additional periodic blowdown is needed to control 

the fines concentration in the scrubber when certain limestone is used. The WWTS is 

designed so the full flow goes first to an equalization tank to attenuate variations in the 

flow rate and then to a primary flocculating clarifier to remove the fines. About 60% of 

the flow is then returned to the scrubber and 40% continues through the rest of the 

wastewater treatment system. The balance of the system is comprised of a transfer tank, 

two reaction tanks in which the chemicals are added (alkali, organo sulfide, and ferric 

chloride), a static mixer for polymer addition and the secondary solids contact clarifiers. 

The physical chemical treated effluent is then directed to a set of sequencing batch 

biological reactors. For the flowrate at this plant, three reactors are used to provide 

parallel operation and adequate redundancy. Associated with the bioreactors are the 

appropriate pre- and post-equalization tanks. Finally, the treated effluent flows to a set of 

continuous backwash gravity sand filters for final suspended solids polishing. The final 

effluent is then discharged. 

All solids generated in the primary and secondary clarifiers and the bioreactors are 

dewatered in a set of filter presses to reduce the amount of water in the solids prior to 

landfill disposal. 

A raw water treatment system was included in the scope of supply to treat secondary 

treated wastewater from a nearby POTW. The treated water will be fed to the FGD 

limestone slurry makeup system and will be the source of service water for the FGD 

wastewater treatment system. The raw water treatment plant will consist of treatment for 

suspended solids reduction and disinfection. The suspended solids removed in the raw 

water sand filters are dewatered with the FGD suspended solids. 

This system is still under construction, thus the lessons learned will be known when start 

up occurs in late 2009. 
Page 17 

Table 4: Characteristics for WWTS with Nitrogen Reduction 

Parameter Specified 

Purge 

Discharge 

Requirements 

pH (S.U.) 4.0 – 6.0 6.0 – 9.0 

Temp (oF) 125 NA 

 

Constituents ppm ppm 

TSS 16,000 30 

TDS 75,000 NA 

Chloride 20,000 – 

30,000 

NA 

Ammonium 26 See Total Nitrogen 

Nitrate Nitrogen 264 See Total Nitrogen 

Total Nitrogen 290 4 

BOD 20 20 

Aluminum 12 NA 

Antimony 0.83 NA 

Arsenic 1.8 0.1 

Beryllium 0.4 NA 

Cadmium 0.5 0.01 

Chromium 0.1 (as Cr3+) 0.1 

Copper 0.76 0.1 

Iron 20 NA 

Lead 4 0.1 

Magnesium 6,000 NA 

Mercury 0.77 0.002 

Molybdenum 0.41 NA 

Nickel 5(90% 

particulate) 

0.35 

Selenium 6.5 NA 

Silver 0.3 0.01 

Thallium 0.21 NA 

Vanadium 1 NA 

Zinc 5 0.1 

NR – Not reported. 

NA – Not applicable, as no discharge limit. 
Page 18 

Figure 6: Flow Diagram for WWTS with Nitrogen Removal
Page 19 

Case Study 3: Wastewater Treatment System for Heavy Metals 

Reduction, Including Special Treatment for Selenium Reduction

A two-unit 1,120 MW coal-fired generating facility in the eastern U. S. was retrofitted 

with a wet FGD system with two absorbers. The FGD purge stream must be treated for 

mercury and selenium reduction prior to final discharge to an ash pond and subsequently 

to a river. 

Table 5: Characteristics for Purge Steam Needing Hg & Se Reduction 

Parameter Specified 

Purge 

Discharge 

Requirements 

pH (S.U.) 5.6 5.3 – 9.0 

Temp (oF) 124 NA 

 

Constituents Ppm ppm 

TSS 16,000 ≤50 

TDS 25,000 NA 

Chloride 12,000 12,100 

Ammonium ≤14 NA 

Antimony 0.02 NA 

Arsenic 0.49 NA 

Barium 7.33 NA 

Beryllium 0.013 NA 

Cadmium 0.10 NA 

Chromium 0.36 NA 

Cobalt 0.42 NA 

Copper 0.54 NA 

Fluorine 39.50 NA 

Lead 0.35 NA 

Manganese 6.52 NA 

Mercury 0.30 0.001 

Molybdenum 0.41 NA 

Nickel 0.76 NA 

Selenium 4.0 0.100 

Silver 0.06 NA 

Thallium 0.2 NA 

Vanadium 0.56 NA 

Zinc 0.9 NA 

NR – Not reported. 

NA – Not applicable, as no discharge limit. 
Page 20 

Many different treatment technologies and combinations were evaluated before the 

customer settled on an approach of physical/ chemical and biological treatment 

combined. The physical/ chemical treatment is for de-supersaturation, pH adjustment, 

much of the heavy metals reduction (including some selenium reduction), suspended 

solids removal, and dewatering. Then an anoxic/anaerobic, fixed-film biological 

treatment system is used for selenium reduction to meet permit requirements. For the 

bioreactors, activated carbon is used as a support media on which proprietary site-specific 

naturally-occurring bacteria cultures will grow and be retained within the bioreactor 

vessels. The microorganisms require a food (carbon) source to support cell growth. A 

molasses-based nutrient is used and is supplemented with micronutrients proven to 

promote stable growth for the target microbial population. The dewatering filter presses 

handle solids from the physical/ chemical and biological processes. The entire system 

has exceeded expectations and is meeting the discharge requirements. 

Figure 7: Wastewater Treatment System with Biological Unit for Mercury and 

Selenium Reduction 
Page 21 

Lessons Learned/Good Engineering Practices for the Physical Chemical, Biological 

and Sludge Dewatering System 

• Selection of appropriate polymer is important for the clarification step. Since 

polymer effectiveness is not an exacting science, jar testing of similar wastewater 

can assist in identifying the polymers that could produce the best success. 

• Use of potable water in the polymer system at this plant was the correct approach, 

rather than filtered service water, to prevent the polymer from reacting with 

compounds in the service water which can increase maintenance activities. 

Proper mixing of polymer is also essential. 

• Drive-through bays on this plant was correct decision for unloading of chemicals 

and loading of dewatered solids. Not every plant will have adequate space to 

implement this design item. 

• Scrubber units will start up at different intervals; so it is important to have a plan 

of operation of the WWTS under partial flow conditions, in order to maintain 

minimum levels in tanks, minimum velocities in pipes, and the chemistry of the 

treatment process. The chemistry of the FGD purge will likely change during the 

startups of the FGD units as their operation is adjusted towards optimization. 

• Plant staff indicated they would prefer to have all pumps located indoors for 

access and maintenance. 

• Provide large lime prep/storage tank to maintain desired slurry concentrate 

• Provide compressed air flushing of polymer lines to ease operator maintenance. 

• Provide automatic/programmed flushing of all slurry service lines. 

SUMMARY 

Proper treatment of the flue gas desulfurization purge stream by a dedicated FGD 

wastewater treatment system is an essential part of the FGD retrofit project, unless direct 

or mixed discharges are allowed for a specific power plant, a situation which is 

uncommon. The required process steps depend on the characterization of the wastewater 

and the specific requirements of the plant’s permit, for both the FGD WWTS discharge 

and for the combined plant discharge. The system may only require physical-chemical 

processes, but might also require biological processes, when selenium, nitrogen, or 

organic compounds must be removed. The selected treatment steps, based on proven 

process technology, integrated into a system with the appropriate equipment components, 

controls, operator interfaces, flexibility of design, redundancy of equipment, and good 

control of the chemistry in the system will result in a reliable operation and ability to 

meet the plant’s discharge requirements. 

The design and planned operation of the FGD WWTS must account for the effect of 

variations in coal composition on purge stream flow rates and composition. The most 

significant coal variation is the chloride content that, for a given absorber equilibrium 

chloride concentration, will directly affect the required purge stream flow rate. This flow 
Page 22 

rate will, in turn, directly affect the concentrations of the other constituents in the purge 

stream, especially the so-called heavy metals. Differences in composition between 

Powder River Basin (PRB) and Eastern Bituminous coals can result in notable changes in 

required purge rates. This impact of coal chloride content is nearly independent of the 

coal sulfur concentration. For similar reasons, operating the plant with coal of different 

composition than the design basis coal, can result in purge rates and characterization 

different than the WWTS design values. 

Startup and operation of the latest generation of WWTS has provided “lessons learned”, 

especially with regard to successfully achieving effluent limits for mercury, selenium, 

and nitrogen, which are more stringent than previous limits. Changes in chemistry may 

be required to achieve the required effluent concentrations. In addition, because FGD 

wastewater characterization may be different than the design basis, due to variations in 

coal and limestone composition, the WWTS operation may have to be adjusted to 

accommodate unanticipated concentrations of certain dissolved species. 

ACKNOWLEGMENTS 

The authors wish to acknowledge We Energies for detailed information on their Pleasant 

Prairie Power Plant, including updated data available since their last paper on the subject. 
Page 23 

REFERENCES 

1. Cotton, A., Wilkinson, G., Murillo, C., (1999). Advanced Inorganic Chemistry

(6th Ed.). New York: Wiley. 

2. Sonstegard, J., Harwood, J., Pickett, T., Full Scale Implementation of GE ABMet 

Biological Technology for the Removal of Selenium from FGD Wastewaters, 

International Water Conference, Orlando, 2007. 

3. Riffe, M., Sequential Batch Reactor Treatment for Nitrogen Reduction, White 

Paper, 2007. 

4. DOE/NETL – 2000/1156, Milliken Clean Coal Demonstration Project: A DOE 

Assessment, Morgantown and Pittsburgh, 2001 

5. Mittal, D., Haskin, J., FGD Blowdown Waste Water Treatment and Zero Liquid 

Discharge, International Water Conference, Pittsburgh, 2006. 

6. Cleveland, R., Mezo, J., Nickrand, W., Ten Eyck, P., Guidos, J., FGD 

Wastewater Disposed of Via Deep-Well Injection at Duke Energy’s Gibson 

Generating Station, International Water Conference, Orlando, 2007. 

7. Eggert, D., Rodgers Jr., J., Hensman, C., Performance of Pilot-Scale Constructed 

Wetland Treatment Systems for Flue Gas Desulfurization Waters, International 

Water Conference, Orlando, 2007. 

8. Murray-Gulde, C., Mooney, D., Huddleston III, G., Rodgers Jr., J., Eggert, D., 

Designing Constructed Wetlands for Mitigating Risks from Flue Gas 

Desulfurization Wastewater, International Water Conference, Orlando, 2007. 

9. Bursik, A., Dieterle, E., FGD Wastewater Treatment – State of the Art in the 

Federal Republic of Germany, International Water Conference, Pittsburgh, 1988. 

10. Kutzora, Pete., FGD Wastewater Treatment using Alkali-Sulfide Process at 

Pleasant Prairie Power Plant, Electric Utility Chemistry Workshop, University 

of Illinois/Champaign, 2008. 

Keywords

Flue gas desulfurization, FGD, wastewater treatment, physical chemical treatment, heavy 

metals, mercury, selenium, nitrogen, biological.