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, 2008Page 2 

### ABSTRACT

In developing a wet limestone flue gas desulfurization (FGD) retrofit program, utilities must also develop a strategy for the associat ed wastewater treatment system (WWTS) to
treat the scrubber chlori de purge stream. This purge st ream is required to control
chloride concentrations and solid s 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, in cluding the coal source, limestone quality, scrubber design, and makeup water quality. Ke y 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 wast ewater treatment system s that resulted from the combination of the FGD purge characteristics and the final treated effluent requirements. Operational procedures and syst em 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 continue s to be a topic of current in terest 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 pr ojects are underway and will be completed in the time frame of 2006–2011; however it appear s retrofits will continue to be made as late as 2015–2020. A significant number of th e 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 contro l (for compatibility
with the scrubber’s materials of construction and to achieve SO 2 removal efficiency). Sometimes the purge stream is necessary fo r fines control in the absorber. The FGD purge stream contains pollutant s from coal, limestone, and make -up water. It is acidic, supersaturated with gypsum, and contains hi gh dissolved solids and suspended solids, comprised of, in addition to gypsum, heavy me tals, chlorides, magnesium, and dissolved organic compounds. “Heavy metals” is a term th at 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 trea tment systems for FGD purge streams.

For some plants located on large rivers or wa ter bodies, it is permissible to direct the FGD purge to the ash pond for suspended solid s settling, pH adjustme nt, 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 th e purge is required prior to discharge to the receiving water body. This is often the case even for plants that have on ce-through cooling water systems available for co-mixing and which di scharge to large rivers. A number of options may be considered:

• Physical/ chemical treatment to reduce to tal suspended solids, adjust the pH, desupersaturate the stream, and reduce hea vy metals. This is the most commonly used system in current FGD retrofit progr ams with about 15 systems installed and operating since 2004 and over 25 under c onstruction in the United States.

• Biological treatment to reduce select ed heavy metals, and/or COD/BOD 5 , and/or total nitrogen. Used in selected plan ts to reduce selenium to low levels, 2 to reduce
COD/BOD 5 resulting from use of organic ac ids, or to reduce total nitrogen 3 (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 ordere d in recent years, with one in construction and one cancelled during construction. 5 Some installations
overseas are installed or under constr uction – no report on performance yet.

• Zero liquid discharge with deep well injec tion. 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 hi gh 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 we re 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 as h quantities could not be assured.

• Stacking the gypsum for either final dispos al or future reclaim will absorb some of the purge in the stack. Runoff of exce ss liquid is collected in holding basins or mixed with ash water. A couple of th ese designs are under construction.

• Constructed wetlands – biologi cal process. Three have been completed with
mixed results. 7,8

Of all of these treatment processes, the mo st commonly used is the physical/ chemical system, which will be the primary subject of this paper.Page 4 

### CHARACTERIZATION OF FGD WASTEWATER

The FGD wastewater to be treated co mes 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 ar e affected by the coal burn rate, scrubber equilibrium chloride concentrations, efficiency of fly ash removal, t ype 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. Powd er River Basin (PRB) coals generally have much lower chloride content than Eastern Bituminous coals. Therefore, for a given target equilibrium ch loride 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 basi s coal, the scrubber might be operated at a chloride purge rate lower than the desi gn value, which could result in higherPage 5 concentrations of other constituents. The in creased concentrations can be significant for some metals, such as selenium, whose con centration 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 targ et equilibrium chloride concentration) and on changes in purge flow rate and characterizati on, due to changes in the actual coal chlorine content, are nearly independent of the coal sulfur concentration.

In addition to the variations in untreated wa stewater characterizati on, the discharge limits can also vary significantly from plant to plant, dependent upon th e state in which the plant is located, the availability of an exis ting high flow rate discharge stream (e.g., oncethrough cooling water), the nature of the r eceiving 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 wastewat er compared to earlier plants within the same regulatory region. Systems have va ried 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 va riety of both purge and discharg e 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 speci fied purge and discharge limits.

Table 1: FGD Wastewater Characteristic s 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 3Page 6 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 ch emical 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 trea tment 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 ma y 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 handli ng capacity of the existing system.

The specific FGD wastewater treatment system must be designed to meet the current wastewater composition and discharge requireme nts, but also have some flexibility for meeting future requirements either as desi gned and constructed or with hopefully minor modifications or add-ons to the system.

A basic building block in the WWTS has been an integrated physica l/ 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 direct ed to an equalization tank, which provides a means to attenuate the flow and chemistry of the purge.Page 7 Figure 2: Process Flow Diagr am of Typical Physical Chemical Treatment System with Three Stage Reaction

The wastewater is pumped to reaction tank #1 for addition of alka li (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 ma gnesium 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-supersatu ration process, which is critical to prevent scale from forming on the downstream equipment. Addition of the alkali also causes precipitation of abunda nt metals, such as aluminum, iron and manganese as metal hydroxides.

Some heavy metals will preci pitate as hydroxides in this reaction step; however metal sulfides have much lower solubility than me tal 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 fe rric chloride is added in reaction tank #3. The iron salt helps denser flocs to form, th us improving clarifier performance. In addition, the iron salt will co-pr ecipitate 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 wast ewater undergoes clarification using a solids contact clarifier, in this particular process de sign. After clarification, the pH is adjusted to neutral using hydrochloric acid (HCl), which is used rather than less expensive sulfuric acid (H 2 SO 4 ). Sulfuric acid would add to the saturation of the treated
effluent and could cause sca ling 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 de watered in filter presses on a batch basis. The sludge tank volume is sized consistent with the utility’s pl ans 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 accumula te sludge for an entire weekend or holiday periods. The most common filter press bei ng 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 dr y 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 ge nerally 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 pre ss 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 pow er plant, additional treatment may be required downstream of the physical/ chemical treatment system. So me 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 di basic acid or other organic acids in the scrubber operation. To reduce these contamin ants 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 wa tershed or fixed film biological treatment systems for selenium reduction at plants in North Carolina. The physical/ chemicalPage 9 system is still needed to perform the ta sks of pH adjustment, de-supersaturation, suspended solids reduction, and removal of h eavy metals. If th ese 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 tr eatment system may be used, including:

Figure 3: Process Flow Di agram 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 si zed 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 tr eatment 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 Di agram 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 locate d prior to a primary clarifier, which is followed by two stage reaction tanks and a seconda ry clarifier. This arrangement is used when the total suspended solids in the FGD pur ge stream could be gr eater than 2 wt% or
when there is a desire to se parate the metal hydroxide from the metal sulfide precipitated solids or when additional periodic blowdown fo r 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 hydrocl ones 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 produc t is unnecessar ily being lost with the wastewater and the gypsum dewateri ng 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 th is arrangement would be if other pl ant 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 wa stewater treatment system is not dedicated to only FGD purge.Page 11 

### Case Study 1: We Energies Plea sant 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 Pr airie 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 sellin g 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 pr ocess 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 E ngineering / Procurement / Construction (EPC) services and Siemens Environmen tal Systems and Services (formerly Wheelabrator) supplied the F GD scrubber equipment. FGD chemistry mass balance modeling used factors such as coal, flue gas, limestone, and makeup water characterization, scrubber performance, mate rials 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 a nd maintenance flexibility. Total mercury concentration in the PPPP FGD absorber blowdown was predicted to be as high as 2,000 ppb. The concentration of merc ury in the FGD wastewater effluent would need to be significantly reduced to meet e xpected discharge limits, so mercury removal became the focus for the WWTS design.

In addition to the usual permit limits on tota l suspended solids, oil and grease, and pH, new limits were set by the Wisconsin Depart ment 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 M onitoring 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 onl y to be discharged when it is combined with cooling tower blowdown, coal pile runof f or other treated wastewater streams to ensure that mercury, chloride, and other pa rameters remain below the discharge permitPage 12 limits. An operational limit has been esta blished that requires a minimum of 1,500 gpm of cooling tower blowdown or other treated wa stewater discharges to allow up to 50 gpm of FGD wastewater discharge.

Table 3: Characteristics for Pl easant 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 -
Se IV ) 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 th e 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 thr ee 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 flow rate 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 Ex perience, and Lessons Learned

Upon startup of the Unit 1 FGD scrubber in November 2006, the WWTS was operated
with all treated FGD wastewat er and filtrate returned to the absorbers (closed loop operation) until system performance met discharge permit requirements. Dewatered sludge from the primary and secondary cl arifiers was confirmed as non-hazardous material and placed in on-site landfill storag e. Lessons learned from the startup and performance testing periods include:

• To better control foaming in the scrubbe r, the WWTS was operated in early 2007 to remove smaller fines (<10 microns) in addition to addi ng a silicon-based antifoam agent to the scrubbe r 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 absorber s. 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 di scharge 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 concentr ations 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 remova l 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 micr on provided consistent performance with typical final mercury concentratio ns 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 de termined the best mercury removal by TMT-15 organo sulfide precip itation 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 instru mentation 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 implemen t. After confirming secondary clarifier performance, the absolute rated cartridge filters were removed from service in February 2008.

• For the primary clar ifier, it was determined the sludge was denser at pH 8.6, rather than allowing to operate as hi gh as 9.2. This lower pH reduced the consumption of both hydrated lime a nd hydrochloric acid. Also, a more continuous transfer of sludge from th e clarifier to reaction tank no. 1 was considered to be better, in order to e liminate the interrupti on of flow from the clarifier during sludge transfer in low WWTS flow conditions such as when only one absorber is online during initial pha sed 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 caus e of the excessive gypsum scaling in the WWTS is the much higher concentrati on of sulfate (20,000 to 30,000 ppm) in the absorber slurry and wastewater than orig inally specified. The cause of this high
sulfate concentration is s till being investigated, alt hough reactive magnesium in the limestone is one suspected cause. The FGD system was designed for limestone containing up to about 2% MgCO 3 of non-reactive magnesium. We Energies is evaluating limestone quality and is considering modifications to Reaction Tank 1 chemistry to reduce the sc aling potential of the wastewater and considering an anti-scalant prior to the sand filters. The sand filters were takenPage 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 poly mer 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 merc ury, since it was found to corr elate 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 flus h 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 plan t 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 Easter n 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 othe r 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 solid s (TSS) and nitrogen. In order to comply with state standards for water qualit y, the FGD purge will require treatment prior to discharge.

Additionally, it was determined that an additi onal 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 equa lization tank to attenuate variations in the flow rate and then to a primary flocculating cl arifier 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 ar e added (alkali, organo sulfide, and ferric chloride), a static mixer for polymer additi on and the secondary solids contact clarifiers.

The physical chemical treated effluent is th en directed to a se t of sequencing batch biological reactors. For the fl owrate at this plant, three reactors are used to provide parallel operation and adequa te redundancy. Associated wi th the bioreactors are the appropriate pre- and post-equaliz ation tanks. Finally, the treate d 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 sec ondary 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 RemovalPage 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 facili ty 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 fina l 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 co mbinations were evaluated before the customer settled on an approach of phys ical/ chemical and biological treatment combined. The physical/ chemical treatment is for de-supersaturation, pH adjustment, much of the heavy metals reduction (incl uding some selenium reduction), suspended solids removal, and dewateri ng. Then an anoxic/anaerobi c, fixed-film biological treatment system is used for selenium reduction to meet permit requirements. For the bioreactors, activated carbon is used as a suppo rt media on which prop rietary 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 s upplemented with micr onutrients proven to promote stable growth for the target microbi al population. The dewa tering filter presses handle solids from the physical/ chemical and biological proce sses. The entire system has exceeded expectations and is meeting the discharge requirements.

Figure 7: Wastewater Treatment System with Biological Unit for Mercury and
Selenium ReductionPage 21 Lessons Learned/Good Engineering Practice s for the Physical Chemical, Biological and Sludge Dewatering System

• Selection of appropriate polymer is impor tant 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 inte rvals; 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 velociti es in pipes, and the chemistry of the treatment process. The chemistry of th e FGD purge will likely change during the startups of the FGD units as their oper ation 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 flushi ng of all slurry service lines.



### SUMMARY

Proper treatment of the flue gas desulfur ization purge stream by a dedicated FGD wastewater treatment system is an essential pa rt of the FGD retrofit project, unless direct or mixed discharges are allowed for a spec ific 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. Th e system may only require physical-chemical processes, but might also require biological processes, when selenium, nitrogen, or organic compounds must be removed. The se lected treatment steps, based on proven process technology, integr ated into a system with the a ppropriate 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 FG D WWTS must account for the effect of variations in coal composition on purge str eam flow rates and composition. The most significant coal variation is the chloride c ontent that, for a given absorber equilibrium chloride concentration, will direct ly affect the required purge stream flow rate. This flowPage 22 rate will, in turn, directly affect the concentr ations of the other constituents in the purge stream, especially the so-called heavy me tals. Differences in composition between Powder River Basin (PRB) and Eastern Bitumi nous coals can result in notable changes in required purge rates. This impact of coal chloride content is near ly independent of the
coal sulfur concentration. For similar reasons , operating the plant wi th coal of different composition than the design basis coal, can re sult in purge rates and characterization different than the WWTS design values.

Startup and operation of the la test generation of WWTS has provided “less ons learned”, especially with regard to successfully achievi ng effluent limits for mercury, selenium, and nitrogen, which are more st ringent 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 ope ration 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 avai lable since their last paper on the subject.Page 23 REFERENCES
1. Cotton, A., Wilkinson, G., Murillo, C., (1999). Advanced Inorganic Chemistry (6 th Ed.). New York: Wiley.

2. Sonstegard, J., Harwood, J., Pickett, T., Full Scale Implementation of GE ABMet Biological Technology for th e 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 Demons tration Project: A DOE Assessment, Morgantown and Pittsburgh, 2001

5. Mittal, D., Haskin, J., FGD Blowdown Waste Water Treatment and Zero Liquid Discharge, International Water C onference, Pittsburgh, 2006.

6. Cleveland, R., Mezo, J., Nickran d, 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 II I, G., Rodgers Jr., J., Eggert, D., Designing Constructed Wetlands for Mitigating Risks from Flue Gas Desulfurization Wastewater, International Water C onference, 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.