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MEMORANDUM

TO: Steam Electric Rulemaking Record

FROM: Dan-Tam Nguyen

DATE: September 30, 2015

SUBJECT: Comparison of Once-Through FGD Systems to Recirculating FGD Systems

ERG is providing technical support to the U.S. Environmental Protection Agency’s 

Office of Water, Engineering and Analysis Division (EAD) for development of effluent 

limitations guidelines and standards (ELGs) for the Steam Electric Power Generating Industry. 

This memorandum describes ERG’s review of once-through FGD systems and how they

compare to recirculating FGD systems. 

1. INTRODUCTION

During the public comment period, EPA received comments from the Utility Water Act 

Group (UWAG) and the Tennessee Valley Authority (TVA) claiming that the operation of oncethrough FGD systems differs significantly enough from the operation of recirculating FGD 

systems that implementing a uniform BAT requirement is impractical due to the differences in 

the resulting wastewater characteristics. These commenters also argued that differences between

the two systems related to materials of construction needed to accommodate high chloride levels 

can obviate the ability of facilities with once-through FGD systems to employ the flow 

minimization scheme that EPA used in its cost estimates. Both UWAG and TVA argue that this 

difference warrants a subcategorization for once-through FGD systems. 

ERG compared the wastewater characteristics, the FGD blowdown flow rates, the 

operating chloride levels, and the materials of construction reported for once-through and 

recirculating FGD systems to evaluate the validity of these claims. No additional data were 

provided by UWAG or TVA or other commenters on once-through FGD systems. Details of 

ERG’s comparison are included in a separate spreadsheet titled “CBI_Once Through FGD 

Analysis” (DCN SE05360).

2. DATA SOURCES FOR ONCE-THROUGH FGD SYSTEMS

This section discusses the available data used to evaluate wastewater and operational 

characteristics of once-through FGD systems.
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2.1 Wastewater Characteristics

To represent the once-through FGD blowdown wastewater characteristics, ERG used the 

one-day grab sampling data from the Widow’s Creek Plant collected by EPA in 20071(DCN 

SE02105). Analytical data for FGD blowdown included total and dissolved metals and classical 

analytes. As part of the Widow’s Creek sampling episode, EPA analyzed some analytes using 

both EPA Method 200.8 and EPA Method 1638 and analyzed for both total and dissolved

fractions. For this comparison, where ERG had data from two different analytical methods, ERG 

used the average of the two results. ERG used the J-values as they were reported by the 

laboratory for values measured between the method detection limit (MDL) and the reporting 

limit (RL) and used the MDL for non-detected values below the MDL. Where appropriate, ERG 

calculated the average result as an arithmetic average of the two results. 

2.2 FGD Blowdown Flow Rates

ERG used industry responses from Question B5-2 of the Questionnaire for the Steam 

Electric Power Generating Effluent Guidelines (Steam Electric Survey) (DCN SE05903) for 

plants reporting blowdown flow rates for once-through systems (termed “single-pass” in the 

survey responses). The reported flows range from 3,000 to 2,600,000 gallons per day. 

2.3 Operating Chlorides and Materials of Construction

ERG used industry responses from Questions B4-2 and B4-3 of the Steam Electric 

Survey for once-through FGD systems reporting the minimum and maximum operating chlorides 

in the absorber and for plants reporting the materials of construction for their absorbers. The 

range of operating chloride concentrations in the absorber reported in the survey is 15 to 50,000 

ppm. The materials of construction are CBI and included in the spreadsheet (SE05360). 

3. DATA SOURCES FOR RECIRCULATING FGD SYSTEMS

This section discusses the available data used to evaluate wastewater and operational 

characteristics of recirculating FGD systems. 

3.1 Wastewater Characteristics

ERG accessed data for FGD blowdown from recirculating FGD systems using 

monitoring data from the Steam Electric Survey (Part B, Section 6) and from the EPA Sampling 

data.

 Monitoring Data - The minimum and maximum values for each analyte were 

tabulated to provide a concentration range by analyte and grouped by total metals, 

dissolved metals, and classicals.

 

1 At this time, ERG does not have any other data from once-through FGD systems. No data were provided as part of 

the comments. All of the data submitted in Part B Section 6 of the Steam Electric Survey is qualified as "clarified or 

settled" in Table B-9 and do not represent FGD blowdown (for once-through FGD systems).
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 EPA Sampling Data – ERG evaluated FGD purge data from the EPA Sampling 

and CWA 308 Monitoring Programs for the following plants: Belews Creek, 

Allen, Miami Fort, Keystone, Pleasant Prairie, Hatfield’s Ferry, and Dickerson. 

The minimum and maximum values for each analyte were tabulated to provide a 

concentration range by analyte and grouped by total metals, dissolved metals, and 

classicals.2

3.2 FGD Blowdown Flow Rates

ERG used industry responses from Question B5-2 of the Steam Electric Survey for plants 

reporting blowdown flow rates for recirculating systems. The reported flows range from 1,500 to 

4,400,000 gallons per day. 

3.3 Operating Chlorides and Materials of Construction

ERG used industry responses from Questions B4-2 and B4-3 of the Steam Electric 

Survey for recirculating FGD plants reporting the minimum and maximum operating chlorides in 

the absorber and for plants reporting the materials of construction for their absorbers. The range 

of operating chloride concentrations in the absorber reported in the survey is 10 to 60,000 ppm. 

The materials of construction are CBI and included in the spreadsheet (SE05360). 

4. COMPARISON AND CONCLUSIONS

This section provides a summary of ERG’s comparison of FGD once-through and 

recirculating systems. 

4.1 Wastewater Characteristics

ERG compared the concentrations from the detailed sampling data for once-through 

systems to concentration ranges from both the monitoring data and sampling data for 

recirculating systems. Table 1 summarizes the number of analytes from the data for once-through 

systems where the concentrations fell within, above, or below the range of concentrations from 

the EPA sampling and monitoring data for recirculating FGD systems. Based on this analysis, 

ERG determined that most of the data for once-through FGD systems were within the range of 

concentrations for recirculating FGD systems. 

When comparing the total metals concentrations, ERG found that only two analytes in 

once-through FGD scrubber wastewater were outside the concentration range of the EPA 

sampling data for recirculating systems (i.e., the metals concentrations were less than one percent 

lower than the minimum concentration for recirculating systems). None of the analytes were 

 

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In this comparison, ERG used the same methodology for handling J-values and non-detects below the MDL as 

was used for the Widows Creek data; ERG used the J-values as they were reported by the laboratory for values 

measured between the MDL and RL and used the MDL for non-detects below the MDL. 
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above the range from either dataset. For total metals, a majority of the analytes appear on the 

lower end of the concentration range for recirculating FGD systems. 

When comparing the dissolved metals concentrations, ERG found that Widow’s Creek 

concentrations for two analytes (arsenic and chromium) were slightly greater than EPA sampling 

data and one analyte (molybdenum) was four times greater than the monitoring data. Dissolved 

metals concentrations were somewhat lower in once-through FGD systems (or generally on the 

lower end of the concentration range for recirculating systems). Seven analytes were below the 

concentration ranges in both data sets. Classical analyte concentrations were generally 

comparable between the two types of systems with nitrate/nitrite levels slightly lower in oncethrough systems. The data for recirculating systems did not contain all classical analytes 

collected for once-through systems during the detailed study; therefore, concentrations for 

several classical analytes were not compared.

4.2 FGD Blowdown Flow Rates

Compared to FGD blowdown flow rates for recirculating systems, the flow rates for 

once-through FGD systems fell within the range of flow rates for recirculating systems, with an 

average flow rate that is also comparable to recirculating systems. See Table 2 for a summary of 

flow rates for once-through and recirculating systems. 

4.3 Operating Chlorides and Materials of Construction

The range of operating chloride concentrations for once-through FGD systems falls 

within the range of operating chloride concentrations for recirculating FGD systems, as reported 

in the Steam Electric Survey. Although only a few plants reported maximum operating chloride 

levels greater than 10,000 ppm for once-through FGD systems, those plants report using

materials of construction similar to what is used for recirculating systems. See Table 2 for a 

summary of operating chloride concentrations for once-through and recirculating systems. 

Information on the materials of construction is CBI in the Steam Electric Survey and presented 

in the accompanying CBI spreadsheet (DCN SE05360). 
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Table 1. Comparison of Analyte-Specific Concentrations between Once-Through and Recirculating FGD Systems

Recirculating Systems: Monitoring Data Recirculating Systems: EPA Sampling Data

Once-Through Systems: 

Sampling Data

Number of 

Analytes Analytes Outside Range or No Data

Number of 

Analytes Analytes Outside Range or No Data

Total Metals

Above 0 0

Within 26 24

Below 0 2 Magnesium, Manganese

No Analyte Data 1 Yttrium 1 Yttrium

Total Analytes 27 27

Dissolved Metals

Above 1 Molybdenum 2 Arsenic, Chromium

Within 16 14

Below 9

Boron, Cadmium, Cobalt, Copper, 

Magnesium, Manganese, Silver, Titanium, 

Zinc 11

Hexavalent Chromium, Cobalt, Iron, Lead, 

Magnesium, Manganese, Nickel, Silver, Tin, 

Titanium, Zinc

No Analyte Data 2 Yttrium, Hexavalent Chromium 1 Yttrium

Total Analytes 28 28

Classicals

Above 1 Sulfate 1 Biochemical Oxygen Demand (BOD)

Within 4 6

Below 2 Nitrate/Nitrite, TDS 2 Nitrate/Nitrite, TDS

No Analyte Data 5

Hexane Extractable Material (HEM), Silica 

Gel Treated HEM (SGT-HEM), Chemical 

Oxygen Demand (COD), Biochemical 

Oxygen Demand (BOD), Total Suspended 

Solids (TSS) 3

Hexane Extractable Material (HEM), Silica Gel 

Treated HEM (SGT-HEM), Chemical Oxygen 

Demand (COD)

Total Analytes 12 12
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Table 2. Summary of Purge Flow Rates and Operating Chlorides for Once-Through and 

Recirculating Systems

FGD System 

Type

Purge Flow Rate (gpd)

Range of Operating Concentration of 

Chlorides in Absorber (ppm)

Minimum Average Maximum Minimum Maximum

Once-Through 3,300 239,395 2,607,000 14 50,000 

Recirculating 1,520 228,078 4,413,600 6 58,000