Document ID: EPA-HQ-OW-2009-0819-5714
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2015-11-03T05:00Z

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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.  
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 once-through 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).

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.
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 2007 (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. 

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.  
      
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).  

Data Sources for Recirculating FGD Systems
	This section discusses the available data used to evaluate wastewater and operational characteristics of recirculating FGD systems. 

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.

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.

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.  
      
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).  

Comparison and Conclusions
	This section provides a summary of ERG's comparison of FGD once-through and recirculating systems. 
      
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 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 once-through 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.

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. 

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). 

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
                                       

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