MEMORANDUM

DATE: November 18, 2012

SUBJECT: Assessment of Effluent Limitations and Standards with No Baseline Substitution 

for the Steam Electric Rulemaking

TO: Steam Electric Rulemaking Record, USEPA

FROM: Ronald Jordan and Cuc Schroeder, USEPA

Cc: John Rogers, Westat

Section 13 of the Technical Development Document (TDD) explains EPA’s procedure for 

calculating effluent limitations and standards (hereafter referred to as limitations). This 

memorandum addresses the sensitivity of the limitations to the substitution of certain low 

concentration values with pollutant-specific “baseline” values (i.e., baseline substitution). For 

each pollutant proposed to be regulated for each treatment technology option1 for flue gas 

desulfurization (FGD) wastewater2 and gasification wastewater, this memorandum provides a

comparison between what the limitations would be using all valid reported results without 

baseline substitution and the limitations calculated with baseline substitution incorporated.

In developing the limitations for the steam electric rulemaking, EPA used the detected 

values or sample-specific detection limit in the case where the pollutant was non-detected3

. 

However, there were instances where EPA substituted a baseline value for a detected value or 

sample-specific detection limit. EPA used the baseline value in the statistical analysis to 

account for the possibility that certain detected results may be at concentrations that are lower

than can be reliably measured when the analytical method(s) is performed by experienced staff 

in a well-operated laboratory.

4

 1 The option long-term average and option variability factors in this memorandum refer to the treatment 

technology option rather than the regulatory options presented in Section 8 of the TDD.

 This approach is consistent with the way EPA has calculated 

limitations in previous effluent guidelines. See, for example, the rulemakings for the Meat and 

Poultry Products ELG and the Iron and Steel ELG.

2 Since EPA is proposing to establish limitations for combustion residual leachate based on transferring limitations 

calculated for FGD wastewater treatment technology option(s), the evaluation of baseline substitution on FGD 

wastewater limitations similarly applies to the proposed limitations for combustion residual leachate. For brevity, 

combustion residual leachate is referred to in this memorandum as leachate.

3

 The term “non-detected” refers to values that are below the method detection limit (MDL) and those measured 

by the laboratory as being between the MDL and the quantitation limit (QL) in the original data (before adjusting 

for baseline).

4 A well-operated laboratory is a laboratory that routinely monitors performance through quality control analyses, 

control charts, and other measures to rapidly identify and correct deteriorating or poor performance, and with 

analysts experienced with method sample preparation, analysis, and detection procedures.
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EPA performed the baseline substitution as follows: EPA examined the data used to 

calculate the limitations for each technology option and compared each reported result to a 

baseline value. Whenever a detected value was lower than the baseline value, EPA used the 

baseline value instead and classified the value as non-detected even though the actual reported 

result was a detected value. Similarly, whenever a non-detected value had a sample-specific 

detection limit lower than the applicable baseline value, EPA used the baseline value instead 

and continued to classify the value as non-detected. The bullets below present the baseline 

values that were used for each pollutant in this steam electric rulemaking:

o arsenic: 2 ug/L 

o mercury: 0.5 ng/L

o nitrate-nitrite as N: 0.05 mg/L

o selenium 5 ug/L

o TDS: 10 mg/L

EPA determined the baseline values for mercury, nitrate-nitrite as N, and TDS using the 

minimum levels (MLs) established by the analytical methods used to obtain the reported values 

or a comparable analytical method where an ML was not specified by the method.5 The 

baseline values for arsenic and selenium are based on the results of method detection limit 

(MDL) studies conducted by well-operated commercial laboratories using EPA Method 200.8 to 

analyze samples of synthetic FGD wastewater (for a detail discussion, see DCN SE03835).

EPA considers this historical approach of incorporating baseline substitution to be

similarly appropriate for the steam electric ELG rulemaking. Nevertheless, EPA conducted an 

assessment to determine how the use of baseline substitution affected the calculation of the 

limitations.

Appendix B of the TDD explains how EPA accounts for autocorrelation when calculating 

limitations. Also, as explained in detail in Section 13 of the TDD, for all technology options 

except the biological treatment technology option for FGD wastewater, EPA was unable to 

obtain reliable estimates for the autocorrelations since there were too few observations 

available, and thus the autocorrelation value was set to zero. For the biological treatment 

technology option for FGD wastewater, since several years of data were available, EPA was able 

to obtain an autocorrelation value for situations when either baseline substitution is or is not 

incorporated. The autocorrelation values obtained with or without baseline substitution were 

used in calculating the limitations presented in tables below. See DCN SE02001 for the 

autocorrelation values used for each pollutant for these two cases; see DCN SE02002 for a list 

 5 The baseline values for mercury and nitrate-nitrite as N are equal to the MLs specified in EPA Methods 1631E and 

353.2, respectively. The method EPA used to analyze for TDS (Standard Method 2540C) does not explicitly state a 

MDL or ML. However, EPA Method 160.1 is similar to Standard Method 2540C and the lower limit of its 

measurement range is 10 mg/L (i.e., the nominal quantitation limit). Thus, EPA used 10 mg/L as the baseline value 

for TDS.
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of data that were used to calculate the limitations; and see Appendix B of the TDD for the 

statistical model used to obtain the long-term average, variability factors, and limitations.

Table 1 below presents the limitations obtained when incorporating baseline 

substitution for detected and non-detected values reported below the pollutant-specific 

baseline values shown in the bullets above. The limitations presented in Table 1 are the 

limitations proposed for this rulemaking, or calculated for other technology options considered 

for BAT/NSPS/PSES/PSNS. 
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Table 1. Summary of the Option Long-Term Averages and Variability Factors Together with the 

Limitations for Each of the FGD, Gasification, and Leachate Technology Options When Using 

Baseline Substitution.

Treatment Technology Pollutant

Option 

Long-Term 

Average 

Option 

Daily 

Variability 

Factor 

Option 

Monthly 

Variability 

Factor 

Daily 

Maximum 

Limitation4

Monthly 

Average 

Limitation4

Chemical Precipitation 

for FGD

Arsenic (ug/L) 4.483 1.741 1.223 8 6

Mercury (ng/L) 75.404 3.209 1.570 242 119

Chemical Precipitation 

and Biological

Treatment

for FGD 

Arsenic (ug/L) 1 4.483 1.741 1.223 8 6

Mercury (ng/L) 1 75.404 3.209 1.570 242 119

Nitrate-nitrite as 

N (mg/L) 0.110 1.499 1.157 0.17 0.13

Selenium (ug/L) 7.455 2.145 1.321 16 10

Chemical Precipitation 

and Evaporation for 

FGD

Arsenic (ug/L) 4.02 --3 --3 4

5 -- 6

Mercury (ng/L) 17.788 2.192 1.338 39 24

Selenium (ug/L) 5.02 --3 --3 55 --6

TDS (mg/L) 14.884 3.341 1.572 50 24

Vapor-Compression 

Evaporation for 

Gasification

Arsenic (ug/L) 4.02 -- 3 -- 3 4

5 -- 6

Mercury (ng/L) 1.075 1.632 1.194 1.76 1.29

Selenium (ug/L) 146.780 3.083 1.545 453 227

TDS (mg/L) 15.209 2.483 1.389 38 22

Chemical Precipitation 

for Leachate 

Arsenic (ug/L)1 4.483 1.741 1.223 8 6

Mercury (ng/L)1 75.404 3.209 1.570 242 119 1

: Option long-term average, option variability factors, and limitations were transferred from chemical 

precipitation technology option for FGD wastewater.

2

: Long-term average is the arithmetic mean since all observations were non-detected. 3

: All observations were non-detected, so variability factors could not be calculated. 4

: Limitations less than 1.0 are rounded up to the next highest hundredths decimal place.

Limitations greater than 1.0 have been rounded upward to the next highest integer, except for limitations for 

mercury in gasification wastewater based on the vapor-compression evaporation treatment technology option 

which have been rounded up to the next highest hundredths decimal place. 

5

: Limitation is set equal to the detection limit. 6

: Monthly average limitation is not established when the daily maximum limitation is based on the detection limit.

Table 2 below presents what the limitations would be if the limitations are calculated 

using all valid reported results (i.e., no baseline substitution is made for values below the 

baseline) for each pollutant for each treatment technology option. Upon comparison of Tables 

1 and 2, the limitations calculated using baseline substitution are identical to the limitations 

calculated when baseline substitution is not used, except for nitrate-nitrite as N and selenium in 

FGD wastewater based on the biological and gasification wastewater based on the vaporcompression evaporation treatment technology options. The baseline affects the nitrate-nitrite 
Page 5

as N limitation only because the baseline affects the selenium measurements and correlation 

and the selenium correlation is used for calculating the limitation for nitrate-nitrite as N. The 

limitations for all other pollutants are unaffected by baseline substitution since all values for 

these pollutants were above their respective baseline values. However, for completeness, this 

document presents the limitations for all pollutants considered for regulation under the various 

technology options for the rulemaking. 

Table 2. Summary of the Option Long-Term Averages and Variability Factors Together with the 

Limitations for Each of the FGD, Gasification, and Leachate Technology Options When Using No 

Baseline Substitution.

Treatment Technology Pollutant

Option 

LongTerm 

Average

Option 

Daily 

Variability 

Factor

Option 

Monthly

Variability 

Factor

Daily

Maximum

Limitation

4

Monthly 

Average

Limitation

4

Chemical Precipitation 

For FGD 

Arsenic (ug/L) 4.483 1.741 1.223 8 6

Mercury (ng/L) 75.404 3.209 1.570 242 119

Chemical Precipitation 

and Biological 

Treatment For FGD

Arsenic (ug/L)1 4.483 1.741 1.223 8 6

Mercury (ng/L) 1 75.404 3.209 1.570 242 119

Nitrate Nitrite as N 

(mg/L) 0.110 1.500 1.157 0.17 0.13

Selenium (ug/L) 6.992 2.927 1.510 21 11

Chemical Precipitation 

and Evaporation for 

FGD

Arsenic (ug/L) 4.02 --3 --3 45 --

6

Mercury (ng/L) 17.788 2.192 1.338 39 24

Selenium (ug/L) 4.02 --3 --3 45 --

6

TDS (mg/L) 14.884 3.341 1.572 50 24

Vapor-Compression 

Evaporation for 

Gasification 

Arsenic (ug/L) 4.02 --3 --3 45 --

6

Mercury (ng/L) 1.075 1.632 1.194 1.76 1.29

Selenium (ug/L) 146.480 2.222 1.331 326 195

TDS (mg/L) 15.209 2.483 1.389 38 22

Chemical Precipitation 

for Leachate

Arsenic (ug/L) 1 4.483 1.741 1.223 8 6

Mercury (ng/L) 1 75.404 3.209 1.570 242 119 1

: Option long-term average, option variability factors, and limitations were transferred from chemical 

precipitation technology option for FGD wastewater. 2

: Long-term average is the arithmetic mean since all observations were non-detected. 3

: All observations were non-detected, so variability factors could not be calculated. 4

: Limitations less than 1.0 are rounded up to the next highest hundredths decimal place.

Limitations greater than 1.0 have been rounded upward to the next highest integer, except for limitations for 

mercury based on the vapor-compression evaporation treatment technology option for gasification wastewater 

which have been rounded up to the next highest hundredths decimal place. 

5

: Limitation is set equal to the detection limit.

6

: Monthly average limitation is not established when the daily maximum limitation is based on the detection limit.