MEMORANDUM 

TO: Steam Electric Rulemaking Record 

FROM: Dan-Tam Nguyen 

DATE: September 30, 2015 

SUBJECT: Revised Methodology to Calculate Average Effluent Concentrations for 

Chemical Precipitation and Biological Treatment Systems for FGD 

Wastewater 

 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 summarizes revisions to EPA’s methodology for calculating FGD wastewater 

effluent concentrations from chemical precipitation and biological treatment systems. EPA 

evaluates these concentrations and the treatment effectiveness of each control technology to 

determine the pollutant concentrations that are used to calculate pollutant loadings and removals 

for each technology option. The pollutant loadings calculation methodology is detailed in 

Incremental Costs and Pollutant Removals for the Final Effluent Limitations Guidelines and 

Standards for the Steam Electric Power Generating Point Source Category (DCN SE05831). 

1. INTRODUCTION 

 EPA updated the calculation methodology because EPA obtained additional effluent 

concentration data to supplement the existing data EPA used in the proposed rulemaking. EPA 

received effluent concentration data from plants that were identified as operating best available 

technology (BAT). The BAT technology basis for FGD wastewater in the Final ELGs is 

hydroxide precipitation, sulfide precipitation (organosulfide), and iron coprecipitation, followed 

by an anoxic/anaerobic fixed-film biological treatment system designed to remove heavy metals, 

selenium, and nitrates. For more discussion on EPA’s selection of the technology basis that 

represents BAT in the Final ELGs see the preamble section VIII. 

 The data sources are described in detail in Development Memorandum for the Steam 

Electric Analytical Database for the Final Rule (DCN SE05876). The data are included in the 

Steam Electric Analytical Database for the Final Rule (DCN SE05359). 

2. CALCULATION OF AVERAGE EFFLUENT POLLUTANT CONCENTRATIONS FOR THE 

PROPOSED RULEMAKING

 In the proposed rulemaking, EPA identified three plants operating BAT chemical 

precipitation systems (Hatfield’s Ferry Power Station, Keystone Generating Station, and Miami 

Fort Station) and two plants operating BAT biological treatment systems (Allen Steam Station 

and Belews Creek Steam Station). EPA collected effluent concentration data for these control 
Memorandum 

September 30, 2015 

Page 2 

technologies from the plants during EPA’s field sampling program, EPA’s CWA 308 monitoring 

program, and Duke Energy’s plant monitoring data1. EPA collected composite samples on four 

consecutive days during EPA field sampling, as compared to the composite samples that the 

industry collected under the CWA 308 monitoring program, which were collected on four nonconsecutive days. Industry also provided plant monitoring data for the BAT biological treatment 

system effluent, which were collected on either a daily, weekly, or intermittent basis. 

 To minimize the variability in concentrations as a result of the different frequencies of 

samples collected between data sources, EPA conducted an analyte-specific analysis to 

determine the total number of data sources for each analyte and used two separate approaches to 

calculate the average effluent concentrations: 

1. For analytes with EPA field sampling and CWA 308 monitoring data only, the smaller 

data set means the variability in concentrations can be biased if a straight average (i.e., 

arithmetic mean) is calculated without consideration for the EPA field sampling data 

which were collected on four consecutive days and the CWA 308 monitoring data which 

were collected on four non-consecutive days. For this reason, EPA put less emphasis on 

the variability in the EPA field sampling data by first averaging the four EPA field 

samples and the four CWA 308 monitoring samples separately; then, EPA averaged the 

EPA field sampling average concentrations and the CWA 308 average concentrations to 

determine the average effluent concentration. 

2. For analytes with EPA field sampling, CWA 308 monitoring, and industry-submitted 

plant monitoring data, EPA had a larger data set that minimized the bias from any one 

data set. On days where multiple sample results are reported across data sources, EPA 

calculated daily averages using all three data sets. EPA then calculated the average 

effluent concentrations using the daily averages. The addition of monitoring data limited 

the variability impact of EPA’s four consecutive days of sampling on the calculated 

average effluent concentrations. 

3. SUPPLEMENTED CALCULATIONS OF AVERAGE EFFLUENT POLLUTANT 

CONCENTRATIONS FOR THE FINAL RULEMAKING

EPA did not identify Pleasant Prairie Power Plant as a BAT plant in the proposed 

rulemaking. EPA did not use data from the two-stage system at Pleasant Prairie in developing the 

limitations because EPA’s basis for the technology option is a one-stage system. However, in 

consideration of public comments received on the proposed rule, EPA now includes Pleasant 

Prairie as a BAT chemical precipitation plant in the basis for the final rulemaking. The 

technology basis that the final ELGs establish as BAT is a chemical precipitation system that 

employs hydroxide precipitation, sulfide precipitation (organosulfide), and iron coprecipitation, 

followed by an anoxic/anaerobic fixed-film biological treatment system designed to remove 

heavy metals, selenium, and nitrates. EPA determined that Pleasant Prairie does operate a 

treatment system that represents BAT for chemical precipitation. 

 EPA included EPA field sampling and CWA 308 monitoring data for Pleasant Prairie in 

calculating the average effluent concentrations for the final rulemaking. Additionally, EPA 

 

1

 Duke Energy’s plant monitoring data includes data collected in 2011 in support of the proposed rulemaking. 
Memorandum 

September 30, 2015 

Page 3 

received supplemental effluent concentration data from the four plants operating BAT chemical 

precipitation and from the two plants operating BAT biological treatment. The monitoring data 

vary between daily, weekly, and monthly results and by the number of results among analytes. 

For the final rulemaking, EPA calculated the average effluent concentrations as follows: 

 For all samples collected on the same days as the EPA field samples, EPA calculated 

daily averages for each sampling day. EPA then calculated four-day average effluent 

concentrations using the daily averages. EPA calculated the average effluent 

concentration by averaging the four-day average effluent concentrations and the 

remaining sample concentrations from the other data sources.