### MEMORANDUM

TO: Steam Electric Rulemaking Record

FROM: Dan-Tam Nguyen

DATE: September 30, 2015

SUBJECT: Revised Methodology to Calculate Aver age Effluent Concentrations for Chemical Precipitation and Biol ogical 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) fo r the Steam Electric Power Generating Industry.

This memorandum summarizes revisions to EP A’s methodology for calculating FGD wastewater effluent concentrations from chemical precip itation and biological treatment systems. EPA

evaluates these concentrations and the treatment effectiveness of each control technology to determine the pollutant concentrations that are us ed to calculate pollutant loadings and removals for each technology option. The pollutant loadings calculation methodology is detailed in Incremental Costs and Pollutant Removals for th e Final Effluent Limitations Guidelines and Standards for the Steam Electric Powe r Generating Point Source Category (DCN SE05831).

1. I NTRODUCTION

EPA updated the calculation methodology b ecause 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 fo r FGD wastewater in the Final ELGs is hydroxide precipitation, sulfide pr ecipitation (organosulfide), a nd iron coprecipitation, followed by an anoxic/anaerobic fixed-film biological trea tment 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 ELG s 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. C ALCULATION OF A VERAGE E FFLUENT P OLLUTANT C ONCENTRATIONS FOR THE
P ROPOSED R ULEMAKING

In the proposed rulemaking, EPA identifie d three plants operating BAT chemical precipitation systems (Hatfield’s Ferry Power St ation, Keystone Generating Station, and Miami Fort Station) and two plants operating BAT bi ological treatment systems (Allen Steam Station and Belews Creek Steam Station) . EPA collected effluent concen tration data for these controlMemorandum September 30, 2015 Page 2

technologies from the plants during EPA’s fi eld sampling program, EPA’s CWA 308 monitoring program, and Duke Energy’s plant monitoring data 1 . EPA collected composite samples on four consecutive days during EPA fiel d sampling, as compared to the composite samples that the industry collected under the CWA 308 monitoring program, whic h 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 cons ideration 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 da ta by first averaging the four EPA field samples and the four CWA 308 monitoring samp les separately; then, EPA averaged the EPA field sampling average concentrations and the CWA 308 average concentrations to determine the average e ffluent concentration.

2. For analytes with EPA field sampling, CW A 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 da ta sets. EPA then calculated the average effluent concentrations using the daily averag es. The addition of monitoring data limited the variability impact of EPA’s four cons ecutive days of samp ling on the calculated average effluent concentrations.

3. S UPPLEMENTED C ALCULATIONS OF A VERAGE E FFLUENT P OLLUTANT C ONCENTRATIONS FOR THE F INAL R ULEMAKING

EPA did not identify Pleasant Prairie Powe r Plant as a BAT plant in the proposed rulemaking. EPA did not use data from the two-stag e system at Pleasant Prairie in developing the

limitations because EPA’s basis for the technolo gy 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 pr ecipitation plant in the basis for the final rulemaking. The technology basis that the final ELG s establish as BAT is a chemi cal precipitation system that employs hydroxide precipitation, sulfide precipitation (organosulfi de), and iron coprecipitation, followed by an anoxic/anaerobic fixed-film biol ogical treatment system designed to remove heavy metals, selenium, and ni trates. 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 concentrati ons 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 re sults among analytes. For the final rulemaking, EPA calculated the averag e effluent concentratio ns 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 av erage effluent concentrations and the remaining sample concentrations from the other data sources.