Final Notes from Site Visit at We Energies’ Pleasant Prairie Power Plant

on March 2, 2010

EPA conducted a site visit at We Energies’ Pleasant Prairie Power Plant (Pleasant 

Prairie) located in Pleasant Prairie, WI on Tuesday, March 2, 2010 to learn about the operations 

at the plant, specifically the flue gas desulfurization (FGD) wastewater treatment system. These 

notes describe the information collected during the site visit. Table 1, located at the end of these 

notes, identifies the site visit participants. An FGD wastewater flow diagram and an overall plant

water balance diagram are provided as Attachments 1 and 2, respectively, to these notes.

Plant Overview

• Pleasant Prairie operates two coal-fired generating units at the power plant. Units 

1 and 2, which began operation in 1980 and 1985, respectively, each have a 

capacity of 615 megawatts (MW). Both units are operated as baseload units.

• Pleasant Prairie retrofitted Units 1 and 2 with FGD scrubber systems to control 

the sulfur dioxide (SO2) releases from the plant. The FGD scrubber systems were 

installed in November 2006 (Unit 1) and March 2007 (Unit 2). With the addition 

of the FGD scrubber systems, Pleasant Prairie also installed an FGD wastewater 

treatment plant to control the discharges of the wastewater generated during the 

operation of the FGD scrubbers.

• SCR systems were retrofit additions to the plant and were installed on Unit 2 in 

2003 and Unit 1 in 2006. The FGD scrubber system and SCR system for Unit 1 

were installed and placed into service at the same time. SCRs are used to control 

nitrogen oxide (NOx) emissions. The plant operates the SCR systems year round. 

• Pleasant Prairie operates cold side electrostatic precipitators (ESPs) on both Units 

1 and 2. The ESPs remove approximately 99.7% of the particulates in the flue 

gas. Pleasant Prairie operates a dry fly ash handling system.

• Pleasant Prairie recently installed a calcium bromide injection system for the 

boiler. The bromide injection system has been achieving 60-70% mercury 

removal, or better. A liquid form of 50% calcium bromide is sprayed on to the 

coal at the inlet to the ball mills used to pulverize the coal. The bromide addition 

results in conditions that lead to the oxidation of mercury into a form that 

increases its removal in the wet FGD system. Mercury is ultimately removed 

from the wet FGD blowdown at the treatment facility and the dried wastewater 

sludge is landfilled. Increased amounts of flue gas mercury removal (adsorbed to 

the fly ash) in the precipitators has also been observed. 

• Pleasant Prairie burns Powder River Basin (PRB) coal in both units. The plant 

burns approximately 5.5 million tons of coal per year. 
Page 2

• Pleasant Prairie is located about 5 miles from Lake Michigan. The plant

discharges to Lake Michigan.

• During the site visit, Unit 1 was in a scheduled maintenance outage. During this 

outage, the air heater baskets were serviced and the annual boiler work was 

completed. 

Carbon Capture Study

• Pleasant Prairie participated in a pilot-scale carbon capture technology study in 

collaboration with EPRI and Alstom that started in 2007 and ended in October 

2009. The study used a slip stream of flue gas from either Unit 2 or Unit 1 to test 

Alstom’s chilled ammonia carbon capture technology. Alstom and EPRI operated 

the pilot-scale system at the plant for approximately two years before the study 

was completed. The carbon capture pilot was initially set up to obtain flue gas 

from only Unit 2. A back up connection was later added to allow the carbon 

capture pilot to remain in service when Unit 2 was shut down for its annual 

scheduled maintenance outage, or was off line due to a forced outage.

• Plant personnel stated that the carbon capture system was successful in removing 

up to 90% of the carbon dioxide from the flue gas. Following capture the carbon 

dioxide was routed to the flue gas and emitted to the stack. This project evaluated 

carbon dioxide capture, and did not involve an evaluation of the potential to store 

carbon dioxide below the plant.

• Plant personnel stated that the lessons learned from the study at Pleasant Prairie 

were incorporated into the pilot study that is taking place at AEP’s Mountaineer 

plant in New Haven, WV. 

• Plant personnel explained that the system at the Pleasant Prairie site utilized 

aqueous ammonia and sulfuric acid for pH adjustment in the absorber. The

process generated a waste stream consisting of an ammonium sulfate solution. 

The plant’s wastewater treatment system does not treat for ammonia; therefore, 

the wastewater stream from the study was collected and transferred off site. The 

generated non-hazardous wastewater was transferred off-site and treated at 

Beaver Oil Company in Illinois. Beaver Oil Company received the dilute 

ammonium sulfate waste stream from Alstom Power's Chilled Ammonia Carbon 

Capture Pilot at Pleasant Prairie, and placed it "into their general waste treatment 

feed." It was mostly water and was used for pH adjustment of incoming feeds to 

allow for precipitation in their batch treatment tanks.

• Pleasant Prairie stated that because the carbon capture system was operated by 

Alstom and EPRI, they were uncertain of the generation rate for the wastewater

stream. Additionally, Pleasant Prairie does not have any analytical results 

characterizing the waste stream; however, the plant believes that EPRI should 

have characterization data. Pleasant Prairie stated that they do not know if the 
Page 3

characterization data will be included in the final report prepared for the study. 

The letter that was submitted by Pleasant Prairie to the Department of Natural 

Resources (DNR) characterizing the waste stream and the DNR’s response to this 

letter are provided in Attachment 3.

• EPRI collected water and air samples from the system during the study. Plant 

personnel stated that a final report, from EPRI, may be available by the end of 

2010. 

FGD Operation

• Pleasant Prairie has two identically designed FGD scrubber systems on Units 1 

and 2.

• The FGD spray tower has three spray header levels; however, the plant typically 

only operates two of the spray levels at one time. The FGD scrubber systems are 

also equipped with one dual flow tray. 

• The plant does not add any organic acids to the scrubber (e.g., dibasic acid 

(DBA)) because the plant’s FGD wastewater treatment system does not have the 

capability to remove the BOD from the system. Pleasant Prairie stated that if they 

did add organic acids they would have to send the wastewater to a publicly owned 

treatment works (POTW) for treatment.

• The plant has experienced some problems with foaming in the absorber; therefore, 

they have been adding a silicone-based antifoamant to control this issue. The 

plant adds between 10 and 20 gallons of the antifoamant per day.

• The plant uses a limestone slurry as the sorbent for sulfur dioxide (SO2)

absorption. The two scrubber systems share two ball mills where the limestone is 

ground and mixed with water to 40 to 50% solids by weight. The plant uses 

hydrocyclones to classify the limestone and ensure the correct size particles are 

used in the FGD scrubber system.

• Pleasant Prairie maintains the solids concentration in the FGD scrubber systems 

between 18 and 21% solids. When the solids concentration exceeds 18%, the 

plant blows down from the FGD scrubber. Plant personnel estimated that each 

unit blows down for about 8 to 10 hours each day. The plant tries to stagger the 

blow down from the two units.

• Pleasant Prairie also maintains the chloride concentration in the FGD scrubber 

systems between 3,000 and 10,000 ppm. The plant tries to keep the chlorides 

concentration around 6,000 ppm, or lower. The maximum design chlorides 

concentration is 15,000 ppm.
Page 4

• Plant staff indicated that the chloride concentration in Unit 1 tends to be higher 

than in Unit 2 because water from Unit 2 is recycled to Unit 1.

• The blowdown for each FGD scrubber system is transferred to its primary 

hydrocyclone. Each FGD scrubber system has a dedicated hydrocyclone. The 

underflow from the primary hydrocyclones, which is 50% solids, is transferred to 

the vacuum belt filters where the plant washes the gypsum to lower the chlorides 

concentration in the gypsum. Each primary hydrocyclone feeds either of the two 

common vacuum belt filters. The vacuum belt filters dry the gypsum to less than 

10% moisture. Pleasant Prairie sells the gypsum to a wallboard manufacturing in 

Waukegan, IL, as well as for agricultural uses.

• The filtrate from the vacuum belt filters is collected and transferred back to the 

FGD scrubber systems. All the filtrate is typically transferred back to the Unit 1 

FGD scrubber system; however the plant has the capability to also send the 

filtrate to the Unit 2 FGD scrubber system.

• The primary hydrocyclone overflow from the two units are commingled together

and transferred to a common secondary hydrocyclone. The underflow from the 

secondary hydrocyclone is recycled back to the FGD scrubber systems. The 

overflow from the second hydrocyclone is transferred to the FGD wastewater 

treatment system.

FGD Wastewater Treatment System

• The FGD wastewater treatment system is designed to receive intermittent 

discharges of FGD wastewater from the secondary hydrocyclones. Normal system 

flow is approximately 40 gpm. The design flow rate for the FGD wastewater 

treatment system is 75 gpm.

• The secondary hydrocyclone overflow is transferred directly to the equalization 

tank, where intermittent flows from the hydrocyclone are equalized allowing the 

plant to pump a constant flow rate of FGD wastewater through the treatment 

system. The equalization tank also receives wastewater from the filtrate sump, 

which consists of filter press filtrate and treated effluent recycle. 

• The wastewater flows from the equalization tank to reaction tank 1, where 

hydrated lime is added to raise the pH of the wastewater from 5.6 to about 8.5 

S.U. Reaction tank 1 also receives some of the sludge from the downstream 

primary clarifier which provide seed crystals for gypsum nucleation.

• From reaction tank 1, the wastewater flows to the primary clarifier. The clarifier 

contains approximately 2 to 4 feet of settled solids in the bottom of the clarifier. 

Pleasant Prairie adds a polymer to the primary clarifier to aid the settling of the 

solids. 
Page 5

• The overflow from the primary clarifier flows to reaction tank 2, where the plant 

adds organosulfide and hydrochloric acid. When the plant first began operating 

the treatment system, they used TMT 15® as the organosulfide. Since that time, 

the plant has switched to using Nalmet® 1689, which is an organosulfide that has 

a longer polymer chain, and therefore, a higher molecular weight, which helps aid 

in the settling of precipitates. Pleasant Prairie also began adding hydrochloric 

acid to reaction tank 2, which wasn’t in the initial design of the treatment system, 

because the plant determined that the they got better removals of mercury when 

the treatment system operated at a lower pH. The pH in reaction tank 2 is

targeted at 7.0 to 7.2 S.U.

• The wastewater flows next to reaction tank 3, where ferric chloride is added. The 

wastewater then flows to the flash mix tank, where polymer is added. From the 

flash mix tank, the wastewater flows to the flocculation tank where a gentler 

mixing takes place.

• The wastewater from the flocculation then flows to the secondary clarifier. The 

secondary clarifier settles the solids created from chemical conditioning steps. 

The secondary clarifier is an inclined-plate lamella separator type clarifier. 

Hydrochloric acid can be added to the overflow from the secondary clarifier to 

meet discharge standards for pH. Under normal operation, hydrochloric acid is 

only added to reaction tank 2.

• The treated wastewater is collected in two effluent tanks prior to discharging to 

Lake Michigan.

• Pleasant Prairie commingles the effluent from the FGD wastewater treatment 

system with other plant wastewaters, typically cooling tower blowdown, at a ratio 

of 30:1 to meet the final outfall mercury and chloride limits, 80 ng/L and 1,514 

mg/L, respectively.

• The plant stated that when they chlorinate the cooling system, they do not 

discharge cooling tower blowdown. Because there is no discharge of cooling 

tower blowdown during chlorination, the plant does not discharge the treated 

FGD wastewater because there is not enough other plant wastewaters to 

commingle with the FGD wastewater to meet the final outfall mercury limit. The 

treated wastewater is held in the effluent tank during chlorination until 

discharged. The working volume of the two treated effluent tanks is about 18,000 

gallons so the plant can interrupt discharge of treated effluent for about 8 hours 

before needing to go into recirculation mode.

• The FGD wastewater treatment system typically operates Monday through 

Saturday. The plant performs weekly maintenance on the system starting Saturday 

evening. This maintenance includes draining and flushing the second clarifier, to 

avoid flocculation build up, which can impact its performance.
Page 6

• The first FGD scrubber, installed in November 2006, ran for several months with 

100% recirculation before blowing down to the FGD wastewater treatment 

system. The plant had several start up and performance issues with the FGD 

wastewater treatment system and did not discharge from the system until the end 

of calendar year 2007. Pleasant Prairie was able to operate the wastewater 

treatment system in recirculation mode for a long period of time because the 

chlorides in the system do not build up quickly due to the low chlorine content in 

the PRB coal.

• When the plant does not blowdown for several weeks, fines and colloidal particles

begin to build up in the FGD wastewater treatment system because they are being 

recirculated throughout the system without being discharged. As the fines 

concentration begins to increase, the mercury concentration also begins to 

increase. If the mercury concentration rises above the NPDES permit limit, the 

plant is unable to discharge and, therefore, the mercury concentration would 

continue to rise. To help remove the fines and associated mercury from the 

wastewater, Pleasant Prairie installed a set of cartridge filters that can be used as a 

polishing step. The plant used the cartridge filters to remove the mercury from the 

FGD wastewater treatment system after the initial start up phase. The filters were

replaced every 24-36 hours depending on flow rate and solids loading. The plant 

is still able to treat the wastewater through the cartridge filters, but they try not to 

do this unless necessary because the filters are expensive. Currently the plant can 

stop discharging for approximately a week before they can notice chlorine 

concentrations increases. 

• Pleasant Prairie has made a few modifications to the FGD wastewater treatment 

system since the system first began operating in early 2007. The following is a list 

of the modifications Pleasant Prairie has made to the FGD wastewater treatment 

system:

• Changed the type of organosulfide used in the system from TMT

15® to Nalmet® 1689;

• Added a hydrochloric acid addition point to reaction tank 2 

because the system achieves better mercury removals at a lower 

pH;

• Bypassed a set of sand filters which were located after the 

secondary clarifiers because of scaling issues; and

• Added the capability to use cartridge filtration to remove colloidal 

particles from the wastewater after the effluent tanks, which may 

build up in the system if the plant recirculates the wastewater 

without discharging for several weeks; however the cartridge 

filtration is not used regularly.
Page 7

Wastewater Treatment System Performance

• Pleasant Prairie is required to sample mercury twice per week, according to their 

NPDES permit. The plant also collects samples Monday through Friday off the 

secondary clarifier for confirmation of system performance. The plant uses a 

Method DMA 80 Hg analyzer to test mercury, which is comparable to Method 

1631E, but is usually biased high. For example, some samples analyzed using 

Method 1631E resulted in mercury concentrations of 0.22 to 0.25 ppb, and the 

DMA 80 resulted in mercury concentrations of 0.35 to 0.4 ppb.

• When mercury concentrations exceed 1.1 ppb, the plant will begin discharging 

from the FGD wastewater treatment system in batches. The wastewater is

transferred to one of the effluent tanks and then isolated when it is full. The plant 

will then analyze a sample for mercury to ensure it is below the permit limit of 1.5 

ppb. Once the mercury concentration is confirmed, the plant discharges the 

wastewater from the effluent tank. 

• The plant staff has been working to optimize the performance of the system and 

typically achieves mercury concentrations below 0.4 ppb. The wastewater 

treatment system is currently achieving the lowest mercury concentrations since 

the installation of the system without the use of polishing filtration, 0.25 ppb.

• FGD wastewater from units burning PRB coal has a higher influent mercury 

concentration, compared to bituminous coal. PRB coal produces an FGD 

wastewater treatment influent with a mercury concentration of 2,000 ppb ± 500, 

compared to 500 ppb for an influent concentration from bituminous coal. 

• Pleasant Prairie believes that high mercury levels in the FGD wastewater are a 

result of the scrubber chemistry and burning PRB coal. Pleasant Prairie stated that 

the mercury concentration measured in the effluent from their treatment system is 

an order of magnitude higher than the effluent from a similar treatment system 

burning bituminous coal at another of We Energies facility. The average mercury 

concentration reached after treatment for bituminous coal is 0.03-0.04 ppb; some 

even reaching 14 ppt.

• Pleasant Prairie has performed pilot tests using microfiltration for the reduction of 

mercury and other heavy metals in the final effluent. In September/October 2008, 

the plant performed a series of membrane pilot tests. The pilot tests used a 1 gpm 

slip stream from the primary clarifier effluent as the basis for the study. The pilot 

tests included the following:

• Pleasant Prairie tested Siemens’ Memcor® hollow fiber microfiltration 

system. The system achieved effluent mercury concentrations of 0.1 ppb. 

The system was only tested over a one week period because it experienced

significant scaling issues which started to develop within less than 24 
Page 8

hours of operation. This pilot study is described in detail in EPRI report 

1020576.

• Pleasant Prairie tested GE’s Zeeweed membranes during a 2-3 week 

period, but not the entire GE Zeeweed microfiltration system. The entire 

Zeeweed system is a microfiltration unit that includes an integrated air 

scour and chemical cleaning system. This standard GE Zeeweed unit was 

not tested because of potential corrosion concerns. The pilot test involved 

the use of organosulfide, iron co-precipitation, and microfiltration that 

achieved effluent mercury concentrations of 0.1 ppb. The microfiltration

system incorporated a small Zeeweed hollow fiber membrane module (0.1 

micron) that operated at about 1 gpm. The Zeeweed membranes were 

removed and manually cleaned to remove scale several times during the 

duration of the pilot test. Pleasant Prairie stated that the system performed 

well and appeared to be more promising than the Memcor® system. 

However, the plant stated that they would want the system to be tested on 

a larger scale before they would purchase the system. More information 

about this study can be found in EPRI report 1016813. 

• Pleasant Prairie also tested an absorptive media system and an iron 

cementation system.

• Selenium levels are also higher in PRB coal than bituminous coal. Total selenium

in the influent FGD wastewater at Pleasant Prairie is about 15,000 ppb, mostly in 

the solids phase. The effluent from the system is between 3,000 and 5,000 ppb. 

Pleasant Prairie believes that the selenium in their wastewater is difficult to treat 

because there is more selenate (which has a higher solubility) in the system, than 

selenite. Pleasant Prairie believes the presence of more selenate is due to the 

scrubber chemistry associated with PRB coal. 

Information for Sampling

• EPA identified the FGD wastewater treatment system influent sampling point as 

the tap off the secondary hydrocyclone. EPA would use this sample to represent 

the influent to the FGD wastewater treatment system. Pleasant Prairie stated that 

they could install a smaller hose barb on the tap to accommodate the sample 

collection. There is plenty of space surrounding the secondary hydrcyoclone in 

order to set up tables and store coolers/supplies.

• EPA identified the FGD wastewater treatment system effluent sampling point as 

the tap located after the effluent tank, prior to recirculation or discharge, at the 

sample panel. Prairie stated that they could install a smaller hose barb on the tap 

to accommodate the sample collection. There is available space in the FGD 

wastewater treatment building to set up tables and store coolers.
Page 9

• Pleasant Prairie stated that all on-site sampling personnel would be required to 

attend a 30 minute health and safety training session. In case of emergencies 

during the sampling episode, the samplers should contact the control room 

immediately. Additionally, all personnel will be equipped with key phone 

numbers and sound awareness cards for any sound signals occurring at the plant 

during sampling.
Page 10

Table 1. Site Visit Attendees

Name Organization (Representing) Telephone E-mail

Elizabeth Hellman We Energies 414-221-3235 elizabeth.hellman@we-energies.com

Pete Kutzora We Energies 414-221-3926 pete.kutzora@we-energies.com

David Lee We Energies 414-221-2158 david.lee@we-energies.com

Ed Morris We Energies 262-947-5625 ed.morris@we-energies.com

Donna Hill Southern Company/UWAG 205-587-4846 dbhill@southernco.com

Jezebele Alicea US EPA 202-566-1755 alicea.jezebele@epa.gov

Ron Jordan US EPA 202-566-1003 jordan.ronald@epa.gov

TJ Finseth ERG 703-633-1698 thomas.finseth@erg.com

Elizabeth Sabol ERG 703-633-1659 elizabeth.sabol@erg.com