## Full Scale Operation of GE ABMet® Biological Technology for

the Removal of Selenium from FGD Wastewaters

Jill Sonstegard, Tim Pickett GE Water & Process Technologies 265 Crossroads Square Salt Lake City, UT 84115, USA jill.sonstegard@ge.com tim.pickett@ge.com

James Harwood GE Water & Process Technologies

3239 Dundas Street West Oakville, Ontario L6M 4B2, Canada james.harwood@ge.com

Danny Johnson Progress Energy Carolinas, Inc.

P.O. Box 1551 Raleigh, NC 27602, USA danny.johnson@pgnmail.com

IWC-08-31

Keywords: FGD wastewater, biological reduction, bacteria, seleni um, mercury, nitrate, heavy metals

Summary: To reduce sulfur dioxide emissions from the flue gasses produced while burning coal, Wet Flue-Gas Desulfurization (WFGD) systems are being em ployed at coal-fired power plants across the globe. These WFGD syst ems transfer sulfur and other compounds released during coal combustion from a gaseous phase to a liquid phase and then collectively contains them in a wastewater stream. These sa me facilities may also utilize Dibasic Acid (DBA) to increase WFGD efficiency. Treatment of the re sulting wastewater in order to comply with increasingl y stringent discharge limits pres ents itself as a formidable challenge. The untreated waste st ream concentrations of solubl e selenate can reach over 10 ppm as well as contain elevated levels of mercury, arsenic and other solublized heavy metals, all of which require removal prior to environmental discharge. Two power plants at Duke Energy and Progress Ener gy in North Carolina have employed ABMet® biological technology for the removal of these contami nants from WFGD blowdow n. This paper will discuss the fundamentals of the biological r eduction process; its demonstrated removal efficiency of selenium, nitrate, and other tr ace metals; the design and commissioning of the first two facilities; as well as the data compiled during the initial months of full-scale operation with an emphasis on the Progress Energy Roxboro Station.‐‐ 2 ‐‐ BACKGROUND

In 2002, the state of North Carolina adopted the Clean Smokestacks Act requiring power companies within the state to reduce their smog and haze-forming emissions by approximately 75% over the following decade. As part of this legislation, a 73 percent cut in sulfur dioxide (SO 2 ) emissions from 2002 levels is required by

2013 1 . In order to achieve this target, Progress Energy and Duke Energy have

both retro-fitted Wet Flue-Gas Desulfurization technologies into some of their coal-fired generating facilities.

Progress Energy’s lar gest coal generating facility, the 2,443 MW, four-unit Roxboro

Station 2 is the second of Progress’ stations

to be retro-fitted with WFGD scrubbers under the new act. Located just outside of Roxboro, NC a nd about 75 miles north of Raleigh, NC, Roxboro is the tenth largest

coal-fired station in North America, and the largest in North Carolina. In order to meet

the standards of the act, Progress Energy

turned to Babcock & Wilc ox to provide their WFGD technology to achieve over 97%

SO 2 removal. At full design capacity, the WFGD scrubbers are expected to produce 1,400 gpm of blowdown flow.

Roxboro Station is lo cated on the shore of Hyco Lake which is used to supply cooling water and WFGD make-up water to the plant. Hyco Lake also receives all of the plant’s water discharge s, including the new

blowdown from the scrubbers. The lake is home to a large fish population and is a very popular recreational destination in the summer months for anglers and cottagers. Special care needed to be taken to ensure

that harmful levels of contaminants such as selenium, nitrate and other trace metals

were eliminated from an environment that could affect the loca l fish and animal populations.

While all of these contaminants are potentially harmful in high enough levels, selenium and mercury in particular must be controlled to very low levels as these elements can bio-accumulate in living tissue. Long-term exposure to elevated

concentrations can cause serious health issues over several years.

WET FLUE-GAS DESULFURIZATION

Wet Flue-Gas Desulfurization scrubbers,

such as those from Babcock & Wilcox at Roxboro, remove SO 2 from the flue gas by passing the flue gas through a large chamber (spray or ab sorber tower).

WFGD systems utilize a limestone reagent mixed with water to form a slurry. As the gas travels through the tower, it is scrubbed with a spray made of the limestone mixture. Multiple levels of sprayers are utilized to

obtain the most efficient contact between the limestone and the flue gas. As the flue gas contacts the spray, the limestone (calcium carbonate - CaCO 3 ) in the

limestone reacts with the SO 2, removing the majority of the SO 2 from the flue-gas before it exits the top of the stack. The rest of the slurry collects in the bottom of the tower

where it is mixed and aerated in order to oxidize the bisulfite ions and produce calcium sulfate dehydrate (CaSO 4 • 2H 2 O), or gypsum, which then precipitates out of

solution.

The slurry in the bottom of the tower is constantly recirculated back through the sprayers. A portion of the slurry mixture containing the precipitat ed gypsum is bled‐ 3

‐ off of the bottom of the tower (blowdown). This blowdown is sent to a pair of Hydroclones to remove the bulk of the precipitated gypsum fr om the stream. The removed gypsum is then sent through

vacuum belt filters to produce a dry cake for

sale to a wallboard manufacturer.

Figure 1 - Generic WFGD Schematic

After the gypsum is recovered from the blowdown, the wastewater is still laced with

various contaminants wh ich originate from

the coal itself and are also captured in the WFGD process. These contaminants must undergo further treatment before they are allowed to be discharged into the

environment.

TREATMENT OPTIONS

Significant concentrations of many trace elements and heavy meta ls pose a unique challenge to treatment, as high levels of TDS, Cl-, Ca++, Mg++ and SO4= make barrier filtration using RO or NF difficult. To add to this challenge, the majority of the selenium present in t he blowdown stream is in the form of soluble selenate (SeO 4 2- ).

Several options were considered by Progress Energy to ma ke their blowdown suitable for discharge, such as conventional physical/chemical systems, constructed wetlands and biologica l treatment.

Without additional tr eatment processes, conventional physical/chemical systems using chemical precipitation and

clarification have not been shown to consistently meet the required effluent criteria with regards to selenium in FGD applications. Progress Energy was also sensitive to consuming vast quantities of chemicals and disposing of large amounts of sludge.

Constructed wetlands were strongly considered as a potent ial alternative. However, this approach would have required approximatel y 100 acres of land for the cells, dilution of the blowdown stream in order to mitigate potential chloride toxicity, and had not demonstrated the ability to remove se lenium at the high removal rates required.

Anoxic biological treatment had not yet

been demonstrated in a full-scale WFGD application during the evaluation; however,

it seemed to offer the best solution over the alternatives. With biological treatment, the foot-print would be <1 acre for a system of Roxboro’s size, the am ount of chemical required would be mini mal, sludge volumes

would be reduced and chloride tolerances of the microbes could handle up to 20,000 mg/l without adverse reactions.

PRETREATMENT

Even after gypsum recovery via Hydroclones, WFGD blowdown may still‐ 4

‐ contain over 4,000 mg/l of TSS, and may also still be well over maximum operating specifications of 105 O F. Both of these

values need to be reduced in order to treat the blowdown biologi cally. Progress Energy employed the use of a 250 million gallon settling pond. The pond has a retention time of approximately 125 days and has four purposes: settle out suspended solids in the blowdown, equalize the blowdown to mitigate any fluctuations in the chemistry of the stream, cool the blowdown, and provide 30 days of blowdown storage capacity.

Solids as mostly gypsum fines will settle-out in the pond over time. With this settling, it is expected that th e pond will require dredging every 10-20 y ears depending on solids loading and Hydroclone performance. Decant is taken from the far end of the settling pond via a floating skimmer and sent to the biological treatment system.

BIOLOGICAL PROCESS OVERVIEW

Max Min Avg Chlorides, mg/l 7,000 2,000 - TSS, mg/l 250 20 40
Temp. O F 105 60 - pH 6.5 5.5 - Selenium, mg/l 3 1.2 - TDS, mg/l - - 13,500

Table 1 – Design Bi oreactor Influent

Criteria

Following the settling pond, water is sent to a series of fixed-film, biological reactors.

The bioreactors ut ilize a mixture of specifically selected, naturally occurring

bacteria.

Many diverse species of bacteria have been proven capable of reducing dissolved selenium species (selenate and selenite) to particulate elemental selenium in an aqueous reductive environment 3 by linking the oxidation of organic substrates to the dissimilatory reduction of the selenium oxyanions 4 . The oxidized selenium compounds serve as electron receptors during the electron trans port process. This effectively reduces the oxidized selenium compounds to nanospheres of granulated

elemental selenium which accumulate in and around the bacterial cells. The unique physical properties of these granules cannot currently be produced by purely chemical means and exhibit a higher level of stability than other chemically-induced

elemental selenium formations 5

.

Within the reactors, the bacteria are fixed to a granular activated carbon (GAC) bed. The GAC is not used for absorption, but rather as a substrate for bacterial growth. This GAC matrix creates a fixed biological film which retains the biomass and prevents it from being washed out of the reactors.

Achieving a reducing env ironment is critical to the success of the process. Wastewater entering the system has a positive ORP in the range of +200 to +300 mV. As wastewater travels through the bioreactors, the ORP drops across the system, causing

several different reactions to occur in various locations throughout the carbon

biomatrix.‐ 5

‐ Figure 2 – Redox Profile

As the ORP approaches 0 mV,

denitrification occurs as the nitrate is reduced to nitrogen gas which is then released to the atmosphere.

NO 3 - + org. C → NO 2 - + org. C → N 2 + CO 2 + H 2 O

As the ORP drops further into the negative range, selenate and/or selenite is then

reduced to an element al state.

SeO 4 2- + org. C → SeO 3 2- + org. C → Se 0 + CO 2 +
H 2 O

Further reductions in OR P will yield sulfide production within th e system, which will precipitate out other tr ace metals such as zinc, copper, nickel, lead and others as metal sulfides. These precipitates are

retained within the biomatrix much the

same as the reduced selenium.

H 2 S + M 2 → MS + 2H + , where M= Metal The precipitated consti tuents are retained within the biomatrix until the system is backflushed, leaving the effluent virtually

free of these contaminants.

Controlling the ORP in the process is achieved by feeding a proprietary

molasses-based nutrient blend into the

reactors as a carbon source for the

bacteria. The nutrient is fed into the influent line of each bioreactor through an injection quill and static mixer to ensure that the system is evenly fed. The dosage rate of the nutrient is dependant upon several factors, including feed nitrate levels, required removal rates and desired ORP

range.

The bioreactors are designed for a plugflow, ensuring even distribution of the feed

water and maximum contact for the bacteria. Roxboro utilizes four parallel bioreactor trains, each with two concrete

bioreactors in series. Each of the bioreactors in the first and second-stages

are identical.

Material selection in this environment is important to the long-term operations of the plant. With such high levels of chlorides, carbon or regular (304, 316) series stainless steels are not appropriate due to the potential for corrosion. All process piping is non-metallic, typically PVC or HDPE depending on the line size and service. Teflon-coated butterfly valves are used for most valve applications and pumps have 2205 duplex stai nless steel wetted parts. The bioreactors are lined with an HDPE sheet to ensure longevity of the

concrete tanks.

Wastewater from the settling pond is pumped via two centrifugal pumps (two duty, one on-line spare) to the first-stage bioreactors. It is then fed into the top of the ORP (mV) Process Electron Acceptors Conditions

O 2 Oxic

Or

Aerobic

NO 3 Anoxic

Anaerobic +300

+200

+100

0

-100

-200

-300

-400 SO 4 Carbonaceous

Organics Fermentative

Anaerobic 1 2 3

4

5 6

7

8

9

1 – Organic Carbon Oxidation 2 – Polyphosphate Development

3 – Nitrification

4 – Denitrification 5 – Polyphosphate Breakdown

6 – Selenium Reduction

7 – Sulfide Formation

8 – Acid Formation

9 – Methane Formation‐ 6

‐ bioreactors and flows evenly across the top

of the GAC biomatrix through a distribution piping grid. Wastewater passes through the biomatrix and is collected via a piping collection grid located in a gravel subfill in

the bottom of the reacto r. The effluent from the first-stage bioreactors is pumped via abooster pump into the top of the secondstage bioreactors. Once the water flows through the second-stage bioreactor, it is sent via gravity to the ash pond discharge

canal.

Figure 3 - Bioreactor Cross-Section

Periodically, the denitrification occurring within the system will c ause nitrogen gas to build-up and form pockets within the GAC beds. These pockets can have a negative effect on the performanc e of the process by allowing for flow to short-circuit around them, and can increase the perceived

head-loss across the bioreactors. A degassing sequence is employed in which a dedicated flush pump pulls water from the feed pond and sends it at a high flux rate

into the bottom of the bioreactor via a dedicated piping distribut ion grid. The flush duration is long enough to loosen the media

so as to allow for trapped nitrogen gas to escape, but short enough to remain below the headspace of the bioreactor.

Differential pressure is monitored acrossthe bioreactors using a combination of tanklevel transmitters and pressure transmitters located on the effluent piping of each

bioreactor. Increases in differential pressure can be used as an indication of

the accumulation of gas pockets within the beds. If a degas does not recover the differential pressure, an accumulation of solids in the bioreactors is likely and a backflush would be required. Degassing may also be done proactively to ensure optimum performance.

Solids in the form of captured TSS, precipitated metals and biomass will also build up over time. When backflushed, the steps are very similar to the degas sequence where the same dedicated flush pump pulls water from the settling pond and

sends it into the botto m of the bioreactor being flushed. During a backflush, the higher flow fluidizes the bed, dislodging the trapped solids, precipitated metals and any excess biomass, and carries it out of the bioreactor as overflow while retaining the activated carbon biofilm. The spent water is sent to the lined flush pond, similar to the settling pond but much smaller. The flush pond remains undisturbed, allowing the

solids to settle and the clean water to decant off of the top of the pond back into the settling pond. The flush pond is sized to hold 20 years of accumulated solids.

START-UP

The first Roxboro WFGD started-up in April 2007, followed by a second WFGD in

November 2007. Blowdown from these two WFG scrubbers was stored in the settling pond while the water level came to a minimum operating hei ght. The biological h 1 – collection and flush distribution systems

in g ravel subfill h 2 – ABMet ®

Biomatrix h 3 – biomatrix expansion

volume h 4 – heads p ace h 5 – reactor cove r‐ 7

‐ system at Roxboro was started-up in late February 2008, which necessitated changes in the seeding process. At the time of commissioning , the settling pond temperature was just below 50 O F, lower than the design value by 10 degrees. For a two-week acclimation period, a heated recirculation loop was used to maintain the reactor temperature at over 80 O F. After acclimation, raw feed water from the settling pond was introduced without supplemental heating. Even at low temperatures, the biol ogical system was fully acclimated to the feed and was consistently producing effluent selenium values below design requirements by week 5 of operations.

OPERATIONAL OBSERVATIONS

Through the beginning of June 2008, total selenium effluent values were consistently less than 10 μg/l with f eed values averaging 1,500 μg/l coming out of the feed pond. This equates to an average removal efficiency of 99.3%. Feed values for selenium have been as expected based on the design basis. The feed TSS loading

has been we ll below design values, indicating that the Hydroclones and the settling pond are performing above expectations. Feed TSS has averaged <10 mg/l throughout the spring of 2008. The feed pH has been well higher than expected, averaging slight ly less than 7.5.

The system was not backflushed until July.

The low levels of TSS in the feed water helped to slow the ne ed for a backflush by limiting the nutrient feed’s contribution to

the solids in the ca rbon beds. Effluent selenium levels slightly increased in June, indicating that the system was in need of a

backflush in order to remove the accumulated biomass and precipitated

solids. Results of backflushing in July 2008 showed a full recovery of the system, with

effluent levels of selenium back below 10 mg/l.

CONCLUSIONS

Overall, the biological system is performing as expected in terms of selenium reduction,

and even better than expected when the start-up feed water temperature is considered. Several key points need to be taken away from this project:

• Facilities with multiple scrubbers will have staggered start-ups and subsequent increases in blowdown flow. The system needs to have the flexibility to handle all flow scenarios.

• The acceptable feed water temperature for acclimation and operations may be more forgiving than initially perceived, with the system starting operations in 50 O F water while the design minimum was originally 60 O F.

• Accurate, factory certified ORP probes are critical. Verification with hand-held ORP meters can be done, provided that a meter is selected with the right electrode material (platinum).

• Proper analytical techniques for lowlevel selenium need to be used.

• A comprehensive sampling protocol with quick analysis turn-around is needed during the first few months of operation in order to ensure that the system is fully optimized for the best performance.

• Changes in blowdown chemistry need to be monitored closely in order to adjust system parameters as

needed.‐8‐Figure 4 - BioreactorProcess Flow DiagramFeedPond1stStageBioreactorsFeedPumpsFlushPondBoosterPumpsFlushPump2ndStageBioreactorsDischarge‐9‐Figure 5 – RoxboroBioreactor System‐10‐Figure 6 – Roxboro BioreactorSelenium Analytical Data010020030040050060070080090010001100120013001400150016001700180019002000 3 / 4 3/11 3/18 3/25 4 / 1 4/8 4 /1 5 4/22 4 /2 9 5/6 5 / 13 5/20 5 / 27 6 /3 6 / 10 6/17 6 / 24 7 / 1 7/8 7/15 7/22 7/2 9

Total Selenium ug/LBioreactor FeedBioreactor Effluent‐ 11

‐ REFERENCES

1 North Carolina Department of Environment and Natural Resources – Division of Air Quality website: http://www.ncair.org/news/leg/cleanstacks.shtml

2 Progress Energy Website: http://www.progressenergy.com/aboutenergy/powerplants/2008generatingplants.pdf

3 Frankenberger, W T., Jr; Karlson, U. Microbial volatilization of selenium from soils and sediments. In: Frankenberger W T, Benson S. , editors; Frankenberger W T, Benson S. , editors. Selenium in the

environment. New York, N.Y: Marcel Dekker, Inc; 1994. p. 369–387

4 De Souza, M.P. et al. 2001. Identification and characterization of bacteria in a selenium-contaminated hypersaline evaporation pond. Applied and Environmental Microbiology. 67:3785-3794

5 Oremland, R. S., et al. 2004. Structural and spectral features of selenium nanoshpheres produced by se- respiring bacteria. Applied and Environmental Microbiology. 70: 52-60