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ABMet® Biological Selenium Removal from FGD Wastewater

Jill Sonstegard, Tim Pickett

Zenon Environmental

P.O. Box 520518

Salt Lake City, UT 84152, USA

jsonstegard@zenon.com

tpickett@zenon.com

IWC-05-72

Keywords: FGD wastewater, biological reduction, bacteria, selenium, arsenic, copper, mercury, zinc

Summary: The biological precipitation and removal of selenium and metals from flue gas desulfurization (FGD)

blowdown utilizing a fixed-film biotreatment process has been successfully pilot tested and demonstrated to

remove dissolved selenium at a > 99% efficiency. Selenium levels less than 0.01 mg/L were achieved both in the

lab and in field tests.

The process has been tested in more than eight FGD wastewaters, and implemented at full scale in other

industries. The wastewaters tested contained up to 500 mg/L Dibasic Acid, (DBA), as well as elevated levels of

chloride, (>15,000 mg/L), and TDS, (>29,000 mg/L). The biofilm maintained its integrity and efficiency in

temperature tests up to 105 °F and at pH levels as low as 5.35. Other metals significantly removed from the FGD

wastewater during the process included arsenic, copper, nickel, mercury and zinc. Minimal sludge volumes were

generated as a waste product as compared to chemical flocculation and precipitation.

A 1,400 gal/min facility using the process is currently under design for full scale operation starting in 2007. This

paper presents a process overview and pilot scale performance data, and a full scale, conceptual design

approach.

INTRODUCTION

Coal-fired power plants produce more than half the

electricity used in the United States. The process of

combusting coal to generate power converts

naturally occurring sulfur contained in the coal to

gaseous sulfur dioxide (SO2), a criteria pollutant and

a precursor to acid rain.

The 1990 Clean Air Act and its subsequent

amendments mandated the reduction of power plant

SO2 emissions. The Best Demonstrated Technology

for reducing SO2 emissions is wet scrubber flue gas

desulfurization (FGD) systems. These systems are

designed to introduce an alkaline sorbent, consisting

of lime or limestone in a spray form, into the exhaust

system of a coal-fired boiler. The alkali reacts with

the SO2 gas and is collected in a liquid form as

calcium sulfite or calcium sulfate slurry. The slurry is

discharged to a treatment system where the calcium

sulfate may be removed by gravity settling or

centrifugation. The pre-treated FGD wastewater,

(blowdown) can contain elevated levels of mercury,

arsenic and selenium as well as other metals and

metalloids found in various coal sources. Directly

following gypsum removal, the blowdown can be

delivered to bioreactor trains for biological

contaminant reduction.

Many diverse species of bacteria have been proven

capable of reducing selenate and selenite in an

aqueous reductive environment (Frankenberger et

al. 1994), by linking the oxidation of organic

substrates to the dissimilatory reduction of the

selenium oxyanions (De Souza, 2001). The

oxidized selenium compounds serve as electron

receptors during the electron transport 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 formations

(Oremland, 2004).
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PROCESS OVERVIEW

The biological treatment technology presented

utilizes biofilms of facultative anaerobic bacteria in

an up-flow bioreactor, through which the wastewater

is passed. Proprietary, site-specific, naturally

occurring bacterial cultures are produced in bulk and

used to inoculate the bioreactors. Microbial

reduction occurs within the confines of the

bioreactor. The elemental selenium and other

contaminant precipitates are retained within the

matrix of the bioreactor until the employment of a

flushing cycle to release and collect the solids for

recycling or disposal.

Implementation of the bio-treatment is a multi-step

process. Initially, site water is qualitatively screened

with microbial stock cultures to select specific

bacteria that demonstrate the ability to metabolize

the target contaminants. Site water is also analyzed

for indigenous microbial populations that have the

ability to effectively reduce metal inorganic

compounds. The selected bacteria are then scaled

up to inoculate bench scale bioreactors for

treatability testing within a laboratory environment.

Bench scale reactors are filled with an inert,

proprietary substrate on which the bacteria form a

high density, fixed biofilm and are therefore retained

within the system. The bioreactors are quantitatively

analyzed for selenium reduction while being

monitored. Site-specific variables such as water

quality, composition, temperature, as well as

nutritional requirements are evaluated.

At the conclusion of bench scale testing, a pilot test

is set up on site. The selected bacterial consortia is

again scaled up and used to inoculate the pilot

bioreactors. Fluctuations in the composition of site

water influent, temperature and nutritional

supplementation are evaluated with respect to

contaminant reduction and hydraulic retention time.

A maintenance schedule for solids removal is

developed.

The parameters established during pilot testing are

incorporated into recommendations for a full-scale

design. Implementation logistics and timelines, as

well as, full-scale design criteria are addressed at

this time.

MATERIALS AND METHODS

Site waters were obtained from several operating

FGD facilities. The FGD wastewaters tested

contained up to 500 mg/L Dibasic acid, (DBA), as

well as elevated levels of chloride, (>15,000 mg/L),

and TDS, (>29,000 mg/L). Inlet Selenium

concentrations ranged from (0.3 – 4.0 mg/L).

Culture Screening

In a series of tests, individual stock cultures were

screened for selenium reduction and chloride

tolerance. The testing procedures involved adding a

stock selenium reducing culture to a test tube filled

with site water and nutrient amended with selenium.

The isolates were allowed to incubate for 48 hours

and selenium reduction was assessed by visually

evaluating the red precipitate in the tube. Isolates

determined to have the greatest selenium reducing

capability in site water were selected for further

screening in chloride amended site water.

Selenium reducing isolates were selected from the

initial selenium reduction screening and grown in

three chloride variables (low = no chloride added to

site water, medium = chloride added to 10 g/L total

chloride, high = chloride added to 20 g/L total

chloride). Isolates were added to test tubes with site

water, nutrient, selenium, and the respective amount

of chloride. Selenium reduction was assessed after

48 hours. Selenium reduction was assessed by

visually rating the red precipitate in the tube.

Isolates that performed well at all three chloride

concentrations were selected for further testing.

Temperature Testing

The cultures previously screened for selenium

reduction, chloride tolerance, and growth potential

were further tested for temperature tolerance. The

testing entailed inoculating activated carbon with the

mixed culture and evaluating the growth of the

culture at several temperatures. The temperatures

tested in the laboratory included room temperature,

85 °F, 90 °F, 95 °F, 100 °F, 105°F, and 113 °F. The

testing procedures involved taking a sample of

inoculated carbon and conducting microbial plate

counts at time zero, then adding the carbon sample

to a flask with nutrient, and allowing the carbon to

incubate in a temperature controlled water bath for

48 hours. After 48 hours in the water bath, microbial

plate counts were conducted on the carbon matrix.

Comparing the T48 hour counts to the initial counts

assessed growth.

Bench Scale Bioreactor Testing

Bench scale bioreactors were set up to test metals

reduction in scaled ABMet® reactors. The

bioreactors have a capacity of 3 liters and operated
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at a 12-hour retention time. The bioreactors were

filled with an inert, proprietary support material and

inoculated with the mixed culture selected from the

culture screening. The bench scale bioreactor

testing consisted of five runs using three different

FGD scrubber waters. The reactors were sampled

daily, and samples were submitted for selenium and

metals analysis at an independent laboratory.

On Site Pilot Testing

The pilot test consisted of a train of two reactors set

up in series, the first being inoculated with sitespecific selenium reducing consortia and the second

with sulfate reducing bacteria. See Figure 1.

Figure 1.

FGD Wastewater Treatment Pilot System Process

Flow Schematic

The bioreactors were fed a proprietary molasses

based nutrient using an automated delivery system.

The influent and effluent of each bioreactor were

sampled three times per week and analyzed for total

selenium. Once per week, the influent and final

effluent were sampled and analyzed for other total

metals arsenic, copper, nickel, mercury and zinc.

RESULTS

The culture screening assessed selenium reduction

capability in site water spiked with 25 mg/L

selenium. All culture screenings used water from

an operating FGD system. Twenty-three isolates

were tested and selenium reduction was rated

between 1 and 5, with 5 being the highest. Results

indicated that selenium reduction occurred in site

water. Eight isolates showing the greatest selenium

reduction capability were chosen for further testing.

See Figure 2.

The eight isolates from Test 1 were screened further

in site water spiked with varying levels of chloride.

Results indicated that selenium reduction occurred

Figure 2.

Culture Screening – Selenium Reduction Test in

FGD Site Water.

at all chloride concentrations and certain isolates

performed well even at high concentrations. It is

also noted that some isolates performed better at

higher chloride concentrations than lower chloride

concentrations. See Figure 3.

Figure 3.

Selenium Reduction in Varying Chloride

Concentrations

Bench scale laboratory analysis included toxicity

evaluations for DBA concentrations, low pH values

and extreme temperature testing. Results of the

tests concluded DBA concentrations up to 2,000

mg/L, native pH of 5.35 and inlet water temperatures
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of up to 105°F did not inhibit selenium reduction or

the viability of the microbial biofilm.

Selenium concentrations can fluctuate daily in FGD

wastewaters due to variations in the coal types

being burned; however, this fluctuation did not have

a detrimental effect on selenium reduction.

Selenium removal efficiencies >99% were

consistently achieved during the pilot test as

illustrated in Figure 4.

Figure 4.

Pilot Test Selenium Reduction Results

In addition to selenium, the system demonstrated

effective removal of arsenic, copper, nickel, mercury

and zinc throughout the test period. See Table 1.

.

Table 1.

Average Metal Concentrations

During FGD Pilot Test

Metals

Inlet

Concentration in

μg/L

Effluent

Concentration in

μg/L

Arsenic 27.63 15.20

Copper 320.86 16.50

Nickel 546.01 20.91

Mercury 2,47 0.08

Zinc 377.00 16.50

.

DISCUSSION

Successful laboratory and pilot scale testing of FGD

wastewaters indicate that the ABMet® process is

suitable for selenium removal from these waters.

Laboratory studies evaluating process operating

variables demonstrated that the ABMet® technology

can perform consistently over a range of operational

scenarios. Specifically, selenium reduction was

constant at native pH of 5.35 and a pH adjusted to

7.5. Additionally, DBA up to 2000 mg/L was not

toxic to the microbes and the presence of DBA did

not affect selenium reduction. Finally, selenium

reduction was evident in water with chloride

concentrations up to 15,000 mg/L, (the maximum

concentration tested).

Effluent concentrations in the extended pilot scale

testing exhibited selenium levels as low as 0.003

mg/L at a retention time of 5 hours. Influent

temperatures to the pilot system ranged from 17 °C

to 32.8 °C (62.6 °F to 91 °F). Results from the

temperature testing in the lab demonstrated that

selenium reduction and good biofilm health were

maintained at temperatures up to 105 °F.

Selenium removal efficiencies >99% were

consistently achieved during the pilot test. In

addition to selenium, the system demonstrated

effective removal of arsenic, copper, nickel, mercury

and zinc throughout the test period. Mercury was

consistently removed in the system to below 150

ng/L.

In extensive bench and pilot testing, successful

biological selenium reduction was demonstrated in

over eight FGD scrubber waters. Based on the

success of the this testing, orders for 2 full scale

facilities have been obtained for start up in 2007. A

simplified full scale flow schematic is provided in

Figure 5.

Figure 5.

Full Scale Process Flow Diagram for FGD

Wastewater

Each facility will treat 1 or 2 Million Gallons/ Day

(MGD) of FGD wastewater. Preliminary design

criteria for full-scale implementation are displayed in

the following table. See Table 2. Both four and eight

hour retention time examples are listed. Full -scale

design retention times will depend on site -specific

treatment targets and inlet water chemistry.
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Table 2.

Preliminary Recommended Full Scale Design

Criteria

Pretreatment for the system may be required to

remove TSS and reduce the solids loading of the

system. It is anticipated that with an inlet feed TSS

concentration of <20 mg/L, the lead reactors will

need to be flushed on a biannual basis.

Post treatment may be necessary to re-aerate

bioreactor effluent and remove residual organics

before discharge. If DBA is present in the FGD

water, it can be removed in the aerobic post

treatment system. Post treatment may include fixedfilm biological systems such as rotating biocontactors and trickling filters, or wetlands.

REFERENCES

De Souza, M.P. et al. 2001. Identification and

characterization of bacteria in a seleniumcontaminated hypersaline evaporation pond. Applied

and Environmental Microbiology. 67:3785-3794.

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.

Oremland, R. S., and J. F. Stolz. 2000. Dissimilatory

reduction of selenate and arsenate in nature, p. 199-

224. In D. R. Lovley (ed.), Environmental metalmicrobe interaction. ASM Press, Washington, D.C.

Oremland, R. S., et al. 2004. Structural and spectral

features of selenium nanoshpheres produced by serespiring bacteria. Applied and Environmental

Microbiology. 70: 52-60.

Stolz, J. F., and R. S. Oremland. 1999. Bacterial

respiration of selenium and arsenic. FEMS Microbiol.

Rev. 23:615-627.[

Flow rate (gpm) 

Parameter 1600 640 150 

System Retention time (hours) 4-8 4-8 4-8 

Operating temperature (°F) 70-105 70-105 70-105 

pH (su) 5.5 – 9 5.5 – 9 5.5 – 9 

Total Reactor Volume (gallons) 1 M – 2 M 0.4 M – 0.8 M 94 K – 188 K