v>EPA
United States
Environmental Protection
Agency
SITE Technology Capsule
Compost-Free Bioreactor Treatment
of Acid Rock Drainage
Abstract
As part of the Superfund Innovative Technology
Evaluation (SITE) program, an evaluation of the
compost-free bioreactor treatment of acid rock drainage
(ARD) from the Aspen Seep was conducted at the
Leviathan Mine Superfund site located in a remote, high
altitude area of Alpine County, California. The
evaluation was performed by U.S. Environmental
Protection Agency (EPA) National Risk Management
Research Laboratory (NRMRL), in cooperation with EPA
Region IX, the state of California, and Atlantic Richfield
Company (ARCO), and the University of Nevada-Reno
(UNR). The primary target metals of concern in the ARD
include aluminum, copper, iron, and nickel; secondary
target metals include selenium and zinc.
Drs. Glenn Miller and Tim Tsukamoto of the UNR have
developed a compost-free bioreactor technology in
which sulfate-reducing bacteria are nurtured to generate
sulfides which scavenge dissolved metals to form metal
sulfide precipitates. Unlike compost bioreactors, this
technology uses a liquid carbon source (ethanol) and a
rock matrix rather than a compost or wood chip matrix
which is consumed by bacteria and collapses over time.
The benefits include better control of biological activity
and improved hydraulic conductivity and precipitate
flushing.
Evaluation of the compost-free bioreactor technology
occurred between November 2003 and July 2005. The
treatment system neutralized acidity and precipitated
metal sulfides from ARD at flows up to 24 gallons per
minute (gpm) on a year-round basis. Multiple sampling
events were conducted during both gravity flow and
recirculation modes of operation. During each sampling
event, EPA collected chemical data from the system
influent and effluent streams and documented metals
removal and reduction in acidity between system
components. Operational information pertinent to the
evaluation of the treatment system was also recorded.
The treatment system was evaluated based on removal
efficiencies for primary and secondary target metals, on
a comparison of effluent concentrations to EPA interim
(pre-risk assessment and record of decision) discharge
standards, and on the characteristics of and disposal
requirements for the resulting metals-enriched solid
wastes. Removal efficiencies of individual unit
operations were also evaluated.
The compost-free bioreactor treatment system was
shown to be extremely effective at neutralizing acidity
and reducing the concentrations of the 4 of the 5 target
metals to below EPA interim discharge standards. Pilot
testing to determine optimal sodium hydroxide addition
resulted in exceedance of discharge standards for iron.
However, after base optimization during gravity flow
operations effluent iron concentrations met discharge
standards. Iron also exceeded discharge standards
during recirculation operations when base addition was
stopped due to equipment failure or lack of adequate
base supply. Although the influent concentrations for the
primary target metals were up to 580 fold above the EPA
interim discharge standards, the treatment system was
successful in reducing the concentrations of the primary
target metals in the ARD to between 1 and 43 fold below
the discharge standards. Removal efficiencies for the
5 primary target metals exceeded 85 percent; sulfate ion
was reduced by 17 percent. The metal sulfide
precipitates generated by this technology were not found
to be hazardous or pose a threat to water quality and
could be used as a soil amendment for site reclamation.
Based on the success of bioreactor treatment at the
Leviathan Mine site, ARCO will continue to use this
technology to treat ARD at the Aspen Seep.
Introduction
In 1980, the U.S. Congress passed the Comprehensive
Environmental Response, Compensation, and Liability
Act (CERCLA), also known as Superfund. CERCLA is
committed to protecting human health and the
environment from uncontrolled hazardous waste sites.
In 1986, CERCLA was amended by the Superfund
Amendments and Reauthorization Act (SARA). These
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amendments emphasize the achievement of long-term
effectiveness and permanence of remedies at Superfund
sites. SARA mandates the use of permanent solutions,
alternative treatment technologies, or resource recovery
technologies, to the maximum extent possible, to clean
up hazardous waste sites. State and Federal agencies,
as well as private parties, have for several years now
been exploring the growing number of innovative
technologies for treating hazardous wastes. EPA has
focused on policy, technical, and informational issues
related to the exploring and applying new remediation
technologies applicable to Superfund sites. One such
initiative is EPA's SITE program, which was established
to accelerate the development, demonstration, and use
of innovative technologies for site cleanups. Technology
Capsules summarize the latest information available on
selected innovative treatment, site remediation
technologies, and related issues. These capsules are
designed to help EPA remedial project managers and
on-scene coordinators, contractors, and other site
cleanup managers understand the types of data and site
characteristics needed to effectively evaluate a
technology's applicability for cleaning up Superfund
sites.
Compost-free bioreactor treatment systems are an
improvement to current wood chip, compost, and
manure based bioreactors in place at many facilities.
This capsule provides information on new approaches to
the use of compost-free bioreactors to reduce the
concentration of toxic metals and acidity in ARD from the
Aspen Seep at Leviathan Mine. The treatment system
implemented by ARCO was specifically designed to treat
low to moderate flow rates of ARD (pH of 3) containing
hundreds of milligrams per liter (mg/L) of toxic metals
that would otherwise be released to the environment.
The mine site also poses operational challenges
associated with its remote location and winter weather
conditions that limit site access and operations from late
fall through late spring. This capsule presents the
following information that documents the evaluation of
the treatment system:
• Project background
• Technology description
• Performance data
• Process residuals
• Technology applicability
• Technology limitations
• Site requirements
• Technology status
• Sources of further information
Project Background
Leviathan Mine is a former copper and sulfur mine
located high on the eastern slopes of the Sierra Nevada
Mountain range, near the California-Nevada border.
The mine occupies approximately 253 acres on the
northwestern flank of Leviathan Peak, at an elevation of
about 7,800 feet. The mine site is drained by Leviathan
and Aspen creeks, which combine with Mountaineer
Creek 2.2 miles below the mine to form Bryant Creek, a
tributary to the East Fork of the Carson River.
Intermittent mining of copper sulfate, copper, and sulfur
minerals since the mid 1860s has resulted in acid mine
drainage (AMD) at Leviathan Mine. During the process
of converting underground workings into an open pit
mine in the 1950s, approximately 22 million tons of
overburden and waste rock were removed from the open
pit mine and placed in the Aspen Creek drainage,
contributing ARD to the Aspen Seep. Oxidation of sulfur
and sulfide minerals within the mine workings and waste
rock forms sulfuric acid (H2SO4), liberating toxic metals
discharged in the ARD.
Historically, the concentrations of four primary target
metals, aluminum, copper, iron, and nickel in the ARD
released from Aspen Seep have exceeded EPA interim
discharge standards up to 580 fold. Release of these
metals has contributed to fish and insect kills in
Leviathan Creek, Bryant Creek, and the east fork of the
Carson River. In 1984 the state of California
significantly reduced the quantity of toxic metals
discharging from the mine site by partially filling and
grading the open pit, building retention ponds to contain
the AMD, building a channel under-drain (CUD) system
to capture ARD, and rerouting Leviathan Creek through
a concrete diversion channel to reduce contact with
waste rock. To further reduce the amount of toxic
metals discharging from the mine site, the state of
California initiated pilot-scale compost bioreactor studies
in 1996 to treat ARD and constructed an active lime
treatment system in 1999 to treat AMD that collects in
the retention ponds. In 2001, ARCO constructed the
semi-passive Alkaline Lagoon treatment system to treat
ARD from the CUD. In 2003, ARCO in conjunction with
UNR constructed the full-scale compost-free bioreactor
treatment system to treat ARD from Aspen Seep.
Technology Description
Biological treatment of ARD relies on the biologically
mediated reduction of sulfate to sulfide followed by metal
sulfide precipitation. Biologically promoted sulfate-
reduction has been attributed primarily a consortium of
sulfate-reducing bacteria, which at Leviathan Mine
utilizes ethanol as a carbon substrate to reduce sulfate
to sulfide. This process generates hydrogen sulfide,
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elevates pH to about 7, and precipitates divalent metals
as metal sulfides. The following general equations
describe the sulfate-reduction and metal sulfide
precipitation processes.
2CH-,CH7OH + 3SO4
3HS" + 3HCO-," + 3H7O
(1)
2CH3CH2OH + S042" -» 2 CH3COO" + HS' + H2O (2)
HS" + M2+-» MS + 2H+
(3)
Here ethanol is the carbon source and SO42" is the
terminal electron acceptor in the electron transport chain
of sulfate-reducing bacteria. Reaction No.1 causes an
increase in alkalinity and a rise in pH, while reaction
No.2 results in the generation of acetate rather than
complete oxidation to carbonate. HS" then reacts with a
variety of divalent metals (M2+), resulting in a metal
sulfide (MS) precipitate.
The reduction of sulfate to sulfide requires 8 electrons:
H2SO4 + 8H+ + 8e~ -» H2S + 4H2O
(4)
Ethanol contributes 12 electrons per molecule oxidized,
assuming complete oxidation to carbon dioxide.
3H20 + C2H5OH -» 12e' + 2CO2 + 12H+
(5)
However, incomplete oxidation of ethanol to acetate
yields only 4 electrons per molecule oxidized.
H2O + C2H5OH -» 4e~ + C2H3OOH + 4H+
(6)
The moles of ethanol consumed per mole of sulfate
reduced in the bioreactors at Leviathan Mine suggest
that incomplete oxidation of ethanol is the predominant
reaction.
Compost-Free Bioreactor System Overview: At
Leviathan Mine, the compost-free bioreactor treatment
system consists of ethanol and sodium hydroxide feed
stocks, a pretreatment pond, two bioreactors, a settling
pond, a flushing pond, and an aeration channel. The
system was designed to treat ARD by gravity flow
through successive sulfate-reducing bioreactors and
precipitation of metal sulfides in the bioreactors as well
as in a continuous flow settling pond (Figure 1). During
the demonstration, an alternative mode of operation
(recirculation) was also evaluated, which involved the
direct contact of ARD with sulfide rich water from the
bioreactors and precipitation of the majority of the metal
sulfides in the settling pond. A portion of the pond
supernatant containing excess sulfate is then pumped to
the head of the bioreactor system to generate additional
sulfides (Figure 2).
The heart of the treatment system is the two compost-
free, sulfate-reducing bioreactors. The bioreactors are
ponds lined with 60 mil high density polyethylene
(HOPE) and filled with 8- to 16-inch river rock (Figure 2
and 3). River rock was selected because of the stability
of the matrix and the ease at which metal sulfide
precipitates can be flushed from the matrix to the
flushing pond. Each bioreactor consists of three 4-inch
diameter influent distribution lines and three 4-inch
effluent collection lines. The distribution and collection
lines are located near the top, in the middle, and just
above the bottom of the bioreactor to precisely control
flow within the bioreactor media. ARD water can be
drawn upward or downward through the aggregate to
one of three effluent collection lines located at the
opposite end of each bioreactor (Figures 2 and 3).
Compost-Free Bioreactor Operation: Influent to the
treatment system consists of ARD discharged from
Aspen Seep. In gravity flow mode (Figure 1), influent
ARD from Aspen Seep passes through a flow control
weir at flow rates ranging from 6.4 to 21.9 gpm, where a
25 percent sodium hydroxide solution (0.26 [ml/L]
milliliter per liter or 83 mg/L) is added to adjust the pH
from 3.1 to approximately 4 to maintain a favorable
environment for sulfate-reducing bacteria and ethanol
(0.43 ml/L or 339 mg/L) is added to provide a carbon
source for reducing equivalents for the sulfate-reducing
bacteria. The dosed influent discharges into a
pretreatment pond (1,000 ft3 [cubic foot], 4 hour
hydraulic residence time [HRT] at 30 gpm) to allow
sufficient time for reagent contact and to stabilize the
flow to the head of Bioreactor No.1. A small volume of
metal precipitation also occurs within the pretreatment
pond. ARD from the pretreatment pond then flows
through Bioreactor No.1 (12,500 ft3 total volume, 5,300
ft3 active volume, 22 hour HRT at 30 gpm) and
Bioreactor No.2 (7,000 ft3 total volume, 3,000 ft3 active
volume, 13 hour HRT at 30 gpm) to reduce sulfate to
sulfide. Excess sulfide generated in the first bioreactor
is passed, along with partially treated ARD water,
through to the second bioreactor for additional metals
removal. Effluent from the second bioreactor discharges
to a continuous flow pond (16,400 ft3, 68 hour HRT at 30
gpm) for extended settling of metal sulfide precipitates.
A twenty-five percent sodium hydroxide solution (0.85
ml/L or 270 mg/L) is added to the bioreactor effluent
prior to the continuous flow settling pond to consume
remaining mineral acidity, convert bisulfide to sulfide
which is necessary for metal sulfide precipitation, and
provide a source of hydroxide ion for metals that do not
form precipitates with sulfide.
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Figure 1-2. Bioreactor Treatment System, Recirculation Configuration Schematic
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Operated in recirculation mode (Figure 2), metal-rich
ARD influent from Aspen Seep passes through a flow
control weir at which point the ARD flow is routed around
the two bioreactors to a flow control vault at the head of
the continuous flow settling pond. The untreated ARD is
mixed with sulfide rich water discharging from bioreactor
No.2 and a 25 percent sodium hydroxide solution (0.5
ml/L or 159 mg/L) and is then discharged to the settling
pond. The combination of a neutral pH condition and
high sulfide concentrations promotes rapid precipitation
of metal sulfides in the settling pond rather than in the
two bioreactors. Precipitation of a majority of the metal
sulfides downstream of the two bioreactors reduces
precipitate formation in the bioreactors and the need for
flushing and the associated stress on the two
bioreactors. A portion of the pond supernatant
containing excess sulfate is then pumped to a holding
pond at flow rates ranging from 30 to 60 gpm (influent to
recirculation ratio of 1:2 to 1:6). Ethanol (0.50ml/L or
395 mg/L) is added to the discharge from the holding
pond, just prior to the head of bioreactor No.1. Sulfate-
rich and metal poor water from the holding pond then
flows through the two bioreactors to promote additional
sulfate reduction to sulfide. The pH of the supernatant
recirculated through the bioreactors is near neutral,
providing optimal conditions for sulfate-reducing bacteria
growth. The system operated in recirculation mode
requires about 49 percent less sodium hydroxide and 14
percent more ethanol than the gravity flow mode of
operation.
In both modes of operation, the effluent from the
continuous flow settling pond then flows through a rock
lined aeration channel (150 feet long by 2 feet wide) to
promote gas exchange (eliminate hydrogen sulfide and
introduce oxygen) prior to effluent discharge. Precipitate
slurry is periodically flushed from the two bioreactors to
prevent plugging of the river rock matrix. The slurry is
sent to a flushing pond (18,000 ft3, 75 hour HRT at 30
gpm) for extended settling. The flushing pond can also
be used for extended settling of the continuous flow
settling pond effluent in the event of a system upset.
Settled solids are periodically pumped out of the settling
and flushing ponds and dewatered using 10- by 15-foot
spun fabric bag filters. The bag filtration process relies
on the build up of filter cake on the inside of each bag to
remove progressively smaller particles. Effluent from
the bag filters, including soluble metals and particles too
small to be captured, flows by gravity back into the
settling pond. Metals in bag filter solids are not
hazardous under Federal or California standards and
can be disposed of on- or off-site. The total system HRT
is 107 hours at maximum design flow of 30 gpm, and
352 hours at an average flow rate of 10 gpm during the
demonstration.
Performance Data
The evaluation of the compost-free bioreactor treatment
systems at Leviathan Mine was conducted between
November 2003 and July 2005; focusing on two primary
objectives. The first objective was to determine the
removal efficiencies for the primary target metals of
concern and the secondary target metals. The second
objective was to determine whether the concentrations
of the primary target metals in the effluent from the
bioreactor treatment system were below EPA interim
discharge standards, as presented in Table 1.
The data evaluation was designed to address both
primary objectives and included both descriptive and
inferential statistics. Descriptive summary statistics of
the data were calculated to screen the sample data for
possible outliers; these statistics included the mean,
median, range, variance, and standard deviation. To
successfully calculate removal efficiencies for each
metal, influent concentrations must be significantly
different than effluent concentrations. A paired t-test
was applied to the data collected during each sampling
event to determine if the influent and effluent
concentrations were statistically different. Where
influent and effluent concentrations for a particular metal
were not statistically different, removal efficiencies were
not calculated for that metal. In addition, removal
efficiencies were not calculated for individual
influent/effluent data pairs when both concentrations for
a metal were not detected.
Tables 2 and 3 present the average and range of
removal efficiencies for filtered influent and effluent
samples collected from the treatment system during both
gravity flow and recirculation modes of operation. A
summary of the average influent and effluent metals
concentrations for each mode of operation is also
presented. The results of a comparison of the average
effluent concentration for each metal to the EPA interim
discharge standards is also presented; where a "Y"
indicates that either the maximum concentration (based
on a daily composite of three grab samples) and/or the
average concentration (based on four consecutive
sampling events) was exceeded; and an "N" indicates
that neither discharge standard was exceeded.
Although the influent concentrations for the primary
target metals were up to 580 fold above EPA interim
discharge standards, both modes of treatment system
operation were successful in reducing the
concentrations of the primary target metals in the ARD
to between 1 and 43 fold below the discharge standards.
Internal trials run to refine base addition requirements
and to evaluate various sources of base addition lead to
significant excursions of effluent iron concentrations
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Table 1. EPA Interim Discharge Standards for Metals of Concern at Leviathan Mine
Target Metals
Maximum (a)
(ug/L)
Average (b)
(ug/L)
Primary Target Metals
Aluminum
Arsenic
Copper
Iron
Nickel
4,000
340
26
2,000
840
2,000
150
16
1,000
94
Secondary Target Metals
Cadmium
Chromium
Lead
Selenium
Zinc
9.0
970
136
No Standard
210
4.0
310
5.0
5.0
210
(a) Maximum concentration based on a daily composite of three grab samples
(b) Average concentration based on four consecutive sampling events
ug/L = microgram per liter
Table 2. Bioreactor Treatment System Removal Efficiencies: Gravity Flow Configuration
Target
Metal
Number of
Sampling
Events
Average
Filtered Influent
Concentration
(ug/L)
Standard
Deviation
Average
Filtered Effluent
Concentration
(ug/L)
Standard
Deviation
Exceeds
Discharge
Standard
(Y/N)
Average
Removal
Efficiency
(%)
Range of
Removal
Efficiencies
(%)
Primary Target Metals
Aluminum
Arsenic
Copper
Iron
Nickel
6
6
6
6
6
37,467
2.1
691
117,167
487
2,011
0.64
51.2
6,242
33.5
103
4.7
4.8
4,885
65.5
78.8
4.0
1.6
4,771
36
N
N
N
Y
N
99.7
NC
99.3
95.8
86.6
99.5 to 99.9
NC
99.1 to 99.7
65.6 to 99.9
72.1 to 92.6
Secondary Target Metals
Cadmium
Chromium
Lead
Selenium
Zinc
6
6
6
6
6
0.61
12.2
3.6
13.9
715
0.27
8.9
2.5
3.1
47.1
<0.21
7.8
4.7
11.2
15.8
0.07
6.6
2.9
2.6
6.8
N
N
N
Y
N
65.3
NC
NC
NC
97.8
42.5 to 79
NC
NC
NC
95.9 to 98.6
NC = Not calculated as influent and effluent concentrations were not statistically different
jxg/L = Microgram per liter
Table 3. Bioreactor Treatment System Removal Efficiencies: Recirculation Configuration
Target
Metal
Number of
Sampling
Events
Average
Filtered Influent
Concentration
(Mfl/L)
Standard
Deviation
Average
Filtered Effluent
Concentration
(ug/L)
Standard
Deviation
Exceeds
Discharge
Standard
(Y/N)
Average
Removal
Efficiency
(%)
Range of
Removal
Efficiencies
(%)
Primary Target Metals
Aluminum
Arsenic
Copper
Iron
Nickel
7
7
7
7
7
40,029
7.4
795
115,785
529
4,837
6.5
187
13,509
34.1
52.7
6.5
4.6
2,704
69.7
25.7
4.9
3.2
3,000
44.2
N
N
N
Y
N
99.9
NC
99.4
97.7
86.8
99.7 to 99.9
NC
98.8 to 99.8
92.8 to 99.7
71 .0 to 96.4
Secondary Target Metals
Cadmium
Chromium
Lead
Selenium
Zinc
7
7
7
7
7
0.60
11.1
4.2
11.5
776
0.50
6.3
2.3
5.1
51.7
<0.20
6.4
2.5
8.5
8.9
0.09
5.2
1.6
3.6
7.4
N
N
N
Y
N
NC
42.5
41.5
NC
98.9
NC
21 .2 to 84.8
22.0 to 57.1
NC
97.7 to 99.8
NC = Not calculated as influent and effluent concentrations were not statistically different
jxg/L = Microgram per liter
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above the EPA interim discharge standards during a
portion of the evaluation. However, after base
optimization during gravity flow operations, effluent iron
concentrations met discharge standards. Iron also
exceed discharge standards during recirculation
operations when base addition was stopped due to
equipment failure or lack of adequate base supply. In
addition, the concentrations of the secondary target
metals, with the exception of selenium, were reduced to
below the discharge standards.
The bioreactor treatment system operated in gravity flow
mode from November 2003 through mid-May 2004
treating 2.44 million gallons of ARD using 2,440 gallons
of sodium hydroxide and 1,180 gallons of ethanol. The
bioreactor treatment system operated in the recirculation
mode from mid-May 2004 through July 2005 treating
5.81 million gallons of ARD using 5,820 gallons of
sodium hydroxide and 2,805 gallons of ethanol.
For the gravity flow mode of treatment system operation,
the average removal efficiency for the primary target
metals was 95 percent over 6 sampling events. For the
recirculation mode of treatment system operation, the
average removal efficiency for the primary target metals
was 96 percent over 7 sampling events. Removal
efficiencies for arsenic were not calculated because the
influent and effluent metals concentrations were not
statistically different. In addition, the concentration of
arsenic in system influent was well below discharge
standards.
Average removal efficiencies for secondary target
metals ranged from 40 to 99 percent in both modes of
operation; however, removal efficiencies were not
calculated for chromium, lead, and selenium as the
influent and effluent concentrations were not statistically
different. In the case of arsenic, cadmium, chromium,
and lead in the ARD, concentrations were near or below
the EPA interim discharge standards in the influent;
therefore, the treatment system was not optimized for
removal of these metals. Sulfate reduction averaged 17
percent, decreasing from an average influent
concentration of 1,567 mg/L to an average effluent
concentration of 1,295 mg/L. There was on average a 9
percent increase in sulfate removal during the
recirculation mode of treatment system operation.
The bioreactor treatment system is extremely effective at
neutralizing acidity and reducing metals content in ARD,
with resulting effluent streams that meet EPA interim
discharge standards for the primary target metals and
the secondary target metals. Based on the success of
treatment system at the site, ARCO will continue to treat
ARD at the site using the bioreactor treatment system in
recirculation mode.
A more detailed evaluation of the compost-free
bioreactor treatment technology, including discussion of
secondary project objectives, will be presented in the
forthcoming Innovative Technology Evaluation Report
(ITER) that is anticipated in the spring of 2006.
Process Residuals
There is one process residual associated with bioreactor
treatment of ARD. The process produces a relatively
small quantity of metal sulfide sludge. During operation
from November 2003 through July 2005, the bioreactor
generated about 14.2 dry tons (49 cubic meters at 80
percent moisture content) of sludge consisting mainly of
iron sulfide. This equals 1.7 dry tons of sludge per
million gallons of ARD treated. The volume of sludge
generated is small in comparison to that generated by
lime treatment of ARD.
The solid waste residuals produced by the treatment
system were analyzed for hazardous waste
characteristics. Total metals and leachable metals
analyses were performed on the solid wastes for
comparison to California and Federal hazardous waste
classification criteria. To determine whether the
residuals are California hazardous waste, total metals
results were compared to Total Threshold Limit
Concentration (TTLC) criteria. To determine whether
the residuals pose a threat to water quality, metals
concentrations in Waste Extraction Test (WET) leachate
samples were compared to Soluble Threshold Limit
Concentration (STLC) criteria. To determine if the
residuals are a Resource Conservation and Recovery
Act (RCRA) waste, Toxicity Characteristic Leaching
Procedure (TCLP) results were compared to TCLP
limits. The waste characteristics determined for the solid
waste stream are presented in Table 4. None of the
solid wastes were found to be hazardous or a threat to
Table 4. Determination of Hazardous Waste Characteristics for Bioreactor Solid Waste Streams
Treatment
System
Bioreactor
Treatment
System
Solid Waste Stream
Dewatered Sludge
Pretreatment Pond
Settling Pond
Flushing Pond
Total Solid
Waste Generated
4.3 dry tons
Moved into Flushing Pond
10 dry tons (estimated)
4.3 dry tons (estimated)
TTLC
Pass or
Fail
P
P
P
P
STLC
Pass or
Fail
P
P
P
P
TCLP
Pass or
Fail
P
P
P
P
Waste Handling Status
Off-site Disposal
Moved into Flushing Pond
Pending Filtration
Pending Filtration
STLC = Soluble limit threshold concentration TTLC = Total threshold limit concentration
TCLP = Toxicity characteristic leaching procedure
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water quality; however, the solids were disposed of off
site pending designation of an on-site disposal area.
Technology Applicability
Bioreactor treatment of ARD at Leviathan Mine was
evaluated based on nine criteria used for decision
making in the Superfund feasibility study process.
Results of the evaluation are summarized in Table 5.
The bioreactor treatment system evaluated was
specifically designed to treat ARD at the mine site to
meet EPA interim discharge standards. In addition to
the five primary target metals of concern, EPA identified
the following metals as secondary target metals:
cadmium, chromium, lead, selenium, and zinc. The
bioreactor treatment system implemented at Leviathan
Mine was also successful at reducing concentrations of
these metals in the ARD, with the exception of selenium,
to below EPA interim discharge standards. The
bioreactor treatment system can be modified to treat a
higher flow rate and ARD with varying metals
concentrations and acidity.
Technology Limitations
In general, the limitations of the bioreactor treatment
system implemented at Leviathan Mine were not related
to the applicability of the technology, but rather to
operational issues due to weather conditions,
maintenance problems, and the remoteness of the site.
The technology is not limited by the sub-freezing
temperatures encountered in the high Sierra Nevada
during the winter months. However, biological activity
did slow resulting in decreased sulfate reduction to
sulfide. Effluent discharge standards continued to be
met during winter months as the flow of ARD entering
the bioreactor treatment system also decreased during
the winter. When designing systems for extremely cold
winters, consideration should be given to constructing
bioreactors of sufficient size to meet winter HRT
requirements and depth to buffer freezing temperatures
near the ground surface. In addition, adjustable
standpipes in below grade vaults should be used to
control the flow of water rather than mechanical valves,
which are subject freezing during the winter.
During extended operation of the bioreactor treatment
system, reagent metering and water recirculation pumps
and the generator that provided power to these pumps
were susceptible to failure. In addition, aboveground
influent ARD transfer and recirculation pipelines were
susceptible to breakage. These limitations are currently
being mitigated by 1) developing wind, solar, and
hydroelectric power sources, 2) installing redundant
pumps, and 3) placing transfer lines below grade.
Overall, the bioreactor treatment system required
minimal maintenance (once a week) in comparison to
maintenance intensive lime treatment systems.
The remoteness of the site also created logistical
challenges in maintaining operation of the bioreactor
treatment system. A winter snow pack from November
through May prevents site access to all delivery vehicles
except for snowmobiles. Consumable materials, such
as sodium hydroxide, ethanol, and diesel fuel (to power
a generator) must be transported to and stored in bulk at
the site during the summer. Sludge transfer from the
settling ponds, dewatering, and on- or off-site disposal
must also be performed during the summer months to
provide sufficient settling pond capacity during the
following winter months. Careful planning is essential to
maintain supplies of consumable materials and
replacement equipment at a remote site such as
Leviathan Mine.
Site Requirements
To conduct full-scale bioreactor treatment of ARD, the
main site requirement at the Leviathan Mine site was
developing adequate space for the treatment system,
staging areas, and support facilities. Space is needed
for reagent storage tanks, a pretreatment pond,
bioreactor ponds, settling ponds, an aeration channel,
and bag filters. Additional space was required adjacent
to the treatment system for storage of spare parts and
equipment, for loading and unloading equipment,
supplies, and reagents, and for placement of operating
facilities such eye wash stations, fuel storage tank, and
power generating equipment. Overall, the space
requirement for the bioreactor treatment of ARD at a
flow rate of 30 gpm at Leviathan Mine is about
0.75 acre.
The main utility requirement for the bioreactor treatment
system is electricity, which is used to operate reagent
delivery pumps, a water recirculation pump, and sludge
transfer pumps, and site work lighting. The bioreactor
treatment system, operated in recirculation mode,
requires less than 0.6 kilowatt (KW) hour of electricity for
continuous operation. Power for recirculation mode of
operation is provided by a 6 KW-hour diesel generator.
Diesel fuel for the generator is stored in a 1,000 gallon
above ground tank. The bioreactor treatment system,
operated in gravity flow mode, requires less than 0.1 KW
hour of electricity for continuous operation as a
recirculation pump is not required. Power for the gravity
flow mode of operation is provided by a solar panel and
storage batteries. Satellite phone service is also required
due to the remoteness of the site.
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Table 5. Feasibility Study Criteria Evaluation for Compost-Free Bioreactor Treatment System at Leviathan Mine
Criteria
Technology Performance
Overall Protection of
Human Health and the
Environment
Bioreactor treatment has been proven to be extremely effective at reducing concentrations of aluminum, copper,
iron, nickel, zinc, and other dissolved metals which can significantly degrade the quality of surface water receiving
ARD at the Leviathan Mine site. The bioreactor treatment system evaluated at Leviathan Mine is effective at
reducing the concentrations of toxic metals in ARD that was historically released to Aspen Creek, to below EPA
interim discharge standards, which were established to protect water quality and the ecosystem in Aspen Creek
and down-stream receiving waters. Resulting metals-enriched wastes were not determined to be hazardous
based on State or Federal criteria or a threat to water quality and can be disposed of on- or off-site.
Compliance with Applicable
or Relevant and Appropriate
Requirements (ARAR)
The bioreactor treatment system is generally compliant with EPA interim discharge standards for the Leviathan
Mine site. However, the effluent from the treatment system did not always meet the EPA interim (pre-risk
assessment and record of decision) discharge standards for the site or the secondary maximum contaminant limit
for iron, which could easily be met with additional sodium hydroxide dosing. No hazardous process residuals are
generated by the treatment system.
Long-term Effectiveness
and Performance
A bioreactor treatment system has been in operation at Leviathan Mine since 1996. The current full-scale
compost-free bioreactor treatment system has been in operation since the summer of 2003. By the fall of 2003,
the entire ARD flow from Aspen Seep was being treated by the full-scale system. The treatment system has
consistently met EPA interim discharge standards, with the exception of iron, since the fall of 2003. The
treatment system operates year round; therefore, discharge of metals-laden ARD has not occurred from the mine
site since initiation of treatment. The treatment system continues to be operated by UNR and ARCO. Long-term
optimization of the treatment system will likely refine sodium hydroxide dosage necessary for iron polishing,
optimize recirculation rates for sulfide generation, and demonstrate whether wind, solar, or a water turbine can
meet the power required for chemical dosage and recirculation pumps.
Reduction of Toxicity,
Mobility, or Volume
through Treatment
Bioreactor treatment significantly reduces the mobility and volume of toxic metals from ARD at Leviathan Mine.
The dissolved toxic metals are precipitated from solution, concentrated, and dewatered removing toxic levels of
metals from the ARD. The bioreactor treatment does produce a solid waste; however, the waste generated has
been determined to be non-hazardous and can be disposed of on- or off-site.
Short-term Effectiveness
The resulting effluent from the bioreactor treatment system does not pose any risks to human health. The sodium
hydroxide solution, ethanol feedstock, and biologically-generated hydrogen sulfide gas, each having potentially
hazardous chemical properties, may pose a risk to site workers during treatment system operation. Exposure to
these hazardous chemicals must be mitigated through engineering controls and proper health and safety
protocols.
Implementability
The bioreactor treatment technology relies on a relatively simple biologically-mediated sulfate reduction and metal
sulfide precipitation process and can be constructed using readily available equipment and materials. The
technology is not proprietary, nor does it require proprietary equipment or reagents. Once installed, the system
can be optimized and maintained indefinitely. System startup and biological acclimation can take up to three
months, depending on target metal concentrations and weather conditions. Routine maintenance is required,
involving a weekly visit by an operator to ensure reagent and recirculation pumps are operational, replenish
reagents as needed, and handle settled metal sulfides as needed. The remoteness of the site also necessitates
organized, advanced planning for manpower, consumables, and replacement equipment and supplies.
Cost
Total first year cost for the construction and operation of the bioreactor treatment system operated in gravity flow
mode was $941,248 and $962,471 operated in recirculation mode. The operation and maintenance costs
associated with the treatment system ranged from $15.28 (recirculation) to $16.54 (gravity flow) per 1,000 gallons
at an average ARD flow rate of 9.45 gallons per minute. The operational costs were incurred during a research
mode of operation. Once the system is optimized an operations mode will be implemented which will reduce
operational labor and reagent costs. Costs for construction and O&M of the treatment system are dependent on
local material, equipment, consumable, and labor costs, required discharge standards, and hazardous waste
classification requirements and disposal costs (if necessary).
Community Acceptance
The bioreactor treatment technology presents minimal to no risk to the public since all system components are
located at and treatment occurs on the Leviathan Mine site, which is a remote, secluded site. Hazardous
chemicals used in the treatment system include sodium hydroxide, ethanol, and for the short term diesel fuel.
These chemicals pose the highest risk to the public during transportation to the site by truck. The diesel
generator creates the most noise and air emissions at the site; again, because of the remoteness of the site, the
public is not impacted. Alternative sources of power are being pilot tested at the site to eliminate the need for the
diesel powered generator.
State Acceptance
ARCO, in concurrence with the State, selected, constructed, and is currently operating a full-scale bioreactor
treatment system at Leviathan Mine, which indicates the State's acceptance of the technology to treat ARD. The
bioreactor treatment system is the only technology operating year round at the mine site. All other treatment
systems at the mine site shutdown for the winter, requiring long-term storage or discharge of ARD and AMD.
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Technology Status
The technology associated with the compost-free
bioreactor treatment system is not proprietary, nor are
proprietary reagents or equipment required for system
operation. The system has been demonstrated at full-
scale and is currently operational at Leviathan Mine.
The treatment system is undergoing continuous
refinement and optimization to reduce the quantity of
alcohol and caustic chemicals required for system
operation. The power required for recirculation of water
to the head of the system is currently provided by a
generator. In 2006, alternative methods of power
generation will be investigated. Based on the success of
bioreactor treatment at the Leviathan Mine site, ARCO
will continue to use this technology to treat ARD at the
Aspen Seep. Application of the technology to other
ARD-impacted sites does not require a pilot-scale
system because the uncertainties related to carbon
availability and sulfate reduction efficiency, matrix
compaction, and solids flushing associated with compost
and wood chip matrices are essentially eliminated. A
simple bench test can be used to optimize the ethanol
dose necessary to reduce sulfate, to optimize the base
type and dose required to neutralize acidity, and to
estimate the volume of precipitate that will be generated.
Sources of Further Information
The ITER for compost-free bioreactor treatment of ARD
at Leviathan Mine is being prepared along with this
Technology Capsule report. The ITER is anticipated to
be available in the spring of 2006. The ITER provides
more detailed information on the treatment technology, a
detailed discussion of capital and operation and
maintenance costs, and a more thorough discussion of
the evaluation results.
EPA Contacts:
Edward Bates, U.S. EPA Project Manager
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Office of Research and Development
26 West Martin Luther King Jr. Dr.
Cincinnati, OH 45268
(513)569-7774
Bates.Edward@epa.qov
EPA Contacts (continued):
Kevin Mayer, U.S. EPA Remedial Project Manager
U.S. Environmental Protection Agency Region 9
75 Hawthorne Street, SFD-7-2
San Francisco, CA 94105
(415)972-3176
Mayer.Kevin@epa.qov
Atlantic Richfield Company Contact:
Mr. Roy Thun, Project Manager
BP Atlantic Richfield Company
6 Centerpointe Drive, Room 6-164
La Palma, CA 90623
(661)287-3855
thunril@bp.com
State of California Contact:
Richard Booth, Project Manager
California Regional Water Quality Control Board
Lahontan Region
2501 Lake Tahoe Blvd.
South Lake Tahoe, CA 96150
(530) 542-5474
RBooth(gjwaterboards. ca.gov
University of Nevada-Reno Contacts:
Drs. Glenn Miller and Tim Tsukamoto
Department of Natural Resources and
Environmental Science
Mail Stop 199
University of Nevada-Re no
Reno, NV 89557-0187
(775)784-4413
gcmiller@unr.edu
timothvt@unr. edu
References:
Tetra Tech EM Inc (Tetra Tech). 2003. 2003
Technology Evaluation Plan/Quality Assurance Project
Plan, Leviathan Mine Superfund Site, Alpine County,
California.
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