United States
Environmental Protection
Agency
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Technology Evaluation Bulletin
Compost-Free Biological Treatment of Acid Rock Drainage
Technology Description: Drs. Glenn Miller and Tim
Tsukamoto of the University of Nevada Reno (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 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.
The U.S. Environmental Protection Agency (EPA), in
cooperation with the state of California, Atlantic Richfield
Company, and UNR evaluated the compost-free
bioreactor treatment of acid rock drainage (ARD) at the
Leviathan Mine Superfund Site located in a remote, high
altitude area of Alpine County, California. The biological
treatment system was evaluated from 2003 though
2005, while operating in both gravity flow and
recirculation modes of operation. The system
neutralized acidity and precipitated metal sulfides from
ARD at flows ranging up to 24 gallons per minute on a
year-round basis.
EPA evaluated the biological treatment systems' ability
to neutralize acidity and to reduce concentrations of
target metals in the ARD to below EPA-mandated
discharge standards. The primary target metals were
aluminum, arsenic, copper, iron, and nickel; and the
secondary target metals were cadmium, chromium, lead,
selenium, and zinc. Historically, the concentrations of
the five primary target metals in ARD released into
Aspen and Leviathan Creeks have exceeded EPA-
mandated discharge levels by up to 580 fold,
contributing to fish and insect kills in the creek and
downstream receiving waters.
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 to 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,
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.
2CH3CH2OH
2CH3CH2OH + SO4 -
HS" + M2+-» MS + 2H+
3S042
2"
» 3HS" + 3HCO3" + 3H2O
2 CH3COO" + HS" + H2O
(2)
(3)
Here ethanol is the carbon source and sulfate (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
(CH3COO~) rather than complete oxidation to carbonate.
Hydrogen sulfide (HS") then reacts with a variety of
divalent metals (M ), resulting in a metal sulfide (MS)
precipitate.
At Leviathan Mine, the compost-free bioreactor
treatment system consists of a pretreatment pond, two
gravity-flow bioreactors, two settling ponds, and an
aeration channel. Operated in gravity flow mode, ARD is
introduced to the pretreatment pond, where sodium
hydroxide is added to adjust the influent pH of 3.1 up to
4.0, and ethanol is added as a carbon source. ARD
from the pre-treatment pond then flows through
Bioreactor No.1 (12,500 total and 5,300 cubic foot active
volume) and Bioreactor No. 2 (7,000 total and 3,000
cubic foot active volume) to biologically 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. Precipitates in effluent from the second
bioreactor are settled in a 16,400 cubic foot continuous
flow settling pond.
Operated in recirculation mode, metal-rich influent ARD
is combined with sulfide-rich water discharged from the
second bioreactor as well as sodium hydroxide to
precipitate metals in the settling pond rather than in the
bioreactors. A portion of the settling pond supernatant
containing minimal residual metals and excess sulfate is
pumped to the first bioreactor and combined with ethanol
feed stock to promote additional sulfate reduction to
sulfide in the two bioreactors.
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The effluent from the continuous flow settling pond then
flows to a 150 foot by 2 foot rock lined aeration channel
to promote degassing of residual hydrogen sulfide and
oxygenation of the effluent prior to discharge.
Precipitate slurry is periodically flushed from the
bioreactors to prevent plugging of the river rock matrix
(gravity flow mode) and is settled in an 18,100 cubic foot
flushing pond. Settled solids from the flushing pond are
periodically dewatered using bag filters.
Waste Applicability: Conventional methods of treating
ARD involve lime addition, which neutralizes acidity and
precipitates metals. Active lime treatment appears to be
applicable in situations where flow rates are moderate to
high and the treatment season is short. However, the
innovative passive compost-free bioreactor is not
constrained by seasonal conditions and can be scaled to
treat low to moderate flows, which are typical of many
ARD sites. The compost-free bioreactor technology
generates relatively small quantities of sludge, in
comparison to the larger sludge yield of lime treatment
technologies.
Evaluation Approach: Evaluation of the compost-free
bioreactor technology occurred between November 2003
and July 2005. Multiple sampling events were
conducted during operation of the bioreactor treatment
system. During each sampling event, EPA collected
chemical data from the system influent and effluent
streams, documented metals removal and reduction in
acidity between the bioreactors, settling ponds, and
aeration channel, and recorded operational information
pertinent to the evaluation of the treatment system. The
treatment system was evaluated independently, based
on removal efficiencies for primary and secondary target
metals, comparison of effluent concentrations to EPA-
mandated discharge standards, and on the
characteristics of and disposal requirements for the
resulting metals-laden solid wastes.
The primary objectives of the technology evaluations
were:
• Determine the removal efficiencies for primary
target metals over the evaluation period
• Determine whether the concentrations of the
primary target metals in the treated effluent are
below the discharge standards mandated in the
EPA Action Memorandum for this site
In addition, the following secondary objectives were
intended to provide additional information that will be
useful in evaluating the technologies:
• Document operating parameters and assess
critical operating conditions necessary to
optimize system performance
• Monitor the general chemical characteristics of
the ARD water as it passes through the
treatment system
• Evaluate operational performance and efficiency
of solids separation systems
• Determine capital and operation and
maintenance costs
Evaluation Results: The compost-free bioreactor
treatment system was shown to be extremely effective at
neutralizing acidity and reducing the concentrations of
the 5 target and 5 secondary metals in the ARD flows at
Leviathan Mine to below EPA-mandated discharge
standards. During the demonstration, pilot testing to
determine optimal sodium hydroxide addition resulted in
exceedance of discharge standards for iron; however,
after optimization iron concentrations in effluent met
discharge standards. The solids generated by this
technology were not found to be hazardous under state
or federal standards or pose a threat to water quality. A
table summarizing average influent and effluent
concentrations and removal efficiencies for the 5 target
metals is provided below.
Key findings from the evaluation of the treatment
system, including complete analytical results, operating
conditions, and a cost analysis, will be published in a
Technology Capsule and an Innovative Technology
Evaluation Report.
For further information contact:
Edward Bates, U.S. EPA Project Manager
EPA 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
Primary Target Metals
Aluminum
(mg/L)
Arsenic
(mg/L)
Copper
(mg/L)
Iron
(mg/L)
Nickel
(mg/L)
Secondary Water Quality Indicator Metals
Cadmium
(mg/L)
Chromium
(mg/L)
Lead
(mg/L)
Selenium
(mg/L)
Zinc
(mg/L)
Gravity Flow Configuration
Influent
Effluent
Removal Efficiency
37.47
0.103
99.7
0.002
0.005
NC
0.691
0.005
99.3
117.2
4.89
95.8
0.487
0.066
86.6
0.0006
<0.0002
65.3
0.012
0.008
NC
0.004
0.005
NC
0.014
0.011
NC
0.715
0.016
97.8
Recirculation Configuration
Influent
Effluent
Removal Efficiency
EPA Standard
40.03
0.053
99.9
2.0
0.007
0.007
NC
0.15
0.795
0.005
99.4
0.016
115.8
2.7
97.7
1.0
0.529
0.070
86.8
0.094
0.0006
<0.0002
NC
0.004
0.011
0.006
42.5
0.31
0.004
0.003
41.3
0.005
0.012
0.009
NC
0.005
0.776
0.009
98.9
0.21
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v°/EPA
United States
PRESORTED STANDARD
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Environmental Protection
Agency
Office of Research and Development
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA/540/S-06/009
March 2006
www.epa.gov
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