&EPA
United States
Environmental Protection
Agency
EPA/540/SR-93/523
September 1993
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Handbook for Constructed
Wetlands Receiving Acid
Mine Drainage
A treatment technology based on
constructed wetlands uses natural
geochemical and biological processes
inherent in the aqueous environment
and designs a system to optimize pro-
cesses best suited to removal of con-
taminants specific to the site. Key
features of this wastewater technology
are that it is a passive treatment sys-
tem, the cost of operation and mainte-
nance is significantly lower than that
for active treatment processes, and the
removal methods try to mock rather
than overcome natural processes. In
this study, the contaminant waters were
metal-mine drainages with low pH (<3.0)
and high concentrations of metals (Al,
Mn, Fe, Ni, Cu, Zn, and Pb).
From studies done at constructed
wetlands at the Big Five Tunnel near
Idaho Springs, Colorado, the important
process for raising the pH and remov-
ing metals was found to be bacterial
sulfate reduction followed by precipita-
tion of metal sulfides. By optimizing
the process and determining how to
properly load the wetland with contami-
nant drainage, the following results
were achieved:
• pH was raised from 2.9 to 6.5.
• Dissolved AI, Cu, Zn, Cd, Ni, and
Pb concentrations were reduced
by 98 % or more.
• Iron removal was seasonal with
99% reduction in the summer.
• Mn reduction was relatively poor
unless the pH of the effluent was
raised above 7.0.
• Biotoxicity to fathead minnows
and Ceriodaphnia was reduced
by factors of 4 to 20.
Once it was found that microbial pro-
cesses were primarily responsible for
contaminant removal, a staged design
process comparable to the design pro-
cess used for other wastewater treat-
ment technologies was devised.
Laboratory studies determine whether
in principle contaminants could be re-
moved and the best substrate combi-
nation for their removal. Bench scale
studies determine the optimum loading
capacity and treatment system configu-
ration. From these studies design of a
reasonably sized module that is spe-
cific to the site can proceed with the
expectation that it will successfully treat
the contaminanted water.
This summary was developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the SITE Emerging Tech-
nology Program that is documented in
a separate report (see ordering infor-
mation at back).
OyD Printed on Recycled Paper
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Introduction
In response to the Superfund Amend-
ments and Reauthorization Act of 1986,
(SARA), the U. S. Environmental Protec-
tion Agency's (EPA) Office of Research
and Development (ORD) and the Office of
Solid Waste and Emergency Response
(OSWER) have established a formal pro-
gram to accelerate the development, dem-
onstration, and use of new or innovative
technologies as alternatives to current con-
tainment systems for hazardous wastes.
This program is called Superfund Innova-
tive Technology Evaluation or SITE.
The SITE program is part of EPA's re-
search into cleanup methods for hazard-
ous waste sites throughout the nation.
Through cooperative agreements with de-
velopers, alternative or innovative tech-
nologies are refined at the bench-scale
and pilot-scale level and then demon-
strated at actual sites. EPA collects and
evaluates extensive performance data on
each technology to use in remediation de-
cision making for hazardous waste sites.
The report summarized here documents
the results of laboratory and pilot-scale
field tests on the applicability of sulfate-
reducing bacteria operating in the anaero-
bic zone within a wetland constructed to
remove contaminant metals associated
with mine drainages. These metals can
include Al, Mn, Fe, Co, Ni, Cu, Zn, As, Ag,
Cd, Hg, and Pb. In the mine drainage
Water used in this study, Mn, Fe, Cu, and
Zn are the primary contaminants in the
water. Also, most mine drainages have an
actdic pH that causes concern and has to
be Increased to effect treatment. In the
water used in this study, the average pH
is 3.0.
The Concept of Constructed
Wetlands
Ecotogists have long understood that
soils in wetlands are often foul because
they naturally accumulate contaminants by:
• filtering of suspended and colloidal
material from the water;
• uptake of contaminants into the roots
and leaves of live plants;
* adsorption or exchange of contami-
nants onto inorganic soil constituents,
organic solids, dead plant material, or
algal material;
• neutralization and precipitation of con-
taminants through the generation of
HCOj'and NH3 by bacterial decay of
organic matter;
« destruction or precipitation of contami-
nants in the aerobic zone catalyzed
by the activity of bacteria; and
• destruction or precipitation of
chemicals in the anaerobic zone
catalyzed by the activity of bacteria.
With so many possible removal pro-
cesses, a wetland, such as depicted in
Figure 1 , is the typical contaminant treat-
ment system in a natural ecosystem. In
addition, it operates in a passive mode
requiring no additional reactants and no
continuous maintenance.
In the last decade, engineers began to
use wetlands to remove contaminants from
water. In some instances, natural wetlands
were used. A natural system however, will
accommodate all the above removal pro-
cesses and probably will not operate to
maximize a certain process. A constructed
wetland, on the other hand, can be de-
signed to maximize a specific process suit-
able for the removing of certain contami-
nants. Engineering and ecological reasons
lead to using a constructed wetland for
contaminant removal rather than using an
existing natural ecosystem.
As an example of constructing a wet-
land to maximize specific removal pro-
cesses, consider the bacterial processes
that are items 6 and 7 in the above list.
Typical microbially mediated reactions that
are possible in the aerobic zone of a wet-
land include:
4 Fe2* + O2 + 10 H2O — >
4 Fe(OH)3 + 8 H+
2 02 + H2S — > S04- + 2 H*
2 H20 + 2 N2 + 5 Oz— > 4 NCy + 4 H*
Typical microbially mediated reactions
that are possible in the anaerobic zone of
a wetland include:
4Fe2*+CO2-i-11 H2O
5 CHO + 4 NO- + 4 H* — >
2 N + 5 C0
7 H20
SO4- +2 CH2O — > H2S
2 HCO-
In these reactions, "CH2O" is used to
symbolize organic material in the substrate.
It is apparent that the anaerobic reac-
tions are approximately the reverse of the
aerobic reactions. Both zones exist in a
wetland. If removal involves aerobic pro-
cesses, then the wetland should be con-
structed so the water remains on the sur-
face. If removal involves anaerobic pro-
cesses, then the wetland should be con-
structed so the water courses through the
substrate. In a natural wetland, the water
primarily remains on the surface.
In the important area of microbially me-
diated removal, the wetland must be con-
structed to maximize removal reactions
and minimize competing reactions. When
removing contaminants from acid mine
drainage, the removal processes should
consume hydrogen ions and, conse-
quently, anaerobic processes are empha-
sized. The research and development at
the Big Five Tunnel site in Idaho Springs,
Colorado has concentrated on understand-
ing the chemistry and ecology involved in
removal and designing structures from
readily available materials that maximize
these processes.
Although this appears to be "low tech-
nology", an intense interdisciplinary effort
and creative engineering skills are needed
to design and perfect systems that maxi-
mize natural processes. For more details
on what should be considered, The full
report cites recent references.
The Big Five Pilot Wetland
The research reported here has involved
studying removal processes from a pilot
constructed wetland designed to receive
metal mine drainage from the Big Five
Tunnel in Idaho Springs, CO. The chemis-
try from the adit drainage is reasonably
constant throughout the year and is sum-
marized in Table 1. After a number of
modifications of the pilot cells, removal
results were excellent.
Table 1. Contaminant Concentration, Big Five
Tunnel Drainage, Averages.
Constituent
Concentration,mg/L
Mn
Fe
Co
Ni
Cu
Zn
Cd
Pb
31
38
0.10
0.15
0.73
9.4
0.03
0.03
Figure 2 shows the removal trends for
a 2-year period as outflow concentrations
over influent concentrations. Cu and Zn
are completely removed; Fe removal
changes with the seasons.
During this 2-year period, analysis of
chemical data accumulated at the site led
to the conclusion that microbial reduction
of sulfate to sulfide followed by precipita-
tion of heavy metal sulfides is the pre-
dominant process accounting for the re-
moval of over 90 % of the Fe, Cu, and Zn
and the rise in the pH from below 3 to
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above 6. Procedures for construction of
wetland ceils that emphasize anaerobic
removal processes has been considered.
Another consideration is how to employ
knowledge of the biochemistry of sulfate
reduction in the wetland design process.
This has been done by using two ideas
particularly suited to wetlands-ideas that
employ anaerobic removal processes such
as sulfate reduction: The limiting reagent
concept, and staged design of wetlands.
The Limiting Reagent Concept
In the design of wetlands for wastewa-
ter treatment, there is a strong emphasis
on determining the loading factor which
gives an indication of how large a wetland
should be to remove the contaminants of
concern. This can be stated as the amount
of square feet of wetland per gallon per
minute of water to be treated or as the
grams of contaminant removed per day
per square meter of wetland. In our expe-
riences at the Big Five site, typical mea-
sures of loading factor do not seem to
explain the removal of metals even though
heavy metals such as Cu and Zn are
Porous
Rock
Dam
Figure 1. Diagram of a typical wetland ecosystem that emphasizes subsurface flow.
Cell E Removal Trends From Sept. 1989
1.00
MnE
FeE
CuE
ZnE
._. SO4E
0.00
Months
Figure 2. Two year removal trends for a subsurface wetland cell located at the Big Five Tunnel in Idaho Springs, Colorado.
3
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reduced by greater than 99 %. We have
discovered that a key factor in sulfate
reduction is to Insure that the optimum
microenvironment for sulfate-reducers is
maintained. The most important environ-
mental conditions are reducing conditions
and a pH of around 7. Since the wetland
cell is receiving mine drainage with pH
below 3 and Eh of above 700 mV, the
water can easily overwhelm the micro-
environment established by the anaerobic
bacteria. This leads to the limiting reagent
concept for determining how much water
can be treated, as an alternative to the
use of typical bading factors.
Consider the following precipitation re-
action:
Fe^-t-S--
->FeS
At high flows of mine drainage through
the substrate, sulfide will be the limiting
reagent, the microbial environment will be
under stress to produce more sulfide, the
pH of the microenvironment will drop, and
removal will be Inconsistent. At low flows
of mine drainage through the substrate,
iron will be the limiting reagent, the ex-
cess sulfide will insure a reducing envi-
ronment and a pH near 7, the microbial
population will remain healthy, and removal
of the metal contaminants will be consis-
tent and complete. Using this idea, load-
ing factors should be set to insure that the
heavy metal contaminants are always the
limiting reagents. The question then is how
much sulfide can a colony of sulfate-re-
ducing bacteria produce per cubic centi-
meter of substrate per day?
Studies by the U. S. Bureau of Mines
wetlands group suggests that a reason-
able figure for sulfide generation is 300
nanomole sutfide/cubic cm/day. This num-
ber, the volume of the wetland cell, and
the metals concentrations in the mine
drainage are used to set the flow of mine
drainage through the wetland cell. Using
this concept in a subsurface wetland cell
to determine the loading factor has re-
sulted in year round complete removal of
Cu and Zn (Rgure 2).
Staged Design of Wetlands
After determining that precipitation of
metals by sulfide generated from sulfate-
reducing bacteria is the important process,
H was realized that establishing and main-
taining the proper environment in the sub-
strate is the key to success for removal.
This means that processes operating on
the surface of the wetland are not that
Important. In particular, plants are not nec-
essary In a wetland emphasizing subsur-
face processes. If this is so, then a large
pilot cell, such as was built at the Big Five
site, is not necessary to determine whether
a wetland that emphasizes anaerobic pro-
cesses for removal will work. Conse-
quently, the study of wetland processes
and the design of optimum systems can
proceed from laboratory experiments to
bench scale studies to design and con-
struction of actual cells. We call this
"staged design of wetland systems".
In current laboratory studies, culture
bottle experiments are used for funda-
mental studies on how to establish simple
tests to determine the production of sul-
fide by bacteria, and of what substrate will
provide the best initial conditions for growth
of sulfate-reducing bacteria. In these ex-
periments, laboratory production of sulfide
at 18 °C has been 1200 nanomole/gm of
dry substrate/day.
Culture bottle tests have shown that in
the case of cyanide, sulfate reduction was
retarded until the concentration of total
cyanide was below 10 mg/L and that Cu
concentrations above 100 mg/L would kill
or retard sulfate-reducing bacteria. How-
ever, other culture bottle tests have also
shown that sulfate reduction was still vig-
orous at Cu and Zn concentrations above
100 mg/L.
For bench scale studies, plastic gar-
bage cans are used to conduct experi-
ments to provide answers necessary to
the design of a subsurface cell, e.g. deter-
mining the optimum loading factor, sub-
strate, cell configuration, and substrate
permeability. In a recent study, garbage
cans filled with substrate were used to
determine whether using the sulfide gen-
eration figure of 300 nanomole sulfide/cm3
of substrate/day could be used to set the
conditions for treating severely contami-
nated drainage that flows from the Quartz
Hill Tunnel in Central City, CO. Contami-
nant concentrations are shown in Table 2.
Using the limiting reagent concept de-
scribed above and the amount of sub-
strate contained in the garbage can, flow
could not exceed 1 ml/min to ensure that
sulfide would always be in excess. Con-
taminant concentrations from the outputs
of three different bench scale cells are
shown in Table 2. For cell A the mine
drainage was passed through the cell with
no delay. For cell B the substrate was
soaked with city water for one week be-
fore mine drainage started passing through
the cell. For cell C, the substrate was
inoculated with an active culture of sul-
fate-reducing bacteria and soaked with city
water for one week before mine drainage
started passing through the cell. Prepara-
tions on cells B and C were done to en-
sure that there would be a healthy popu-
lation of sulfate-reducing bacteria before
mine drainage flowed through the sub-
strate. All cells were run in a downflow
mode of the mine drainage through the
substrate. In all three cells removal of Cu,
Zn, Fe, as well as Mn is greater than
99%. The increase in pH is from about
2.5 to above 7. These results were con-
sistently maintained for over ten weeks of
operation.
The substrate used was a mix of 3/4
cow manure and 1/4 planting soil. The
results from cells B and C show that the
cow manure has an indigenous popula-
tion of sulfate-reducing bacteria that are
quite active. Inoculation with an active cul-
ture of bacteria is not necessary in this
case. Also, since the results from cell A
are comparable to those of cells B and C,
the population of sulfate reducers can with-
stand immediate exposure to severe mine
drainage and still produce sufficient quan-
tities of sulfide. The key to good initial
activity is to ensure that the flow of mine
drainage is low enough that its low pH
does not disturb the micro-environment
established by the bacteria.
These bench scale systems also serve
as permeameters and thus can provide
important information for others aspect of
wetland design. Determination of soil con-
ductivity and how this physical property
changes with time is found to be an im-
portant geotechnical parameter for the de-
sign of subsurface constructed wetlands.
Conclusions
Using constructed wetlands for waste-
water treatment is still a developing tech-
nology. The results from the Big Five Pilot
Wetland study, however, show promising
removal of heavy metals and increase of
pH for acid mine drainage. Conclusions
from the project include:
• Toxic metals such as Cu and Zn can
be removed and the pH of mine
drainage can be increased on a long
term basis.
• The major removal process is sulfate
reduction and subsequent precipitation
of the metals as sulfides. Exchange
of metals onto organic matter can be
important during the initial period of
operation.
• A trickling filter type of configuration
achieves the best contact of the water
with the substrate.
• Removal efficiency depends strongly
on loading factors. In the Big Five
wetland and in bench scale studies,
flow of water should not exceed the
300 nanomoles/cmVday of sulfide that
can be generated by the microbes in
the substrate.
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• Permeability of the substrate is a
critical design variable for successful
operation. Using laboratory and 5.
bench-scale tests, a good indication
of the soil permeability in a
constructed wetland can be
determined.
• As with, any other wastewater removal
technology, design of a constructed
wetland or passive bioreactor is
specific to the site and the water to
be treated. 6.
• A staged design and development
sequence can be used where
laboratory studies are used to
determine the best conditions and
substrate, bench scale experiments
help to determine loading factors and
substrate properties, and pilot
modules test the performance of a
typical field module.
References 7.
1. Reed, S. C., Middlebrooks, E. J.,
and Crites, R. W. Natural Systems
for Waste Management and Treat-
ment. McGraw-Hill, New York,
1988. 308pp.
2. Hammer, D. A. Constructed Wet-
lands for Wastewater Treatment. 8.
Lewis Publishers, Chelsea, Michi-
gan, 1989. 800 pp.
3. Kleinmann, R. L P. (ed.), Proceed-
ings of a Conference on Mine Drain-
age and Surface Mine Reclamation.
Vol. 1. Mine Water and Mine Waste.
U. S. Department of the Interior,
Bureau of Mines Information Circu-
lar 1C 9183, 1988, 413 pp. 9.
4. Wildeman, T. R., and Laudon, L. S.
The use of wetlands for treatment
of environmental problems in min-
ing: Non-coal mining applications.
In: D. A. Hammer (ed.), Constructed
Wetlands for Wastewater Treat-
ment. Lewis Publishers, Chelsea,
Michigan, 1989. p. 221.
Howard, E. A., Emerick, J. C., and
Wildeman, T. R. The design, con-
struction and initial operation of a
research site for passive mine drain-
age treatment in Idaho Springs,
Colorado. In: D. A. Hammer (ed.),
Constructed Wetlands for Waste-
water Treatment. Lewis Publishers,
Chelsea, Michigan, 1989. p. 761.
Machemer, S. D., Lemke, P. R.,
Wildeman, T. R., Cohen R. R.,
Klusman, R. W., Emerick, J. C.,
and E. R. Bates. Passive Treat-
ment of Metals Mine Drainage
through use of a Constructed Wet-
land. In: Proceedings of the 16th
Annual Hazardous Waste Research
Symposium, U. S. EPA, Cincinnati,
OH, 1990. EPA Document No. EPA/
600/9-90-037, pp. 104-114, 1990.
Machemer, S. D., and Wildeman,
T. R., Organic Complexation Com-
pared with Sulfide Precipitation as
Metal Removal Processes from Acid
Mine Drainage in a Constructed
Wetland". Jour. Contaminant
Hydrology,vol. 9, pp. 115-131,1992.
Hedin, R. S., and R. W. Nairn. Siz-
ing and Performance of Constructed
Wetlands: Case Studies. In: Pro-
ceedings of the 1990 Mining and
Reclamation Conference, J.
Skousen, J. Scencindiver, and D.
Samuel eds., West Virginia Univ.
Publications, Morgantown, WV,
1990. pp. 385-392
Wildeman, T. R., Machemer, S. D.,
Klusman, R. W., Cohen, R. R., and
P. Lemke. Metal Removal Efficien-
cies from Acid Mine Drainage in
the Big five Constructed Wetland.
In: Proceedings of the 1990 Mining
and Reclamation Conference, J.
Skousen, J. Scencindiver, and D.
Samuel eds., West Virginia Univ.
Publications, Morgantown, WV,
1990. pp. 417-424.
10. Mclntire, P. E., and H. M. Edenborn.
The use of Bacterial Sulfate Re-
duction in the Treatment of Drain-
age from Coal Mines. In:
Proceedings of the 1990 Mining and
Reclamation Conference, J.
Skousen, J. Scencindiver, and D.
Samuel eds., West Virginia Univ.
Publications, Morgantown, WV,
1990. pp. 409-415.
11. Reynolds, J. S., Machemer, S. D.,
Wildeman, T. R., Updegraff, D. M.,
and R. R. Cohen, 'Determination of
the Rate of Sulfide Production in a
Constructed Wetland Receiving
Acid Mine Drainage". Proceedings
of the 1991 National Meeting of the
American Society of Surface Min-
ing and Reclamation, ASSMR,
Princeton, WV, 1991, pp 175 -182
12. Filas, B., and T. R. Wildeman, The
Use of Wetlands for Improving Wa-
ter Quality to Meet Established
Standards", Nevada Mining Assoc.
Annual Reclamation Conference,
Sparks, NV, May, 1992.
13. Bolis, J. L., Wildeman, T. R., and
R. R. Cohen, ' The Use of Bench
Scale Permeameters for Preliminary
Analysis of Metal Removal from
Acid Mine Drainage by Wetlands".
Proceedings of the 1991 National
Meeting of the American Society of
Surface Mining and Reclamation,
ASSMR, Princeton, WV, 1991, pp
123-136.
14. Wildeman, T. R., Brodie, G. A., and
J. J. Gusek, Wetland Design for
Mining Operations, Bitech Publish-
ing Co. Vancouver, BC, Canada,
1992, 300 pp
Table 2. Constituent concentrations in mg/L in the Quartz Hill Tunnel mine drainage and in effluents from the bench scale tests
Sample
Mine Drainage
Cell A
CellB
CellC
Mine Drainage
CellA
CellB
CellC
Mine Drainage
CellB
Cell C
Days
Operated
24
24
24
24
43
43
43
43
71
71
71
Mn
80.0
0.94
0.91
0.99
80.0
0.97
0.64
1.6
70.0
0.48
1.6
Fe
630.0
1.6
1.9
1.0
640.0
0.87
0.96
0.46
820.0
0.40
0.40
Cu
48.0
0.06
<0.05
<0.05
50.0
<0.05
<0.05
<0.05
70.0
<0.05
<0.05
Zn
133.0
0.27
0.17
0.16
135.0
0.18
0.24
0.14
101.0
0.21
0.25
so4
4240
450
70
412
4300
1080
660
1180
NA
NA
NA
pH
2.4
7.4
7.5
7.4
2.5
7.2
7.4
7.2
2.6
8.0
7.9
•fru.8. GOVERNMENT PRINTING OFFICE: IM3 - 7SO-071/80M*
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Thomas Wildeman is with the Department of Chemistry and Geochemistry,
Colorado School of Mines, Golden, CO 80401
Edward R. Bates is the EPA Project Officer (see below).
The complete report, entitled "Handbook For Constructed Wetlands Receiving
Acid Mine Drainages," (Order No. PB93-233914AS; Cost: $36.50, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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