U.S. Department of the Interior • Bureau of Land Management

Technical Note 409

April 2003
Production services provided by:

Information and Communications Staff

Publishing Staff (303-236-6547)

Janine Koselak: Layout and Design

Linda Hill: Editing

Lee Barkow, Director

National Science & Technology Center

P.O. Box 25047

Denver, Colorado 80225-0047

The Bureau of Land Management’s National Science

and Technology Center supports other BLM offices by

providing a broad spectrum of services in areas such as

physical, biological, and social science assessments; 

architecture and engineering support; library assistance;

mapping science; photo imaging; geographic information

systems applications; and publications support. 

Available at:

www.blm.gov/nstc/library/techno2.htm

Technical Note 409

BLM/ST/ST-02/001+3596
Technical Note 409

April 2003

by:

K.L. Ford

Bureau of Land Management

National Science and Technology Center
PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

SUGGESTED CITATION:

Ford, K.L. 2003. Passive treatment systems for acid mine drainage. Technical

Note 409. BLM/ST/ST-02/001+3596. Bureau of Land Management Webbased report available online at http://www.blm.gov/nstc/library/techno2.htm.
iii

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Chemistry of Acid Mine Drainage . . . . . . . . . . . .3

Types of Passive Treatment Systems . . . . . . . . . . .5

Site Characterization . . . . . . . . . . . . . . . . . . . . . .7

Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . .8

Conceptual Design . . . . . . . . . . . . . . . . . . . . . . .9

Effluent Regulations and Expectations . . . . . . . .10

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Cited References . . . . . . . . . . . . . . . . . . . . . . . .13

1

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Abstract

PASSIVE TREATMENT SYSTEMS provide an alternative method for

removing metals from acid mine drainage. There are several types of passive

treatment systems; they may be used on their own or in combination to treat

difficult effluents. The type of system selected is dependent upon the chemistry of the acid mine drainage and the flow of the discharge, as well as State

and Federal regulations. Passive treatment systems do not require power and

are less expensive than active treatment systems. They also require less

maintenance, which makes them advantageous in remote locations. 

Keywords

Abandoned mines Anaerobic wetland

Acid mine drainage Passive treatment system

Aerobic wetland Water treatment 

3

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Introduction

THERE ARE MANY TYPES of water treatment systems available for removing

metals from acid mine drainage. Passive treatment systems are a relatively new

technology that involves using sulfate-reducing bacteria or limestone or both

to neutralize acidity and precipitate metals. These systems are sometimes

called “wetlands” or “bioreactors.” Passive treatment systems differ from active

systems (water treatment plants), which commonly use power; use more 

hazardous chemicals such as hydrated lime, caustic soda, or ammonia; and are

more expensive. Passive treatment systems are preferred for sites managed by

the Bureau of Land Management (BLM). This Technical Note is intended to

help BLM field personnel objectively evaluate the potential of passive treatment

systems for treating flowing adits with acid mine drainage. It provides a brief

overview of acid mine drainage and describes the types of passive systems

available, type of site characterization needed to evaluate the use of passive

systems, criteria for selecting a passive system, general design parameters, and

expectations for performance.

Chemistry of Acid

Mine Drainage

THE PENNSYLVANIA Department of Environmental Protection (1999)

provides a comprehensive description of basic acid mine drainage chemistry,

which is reprinted here with permission:

There are four commonly accepted chemical reactions that represent the chemistry

of pyrite weathering to form AMD. An overall summary reaction is as follows:

4 FeS2 + 15 O2 + 14 H2O 4 Fe(OH)3 + 8 H2SO4 [1]

Pyrite + Oxygen + Water “Yellowboy” + Sulfuric Acid
4

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

The first reaction in the weathering of pyrite includes the oxidation of pyrite by

oxygen. Sulfur is oxidized to sulfate and ferrous iron is released [equation 2]. This

reaction generates two moles of acidity for each mole of pyrite oxidized.

2 FeS2 + 7 O2 + 2 H2O 2 Fe2+ + 4 SO4

2- + 4 H+ [2]

Pyrite + Oxygen + Water Ferrous Iron + Sulfate + Acidity

The second reaction involves the conversion of ferrous iron to ferric iron 

[equation 3]. The conversion of ferrous iron to ferric iron consumes one mole of

acidity. Certain bacteria increase the rate of oxidation from ferrous to ferric iron.

This reaction rate is pH dependent, with the reaction proceeding slowly under

acidic conditions (pH 2-3) with no bacteria present, and several orders of

magnitude faster at pH values near 5. This reaction is referred to as the “rate

determining step” in the overall acid-generating sequence.

4 Fe2+ + O2 + 4 H+ 4 Fe3+ + 2 H2O [3]

Ferrous Iron + Oxygen + Acidity Ferric Iron + Water

The third reaction that may occur is the hydrolysis of iron. Hydrolysis is a reaction

that splits the water molecule [equation 4]. Three moles of acidity are generated

as a byproduct. Many metals are capable of undergoing hydrolysis. The formation

of ferric hydroxide precipitate (solid) is pH dependent. Solids form if the pH is

above about 3.5, but below pH 3.5, little or no solids will precipitate. [Other

metals such as aluminum, manganese, and trace metals such as arsenic, cadmium,

copper, lead, and zinc will also coprecipitate with iron, to some degree.]

4 Fe3+ + 12 H2O 4 Fe(OH)3 + 12 H+ [4]

Ferric Iron + Water Ferric Hydroxide (yellowboy) + Acidity

The fourth reaction is the oxidation of additional pyrite [or other metals] by

ferric iron [equation 5]. The ferric iron is generated in reaction steps 1 and 2.

This is the cyclic and self-propagating part of the overall reaction and takes place

very rapidly and continues until either ferric iron or pyrite [and other metals] is

depleted. Note that in this reaction, iron is the oxidizing agent, not oxygen.

FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO4

2- + 16 H+ [5]

Pyrite + Ferric Iron + Water Ferrous Iron + Sulfate + Acidity
5

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Types of Passive

Treatment Systems

PASSIVE TREATMENT SYSTEMS provide a controlled environment in

which natural chemical and biological reactions that help in the treatment of

acid mine drainage can occur (Pennsylvania Department of Environmental

Protection 1999). There are several types of passive treatment systems; some of

the more common ones are shown in Figure 1. Each type may be used on its

own, or more than one may be used in sequence to optimize treatment of

difficult effluents. However, the design selected will ultimately depend upon

site characteristics and other specific criteria.

FIGURE 1. Schematic diagram of common passive treatment systems used to treat acid mine

drainage (adapted with permission from Skousen 2001).

A. Aerobic Wetlands

1-3 in. water

1-3 ft. organic matter

B. Anaerobic Wetlands

1-3 in. water

1-2 ft. organic matter

.5-1 ft. limestone

C . Successive Alkalinity Producing 

 Systems (SAPS)

3-6 ft. water

6-12 in. organic matter

1-2 ft. limestone drainage system

D. Anoxic Limestone Drains (ALD)

2-4 ft. soil

20-40 mil plastic liner surrounding or 

covering limestone trench or bed of 

limestone

E. Limestone Pond

3-6 ft. water

1-3 ft. limestone

F. Open Limestone Channel (OLC)

Small or large sized limestone placed 

along sides and in bottom of culverts, 

diversions, ditches, or stream channels
6

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Aerobic wetlands are shallow (1- to 3-foot deep) ponds; they may be lined or

unlined and some are nearly filled with soil or limestone gravel. They facilitate

natural oxidation of the metals and precipitate iron, manganese, and other

metals (equation 4). Anaerobic wetlands are used to neutralize acidity and

reduce metals to the sulfide form (the reverse of equation 2). This reaction

consumes H+ and therefore acidity. Anaerobic wetlands may be lined or

unlined shallow ponds filled with organic matter, such as compost, and underlain by limestone gravel. Water percolates through the compost and becomes

anaerobic where equation 2 is reversed and metals precipitate as sulfides.

Microorganisms facilitate this reaction by first consuming oxygen. Alkalinity

and H2S are produced. If the system is improperly sized, if flow dries up, or

if extended low temperatures are encountered, the microorganisms will die

and the performance will be decreased. Some anaerobic wetlands discharge a

sulfide “sewage” effluent, particularly during the first few years. 

Anoxic limestone drains consist of a buried limestone gravel system that

requires the exclusion of oxygen and aluminum in the water. If oxygen or

aluminum are present, iron and aluminum hydroxides clog the system, causing

failure. Alkalinity producing systems are a combination of an anaerobic wetland and an anoxic limestone drain. Other types of passive treatment systems

include various limestone treatment configurations, ranging from limestone

ponds to open limestone channels in which water flows down a steep slope

with limestone riprap. These systems oxidize and precipitate metals (equation 4)

and add alkalinity to the water.

Another type of passive treatment system uses lime dispensing technology to

neutralize acidity and precipitate metals in a settling pond. These units do not

require power or hazardous chemicals and are inexpensive. BLM is currently

conducting pilot tests on the Aquafix technology.

Passive treatment systems are a valuable option for treating acid mine drainage

at remote BLM locations. The advantages of passive treatment systems are that

they:

• do not require electrical power

• do not require any mechanical equipment, hazardous chemicals, or buildings

• do not require daily operation and maintenance

• are more natural and aesthetic in their appearance and may support plants

and wildlife

• are less expensive
7

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Of course, there are disadvantages with any water treatment system. The

disadvantages of passive treatment systems are that they: 

• may require complex discharge permits unless taking a Comprehensive

Environmental Response, Compensation, and Liability Act (CERCLA) action

• may not meet stringent water-quality-based effluent standards

• may fail because of poor design or severe winter conditions

• are a relatively new technology and an area of active research; as such, there

have been failures along with success stories

All of the passive treatment systems described will accumulate metal precipitates

and will eventually have to be replaced. Skousen (2001) and Hedin et al.

(1994) indicate that these systems can be expected to perform for 20 years.

The precipitate is not normally a hazardous waste. Nonetheless, regular

monitoring, inspection, and maintenance are required, although to a much

lesser extent than with active water treatment systems.

Site Characterization

SITE CHARACTERIZATION is exceedingly important because it is used to

design passive treatment systems. Acid mine drainage is often characterized by

acidic pH and orange-yellow precipitates (ferric hydroxides or “yellowboy”).

The pH ranges from circumneutral to 2.5 and, uncommonly, even lower. It is

not enough to perform routine metals analyses and measure pH to adequately

characterize an acid mine drainage flow for the purpose of evaluating treatment

systems. In addition to dissolved metals (including aluminum and manganese)

and pH, it is necessary to measure dissolved oxygen, major cations, anions

(especially sulfate), and iron couples (Fe+2 and Fe+3) at the mine opening. In

addition, alkalinity and acidity need to be measured. Acidity is contributed

not only by pH, but also by iron, manganese, and aluminum. Alkalinity and

acidity are both measured in mg/L calcium carbonate equivalence. The crucial

determination is net alkalinity. Net alkalinity is total alkalinity minus acidity.

In the worst acid mine drainage sites, net alkalinity may be negative (or acidity)

if acidity exceeds alkalinity. 

In addition to these chemistry parameters, it is crucial to measure flow rates

and get a topographic survey of the site. Flows can be measured by simple
8

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

means, such as bucket and stopwatch, or more complicated methods, such as

sharp-crested weirs, flumes, or current meters. It may be necessary to consult

with a hydrologist to ensure accurate flow rates. Chemistry and flows should

be measured seasonally for at least a year to understand the conditions necessary to select a treatment system. If tailings and rock dumps are present, they

may require additional characterization (Ford 1999).

Selection Criteria

THE CHEMISTRY AND FLOW of the acid mine drainage discharge are

critical to the proper selection of a system. Figures 2 and 3 show a selection

scheme. Generally, net alkaline water is needed for an aerobic wetland. If the

water is net acid but has low dissolved oxygen, ferric iron, and aluminum, an

anoxic limestone drain may be selected. Higher concentrations of dissolved

oxygen, ferric iron, and aluminum are more suited for anaerobic wetlands,

successive alkaline producing systems (SAPS), or limestone channels. Anaerobic

wetlands are normally six times larger (and more costly) than aerobic systems.

FIGURE 2. Passive treatment system selection.

1 Dissolved Oxygen (mg/L)

Anoxic

Limestone

Drains

Anaerobic

Wetland or

SAPS

Open

Limestone

Channels

Anoxic

Limestone

Drains

Aerobic

Wetland

Settling

Ponds

Fe+3, Al+3

Flow (GPM)

Net Alkaline, mg/L Net Acid, mg/L

5

50

1 50

0
PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

9

FIGURE 3. Flowchart for selecting a passive acid mine drainage treatment system based on

water chemistry and flow (adapted from Hedin et al. 1994 and reprinted with permission

from Skousen 2001.)

Conceptual Design

CONCEPTUAL DESIGN is used to evaluate the approximate size and cost

of passive treatment systems. Passive treatment systems are often designed

based on loadings of metal or acidity in grams per square meter per day

Determine O2 content

Ferrous/ferric iron ratio

ANOXIC

LIMESTONE

DRAIN

AEROBIC or

ANAEROBIC

WETLAND or

SAPS

Strip DO,

precipitate Fe +3

Discharge Chemical treatment, recirculate Yes No

Low flow

(< 200 L/min)

50 gpm

SETTLING

POND

Net Acid Water

DO 2-5 mg/L

Fe+3 10-25%

DO < 2 mg/L

Fe+3 <10%

AI+3 < 25 mg/L

DO > 5 mg/L

Fe+3 >25%

High flow

(> 200 L/min)

AEROBIC

WETLAND

SETTLING

POND

Net Alkaline

Water

Meet effluent

limits?

Determine flow rate

Analyze water chemistry

Calculate loadings

OPEN

LIMESTONE

CHANNEL

SETTLING

POND

ANAEROBIC

WETLAND or

SAPS

pH > 4.5

Aerate

pH < 4.5

Net

alkaline

water

Net

acid

water
10

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

(g/m2/d). Grams/day is computed by multiplying the concentration in mg/L

times the flow in gallons per minute times 5.47 (1 mg/L equals 5.47 g/day):

mg/L * 3.8 L/gal * flow gal/min * g/1000 mg * 60 min/hr * 24 hr/day = 5.47 g/day [6]

Aerobic wetlands are sized for 10 g of Fe/m2/d and 0.5 g Mn/m2/d to achieve

3 mg/L iron and 2 mg/L manganese. For example, if the Fe loading was 260

grams/day, dividing by 10 g/m2

/day results in a wetland size of 26 m2. Lower

effluent concentrations can be sized proportionately. In the example, doubling

the wetland size to 52 m2 should reduce iron and manganese concentrations

by half. Manganese is the limiting contaminant, as it requires 20 times more

area per mg/L. Anaerobic wetland area is designed using an acidity loading

factor of 3.5–7 g/m2

/d (Hedin et al. 1994; Pennsylvania Department of

Environmental Protection 1999). Anoxic limestone drain sizing is more complicated; additional details can be found in Pennsylvania Department of

Environmental Protection’s 1999 report. Open limestone channels are sized

using 0.03–0.19 mg/L acidity per meter of channel length (Skousen 2001)

with a slope of greater than 20 percent for optimum performance.

Gusek (1995) reports that the capital cost of passive wetlands ranged from

$0.32–$0.46 per kg of metal removed, while for lime precipitation (an active

system), capital cost ranged from $0.66–$0.89 per kg. (Note: these costs may

not fully account for the added expense of remote locations typical of BLM

sites). However, operation and maintenance costs ranged from 5–10 times

higher for active systems.

Effluent Regulations

and Expectations

THE ENVIRONMENTAL PROTECTION AGENCY (EPA) or the State

may require a National Pollutant Discharge Elimination System (NPDES)

permit for effluent from a passive treatment system entering waters of the

State. The actual permit is not required if BLM is taking a CERCLA

action; however, compliance with the State or EPA substantive requirements 

(e.g., effluent limitations) is required. The regulator has an option of requiring

either a technology based effluent limitation or a water quality standards based
11

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

effluent limitation. Water quality standards based limitations are extremely

stringent and most passive treatment systems will not achieve them, as they

are equal to instream water quality standards for the protection of aquatic life

and may require low parts per billion in metals concentration. (These standards

are usually hardness based, so calcium and magnesium levels will have to be

analyzed). Some States may allow mixing zones for dilution, thus relaxing the

published standards. 

However, technology-based standards may be achievable with a properly

designed passive treatment system. In addition, passive treatment systems are

allowed as a best practical technology (BPT) under the Clean Water Act. They

were developed from the need to treat acid mine drainage from coal mines in

States such as Pennsylvania and West Virginia where acid mine drainage is a

severe problem. EPA has published BPT effluent limitations for treatment of

wastewaters from coal mines in 40 CFR 440.30:

Total Metal Concentrations

Parameter (30-day average)

Iron 3.5 mg/L

Manganese 2.0 mg/L

pH 6–9 pH units

EPA has also published BPT effluent limitations for copper, lead, zinc, gold,

and silver acid mine drainage sites in 40 CFR 440.102:

Total Metal Concentrations

Parameter (30-day average)

Copper 0.15 mg/L

Zinc 0.75 mg/L

Lead 0.3 mg/L

Mercury 0.001 mg/L

pH 6–9 pH units

Note: the regulations for flotation mills stipulate a cadmium limitation of 0.05 mg/L.

Several lessons have been learned from passive treatment systems constructed

in the West with respect to performance expectations. Wetland systems relying

on sulfate reduction are biologically based. Cold winters may kill the organisms
12

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

and reduce the effectiveness of the system. Anaerobic wetlands using compost

or manure materials may generate odors and sewagelike effluent that creates

another water quality problem.

Conclusion

IN SUMMARY, passive treatment systems have application in remote BLM

sites where some treatment is necessary, but where a low-maintenance, costeffective treatment system is desired. Since passive treatment systems are a

relatively new technology, it is important to get an experienced designer to

design the system. The source chemistry and flow over time, as well as the

regulatory environment must be understood in order to make an informed

decision when selecting a passive treatment system. Certain conditions, such

as severe winters or dried-up flow, may affect the performance of certain 

passive treatment systems, especially anaerobic wetlands and successive alkaline

producing systems where sulfate-reducing bacteria may die off. Sites with

extremely low pH (<3) may not be fully amenable to limestone neutralization,

and a more caustic reagent, such as caustic soda (NaOH), may be required.

Since passive treatment systems are a relatively new technology, it may be

desirable to construct a pilot treatment plant first to evaluate the performance

(effluent quality) of the system. Finally, monitoring, inspection, and 

maintenance are still required for passive treatment systems and should not be

overlooked.
13

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

Cited References

Ford, K.L. 1999. Site characterization for abandoned mine/mill sites.

Information Bulletin No. RS-99-108. National Applied Resource Sciences

Center, Denver, CO. 

Gusek, J.J. 1995. Passive treatment of acid rock drainage: what is the bottom

line? Mining Engineering. Vol. 47, No. 3. pp. 250–253.

Hedin, R.S., R.W. Nairn, and R.L. Kleinmann. 1994. Passive treatment of

coal mine drainage. U.S. Bureau of Mines. Information Circular 9389.

Pennsylvania Department of Environmental Protection. 1999. The science of

acid mine drainage and passive treatment. Publication 5400-BKDEP2365. Harrisburg, PA.

Skousen, J. 2001. Overview of passive systems for treating acid mine drainage.

West Virginia University Extension Service.
14

PASSIVE TREATMENT SYSTEMS FOR ACID MINE DRAINAGE

The mention of trade names or commercial products does not constitute endorsement or 

recommendation for use by the Federal Government.