Lesson Plan 10
PASSIVE ACID MINE DRAINAGE
Lesson Plan Created By:
Todd Mills (Grades
Passive Treatment of AMD
As early as 1978, many
variations of AMD passive treatment systems were studied by numerous
organizations on the laboratory bench-testing level. During the last 15
years, passive treatment systems have been implemented on full-scale sites
throughout the United States with promising results. The concept behind
passive treatment is to allow the naturally occurring chemical and
biological reactions that aid in AMD treatment to occur in the controlled
environment of the treatment system, and not in the receiving water body.
conceptually offers many advantages over conventional active treatment
systems. The use of chemical addition and energy consuming treatment
processes are virtually eliminated with passive treatment systems. Also,
the operation and maintenance requirements of passive systems are
considerably less than active treatment systems.
The first passive
technology involved the use of natural Sphagnum
wetlands that could improve the water quality of AMD without causing other
detrimental impacts on the ecosystem. Although this concept had its
limitations, it spawned research and development into other passive
treatment technologies that did not follow the natural wetland paradigm.
Designing a passive
treatment system for AMD requires the understanding of mine water
chemistry, available treatment techniques and experience. Analytical
sampling of the AMD is extremely important in the selection of appropriate
AMD TREATMENT TECHNOLOGIES
An aerobic wetland consists of a
large surface area pond with horizontal surface flow. The pond may be
planted with cattails and other wetland species. Aerobic wetlands can only
effectively treat water that is net alkaline. In aerobic wetland systems,
metals are precipitated through oxidation reactions to form oxides and
hydroxides. This process is more efficient when the influent pH is greater
than 5.5. Aeration prior to the wetland, via riffles and falls, increases
the efficiency of the oxidation process and therefore the precipitation
process. Iron concentrations are efficiently reduced in this system but
the pH is further lowered by the oxidation reactions.
A typical aerobic wetland
will have a water depth of 6 to 18 inches. Variations in water depth
within the wetland cell may be beneficial for performance and longevity.
Although shallow water zones freeze more quickly in winter, they enhance
oxygenation and oxidizing reactions and precipitation. Deeper water zones
provide storage areas for precipitates but decrease vegetative diversity.
Aerobic wetlands are
sized based on the criteria developed by the now defunct U.S. Bureau of
Mines for abandoned mined lands (AML) and compliance. AML criteria for
aerobic wetland sizing is as follows:
Minimum wetland size (ac) =
(lb/day) ¸ 180 (lb/ac/day)] +
(lb/day) ¸ 9 (lb/ac/day)] +
¸ 60 (lb/day/acre)]
calculate loading rates (lb/day), take the flow rate (gpm) x concentration
(mg/l) x 0.012.
The factor of 0.012
converts gallons per minute and milligrams per liter to pounds per day as
(gal/min)(mg/l)(3.8 l /gal)(g/1000 mg)(lb/454 g)(60 min/hr)(24 hours/day)
Compliance criteria are
suggested for wetlands that have to meet a specific National Pollution
Discharge Elimination System (NPDES) effluent limitation. Compliance
criteria are more conservative than AML criteria and results in wetlands
that are approximately twice as large.
or Anaerobic Wetland
Compost wetlands, or anaerobic
wetlands as they are sometimes called, consist of a large pond with a
lower layer of organic substrate. The flow is horizontal within the
substrate layer of the basin. Piling the compost a little higher than the
free water surface can encourage the flow within the substrate. Typically,
the compost layer is made from spent mushroom compost that contains about
10 percent calcium carbonate. Other compost materials include peat moss,
wood chips, sawdust or hay. A typical compost wetland will have 12 to 24
inches of organic substrate and be planted with cattails or other emergent
vegetation. The vegetation helps stabilize the substrate and provides
additional organic materials to perpetuate the sulfate reduction
Anaerobic wetlands are
used to treat AMD from active mine discharges to meet established effluent
requirements. Generally, the design of these wetlands is conservative and
can treat discharges that contain dissolved oxygen, Fe3+, Al3+
or acidity less than 300 mg/l. When treating discharges from
abandoned mines the goal is to reduce the pollution to levels that will
restore the receiving stream. In these cases, wetlands can accept
discharges with an acidity in the 500 mg/l range.
The compost wetland acts
as a reducing wetland where the organic substrate promotes chemical and
microbial processes that generate alkalinity and increase the pH. The
compost removes any oxygen in the system. This allows sulfate to be
reduced and also keeps the metals from oxidizing and armoring or coating
the limestone present in the compost, thereby preventing its dissolution.
Microbial respiration within the organic substrate reduces sulfates to
water and hydrogen sulfide. The anoxic environment within the substrate
also increases the dissolution of limestone.
Anaerobic wetlands are
sized according to U.S. Bureau of Mines criteria for AML sites as follows:
wetland size (m2) = acidity loading (g/day) ¸ 0.7
Open limestone channels may be the
simplest passive treatment method. Open limestone channels are constructed
in two ways. In the first method, a drainage ditch is constructed of
limestone and AMD-contaminated water is collected by the ditch. The other
method consists of placing limestone fragments directly in a contaminated
stream. Dissolution of the limestone adds alkalinity to the water and
raises the pH. Armoring or the coating of the limestone by Fe(CO)3
and Fe(OH)3 produced by neutralization reduces the generation
of alkalinity, so large quantities of limestone are needed to ensure
long-term success. High flow velocity and turbulence enhance the
performance by keeping precipitates in suspension thereby reducing the
armoring of the limestone. Open limestone channels are sized according to
standard engineering practice using the Manning equation and providing
additional freeboard. Impervious liners are sometimes used under the
limestone to prevent infiltration of the AMD into the groundwater table.
Diversion wells are another simple
way of adding alkalinity to contaminated waters. Acidic water is conveyed
by a pipe to a downstream "well" which contains crushed
limestone aggregate. The hydraulic force of the pipe flow causes the
limestone to turbulently mix and abrade into fine particles and prevent
armoring. The water flows upward and overflows the "well" where
it is diverted back into the stream. Diversion wells require frequent
refilling with clean limestone to assure continued treatment.
Limestone Drains (ALD)
An anoxic limestone drain (ALD) is
a buried bed of limestone constructed to intercept subsurface mine water
flows and prevent contact with atmospheric oxygen. Keeping oxygen out of
the water prevents oxidation of metals and armoring of the limestone. The
process of limestone dissolution generates alkalinity. The sole purpose of
an ALD is to provide alkalinity thereby changing net acid water into net
alkaline water. Retaining carbon dioxide in the drain can improve
limestone dissolution and alkalinity generation.
An ALD can be considered
a pretreatment step to increase alkalinity and raise pH before the water
enters a constructed aerobic wetland. In the aerobic wetland, metals can
be oxidized and precipitated. ALDs are limited to the amount of alkalinity
they can generate based on solubility equilibrium reactions. Also, the
effectiveness and longevity of an ALD can be substantially reduced if the
AMD has high concentrations of ferric iron, dissolved oxygen or aluminum.
ALDs are sized based on
the assumption that the drain will produce water between 275 and 300 mg/l
of alkalinity. The amount of alkalinity generated is based on the
solubility of the calcite within the limestone and the retention time
within the ALD. Retention times of 14 to 15 hours are used as standard
practice to balance construction costs and the efficiency of alkalinity
generation. The overall equation to calculate the mass of limestone
necessary for an ALD is as follows:
= (Q r b td / Vv) + (Q C T / x)
M = mass of
limestone in tons
Q = flow rate of
AMD in cubic meters per day
r b = bulk density of the limestone in tons per cubic meter
retention time in days, 0.625 days is standard practice
bulk void ratio expressed as a decimal
C = effluent
alkalinity concentration in tons per cubic meter
T = design life
of the drain in days, typically 9,125days (25 years)
x = CaCO3 content of the limestone as a decimal
Vertical Flow Reactors (VFR)
Vertical flow reactors (VFR)
were conceived as a way to overcome the alkalinity producing limitations
of ALD’s and the large area requirements for compost wetlands. The VFR
consists of a treatment cell with an underdrained limestone base topped
with a layer of organic substrate and standing water. The water flows
vertically through the compost and limestone and is collected and
discharged through a system of pipes. The VFR increases alkalinity by
limestone dissolution and bacterial sulfate reduction. Highly acidic
waters can be treated by running the AMD through a series of VFRs. A
settling pond and an aerobic wetland where metals are oxidized and
precipitated typically follow a VFR plan.
This patented process utilizes site-specific laboratory cultured
microbes to remove iron, manganese and aluminum from AMD. The treatment
process consists of a shallow bed of limestone aggregate inundated with
AMD. After laboratory testing determines the proper combinations, the
microorganisms are introduced to the limestone bed by inoculation ports
located throughout the bed. The microorganisms grow on the surface of the
limestone chips and oxidize the metal contaminants while etching away
limestone, which in turn increases the alkalinity and raises the pH of the
water. This process has been used on several sites in western Pennsylvania
with promising results.
students will be able to work in cooperative learning groups to study
the various methods of Passive Acid Mine Drainage Treatment.
students will be able to read and understand topographic maps.
students will be able to construct a model that applies to a given (by
teacher) AMD problem.
information to the students.
lecture and discussion, make students aware of Acid Mine Chemistry and
Drainage and research conducted by the DOE in this area.
students knowledgeable on the reading of Topographic Maps.
|Introduce and discuss
science and environmental vocabulary.
students with a glossary of terms.
the terms most relevant to the current lesson.
teacher will break the students into groups of 3 or 4 and provide the
students with a topographic map
which shows a stream. Topographic
maps are easily available via Internet
(see Internet Resources). The students will be told that the stream has AMD. Each group
will have a different AMD chemistry and flow rate.
From this information, the
students will design a Passive Treatment System that will be
appropriate for their problem stream.
The students’ design should address type and size
of Passive AMD treatment technology and how well it is suited for the
students will present their design/model to the class.
Each group will have a different
model due to a different situation.
They will explain the type of treatment, size
of treatment and how they addressed their own problem stream.
SAMPLE WATER FLOWS
A) Alkaline water, rapid flow rate
B) Acidic water; 5% Fe; 21 mg/L Al; DO 1mg/L
C) Acidic water; 33% Fe; DO
16 mg/L High Flow
D) Acidic water: 13% Fe; DO
Topographic Maps can be
obtained from The PA Geological Survey.
The BACKGROUND section is
directly from the internet site.
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