REVIEW
OF PASSIVE SYSTEMS FOR TREATMENT OF ACID MINE DRAINAGE
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
3.14.1
May 1996 Revised 1999
EXECUTIVE
SUMMARY
One of the
major issues facing the Canadian mining industry is the treatment
of effluent during and after closure of a mining property. Effluent
treatment may be complicated by the presence of acid mine drainage
(AMD) which even under the best reclamation scenario may require
long term collection and treatment. While chemical treatment or
some other form of active effluent treatment has traditionally been
conducted in Canada, greater consideration has been given recently
to forms of passive treatment.
Passive treatment
systems utilize the chemical, biological and physical removal processes
that often occur naturally in the environment to modify the influent
characteristics. Passive treatment systems were initially considered
attractive to treat acid mine drainage due to their lower costs
of construction, operation and maintenance, and their ability to
operate at remote locations with limited operational requirements.
The objective of this project was to review passive treatment systems,
and make recommendations on their applicability to treat acid mine
drainage in Canada.
Although passive
systems have been proven at many locations around the world, the
Canadian climate and aquatic environments present considerable challenges
to large-scale use in Canada. Biologically driven systems have low
activity in cold temperatures and drought, while storm and spring
"freshet" events demand flexible, strong systems.
In this review,
four major types of passive technologies for the treatment of acid
mine drainage have been examined:
- anoxic limestone
drains;
- constructed
wetlands;
- microbial
reactor systems; and,
- biosorption
systems.
In accordance
with the scope of work, this document provides:
- summary
of known passive treatment technologies;
- maintenance
and monitoring requirements;
- life expectancy
and long term implications (>100 years);
- implications
of treatment product disposal;
- estimate
of costs for the technologies described based on generic cases;
- ability
to meet Canadian Metal Mining Liquid Effluent Regulations, and
to control toxicity;
- descriptions
of case studies including: range of flow, temperature, water chemistry,
and identification of limiting conditions where available in the
literature (Appendix A); and,
- general
assessment of the applicability of current passive treatment systems
to Canadian mine sites.
ANOXIC LIMESTONE
DRAINS (ALD'S)
The basic design
of an ALD is a trench filled with high quality crushed limestone,
sealed under plastic and geotechnical fabric, covered by soil, through
which an unaerated, contaminated effluent stream flows by gravity.
As it flows through the system, the acid mine drainage gradually
dissolves the limestone, releasing calcium as bicarbonate, thus
raising the pH.
Based primarily
on studies conducted at coal mines in the United States, ALD's have
been shown to be most effective for influent with dissolved oxygen,
ferric iron (Fe3+) and aluminum concentrations of less
than 1 mg/L, and sulphate concentrations below 2,000 mg/L. At higher
concentrations the limestone may become armoured with oxides or
gypsum, reducing the rate of limestone dissolution or plugging the
system. In either instance, the ability of the ALD to generate alkalinity
may be significantly reduced, and failure of the system may occur.
As a result
of these strict influent requirements, ALD's are expected to have
only limited application to treatment of acid mine drainage at Canadian
metal mines.
CONSTRUCTED
WETLANDS
Constructed
wetlands are ecological systems designed to optimize a variety of
natural physical, chemical, microbial and plant-mediated processes.
In a constructed wetland, influent AMD drains by gravity through
the wetland, progressively undergoing metal removal and neutralization.
Metals are removed by precipitation, chelation and exchange reactions,
while neutralization is primarily achieved by the activity of sulphate
reducing bacteria (SRB), or the increase in alkalinity from the
chemical and microbial reactions including limestone dissolution.
Passive systems
for AMD treatment have commonly used combinations of natural or
constructed wetlands, Sphagnum peat and open ponds, supplemented
by chemical amendments (mostly limestone) and organic substrate
to increase alkalinity and reduce acidity. Sequential treatment
of AMD to remove iron by oxidation, hydrolysis and settling in the
aerobic stage, followed by SRB activity in an anaerobic stage to
raise pH, is an effective combination.
For either
aerobic or anaerobic cells, the design must maximize contact with
the matrix, which can either be aerated water, or anaerobic substrate.
It is essential that constructed wetlands are managed in terms of
their individual components and their mutual interactions to gain
a desired overall efficiency. Many of the metal removing mechanisms
in a wetland are temporary and reversible, and can reach saturation;
thereby reducing the wetland's efficiency and decreasing their cost
effectiveness. In addition, para-reversibility represents a challenge
for treatment product monitoring or disposal.
Constructed
wetlands have the potential to address AMD treatment at some Canadian
sites where sufficient surface area is available, and can form the
preferred alternative in terms of costs, efficiency and environmental
safety. The 'black box' design approach that has been used in the
past and is still being suggested, is not recommended. The design
should be based on an understanding of the interactions between
the chemical, microbial and plant-mediated components of the system
and the engineering, climate and hydrogeological realities of the
treatment site.
A well designed,
constructed wetland is an efficient accumulator of metals and reaction
products. Key to its efficiency, is the continuous physical, chemical
and biotic matrix in the wetland. This capacity will be limited
during freezing or high flow conditions. As a result, constructed
wetlands may be most applicable to Canadian mines having shorter
and milder winters, and at sites where a constant rate of flow can
be maintained. An alternative treatment method may be required during
the winter and during spring runoff conditions, or else large retention
ponds are required.
MICROBIAL REACTOR
SYSTEMS
Microbial reactor
systems or bioreactors may be in an open or closed configuration,
referring to whether they are exposed to the atmosphere. In either
instance, the microbial reactor shell contains a biodegradable substrate
(usually agricultural products such as mushroom compost or straw)
which supports the growth of micro-organisms, which in turn treat
acid mine drainage.
The cellulose
of the agricultural products is degraded by cellulolytic bacteria
to generate free sugars and other metabolites, which are further
metabolized to provide substrate for the fermentative anaerobes.
Under anaerobic conditions these free sugars are fermented to short
chain organic acids or short chain fatty acids, which are suitable
substrates to support the growth of sulphate reducing bacteria (SRB).
SRB reduce sulphate to hydrogen sulphide which precipitates metal
ions as low solubility metal sulphides. Concurrently, the sulphate
reducing bacteria consume hydrogen ions and produce carbon dioxide
during their metabolism, causing an increase in the pH of the solution
due to reduced concentration of free hydrogen ions and the buffering
effect of the CO2/bicarbonate buffer system.
Pilot plant
data available suggests that bioreactors are a feasible technology
for treatment of small AMD streams. Open bioreactors are expected
to only be applicable at Canadian mines with mild or moderate winters.
Closed bioreactors can operate anywhere that a fairly constant temperature
can be maintained.
BIOSORPTION
SYSTEMS
Micro-organisms,
including bacteria, algae, fungi and yeasts, can efficiently accumulate
heavy metals and radionuclides from their external environment.
Biosorption systems in a wide variety of configurations rely on
this ability to treat acidic drainage. Living cells can be used
to treat effluent where metal concentrations are below toxic levels.
The use of dead biomass in the form of commercial biosorbents eliminates
the problems of metal toxicity, adverse climatic conditions, and
the costs associated with nutrient supply and culture maintenance.
Only a limited
number of studies have been conducted to date in regards to treating
AMD with biosorption systems. While it does not appear that biosorption
systems are an effective stand-alone treatment system for AMD, with
further study they may become an alternative form of treatment of
parts of an effluent stream, or as a final polishing step. The success
of biosorption systems using living biomass during the winter is
expected to be limited. Treatment efficiency will be lowered under
poor growth conditions. Systems employing dead biomass are expected
to have greater applicability, and may not be compromised by winter
conditions as long as flow is maintained.
OVERVIEW
Passive treatment
of acid mine drainage has a future in Canada, but is limited to
applications where:
- flows are
of relatively constant volume
- water temperature
is greater than 7C (eg. mine water or embankment seepage)
- water chemistry
of low to medium strength acidity and metal concentration
- low concentrations
of aluminum and iron
- low sensitivity
of the receiving environment to upsets in the passive treatment
system.
Further research
and field experience is needed to more precisely specify passive
treatments for AMD in Canada to ensure that metal mining liquid
effluent regulations are met all the time. No "passive treatment"
system is truly passive. All systems require monitoring and replacement
of consumed alkalinity or organic-based nutrients for bacteria.
Also, metal precipitates need to be removed and in some jurisdictions
sludge falls under "hazardous waste" regulations and disposal may
be a significant challenge.
SUMMARY
OF PASSIVE TREATMENT TECHNOLOGIES FOR TREATMENT OF ACID MINE DRAINAGE
Technology
|
General
Comments
|
Applicability
/ Limitations
|
Costs
|
Anoxic
Limestone Drains
|
- low
cost form of passive alkalinity addition
- most
research relates to AMD from US coal mines
|
able
to operate year round as long as flow continues
strict
influent quality limits to ensure long-term alkalinity generation
expected
to have limited application to Canada
|
virtually
no maintenance or other operating costs
capital
costs in the range of $4,000 for a flow of 7.5 L/min and $22,500
for a flow of 125 L/min
|
Constructed
Wetlands
|
- designed
to optimize processes that occur in a natural wetland
- internal
cells focus on either aerobic or anaerobic processes
- must
be designed to ensure that the processes are optimized and
do not counteract each other
|
expected
to have the greatest application to mines with moderate winters
having secondary active treatment for the winter and spring
runoff periods
ability
to treat effluent during cold, harsh winters is unknown
|
annual
operating costs estimated at 10% to 20% of capital cost
generic
wetland to treat 60 L/min is estimated at $85,000
literature
suggests capital costs of US $5 to US $32/m2 are
typical
|
Microbial
Reactor Systems
|
- relies
on microbial reactions, supported by a biodegradable carbon
source
- only
short term studies have been completed to date, primarily
under very low flow conditions (<1 L/min)
|
limiting
factor is rate of biodegradation of carbon source
unknown
ability to treat moderate or high flows
may
be applicable to small effluent streams and could complement
other passive or active treatment systems
|
operating
are not significant; new substrate required semi-annually
capital
costs for an open reactor (50 to 60 L/min) are estimated at
near $33,500; a closed reactor system to treat 75 to 100 L/min,
$56,000
|
Biosorption
Systems
|
- metals
are removed from solution by adsorption/absorption to living
cells or non-living biomass
- few
field studies are available; most research is bench scale
only
|
may
have application as a secondary form of treatment when integrated
with another treatment system
success
of systems employing living cells during the winter is doubtful
|
insufficient
information is available to assess at a full-scale level
|
Français
| Contact Us
| Help | Search
| Canada Site
Home | What's
New | CANMET-MMSL
| MMS Site
| NRCan Site
|