James J. Gusek, P.E.
Knight Piésold and Co.
1050 17th Street, Suite 500
Denver, CO 80265-0500
Abstract
There are hundreds of passive treatment systems accepting mining influenced
water (MIW) throughout the world. Some systems do not perform to design
expectations while others, including volunteer systems, have successfully
operated relatively unattended for decades. The primary reasons for
this situation include the common misconceptions that 1) a "cookbook"
approach to design is valid for a wide array of MIW chemistries and
site conditions and 2) low-maintenance means "no maintenance."
Passive treatment systems for MIW are typically man-made ecosystems
that are designed to handle a specific range of metal loading conditions
and MIW geochemistry. Thus, when design conditions are exceeded, the
suite of microbial to macroscopic ecosystems may be slow to recover
or mature. This should be no surprise to designers. But when a particular
system fails, it may be inappropriately attributed to the technology,
not the design. This paper presents a standard "phased" design
protocol that appears to work and provides examples of sub-par performance
of selected passive treatment systems.
Introduction and Background
Natural systems have been removing metals from water for eons; examples
include pyrite fixed into coal beds and bog iron ore deposits. For the
past dozen years, wetlands and bogs have been the natural method of
choice of engineers at many abandoned mine land sites for improving
water quality. Contaminant reductions are being seen through the precipitation
of hydroxides, sul-fides, and carbonates, and pH adjustments. Local
conditions, oxidation state, and water and soil chemistries dictate
whether such natural reac-tions occur under oxidiz-ing (aerobic) or
reducing (anaero-bic) conditions. Man-made or constructed wetlands/bioreactors
employ the same principles as natural wetlands but are designed to optimize
processes occurring in natural wetland ecosystems. The key goal of bioreactors/wet-lands
is the long-term immobilization of metals in the substrate materials.
Metals are precipitated as carbonates or sulfides in the "bioreactor"
substrate (anaerobic cells) and as oxides and hydroxides in aerobic
(rock filter) cells.
Anaerobic bioreactors have been successful at substantially reducing
metal concentrations and favorably adjust-ing pH on metal mine drainages.
It is generally recognized that the bacteria commonly found in cattle
and other domestic animal intestinal tracts include sulfate reducers
and a consortium of other bacteria. Hence, manure from cows or other
animals has been frequently used as bacterial inoculum for anaerobic
biotreatment cells. These same bacteria are found in many natural wetlands
and bogs and in lakes and ocean water. Aerobic biotreatment systems
are similar to "natural" wetlands in that they typically have
shallow depths and support vegetation in the form of algae and emergent
plant species.
Since 1988, there have been rapid advancements in understanding the
functioning of wetland/ bioreactor systems. The first large-scale aerobic
system (7.6 m3 per min. or 2,000 gpm capacity) was built in 1992 by
TVA in Alabama (Brodie, et al., 1993). The West Fork Unit system (4.5
m3 per min. or 1,200 gpm capacity) was constructed in Missouri in 1996
and is the first large-scale anaerobic biotreatment system (Gusek, et
al., 1998 and Gusek, 2000). At West Fork, an aerobic "rock filter"
cell provides polishing treatment for manganese and other parameters.
The passive treatment technique holds promise over typical chemical
treatment methods because large volumes of treatment residuals are not
generated; in fact, residual disposal may be delayed for decades or
longer. One volunteer passive treatment system outside an abandoned
metal mine has been identified in Ireland that has operated unattended
since about 1889, over a century (Beining and Otte, 1997). This volunteer
passive treatment system reportedly has 70% of its total metal removal
capacity remaining.
Return
to AMD Stakeholders' page
