[ Main ] Educator's Guide
Outreach Initiatives
[ Site Map ]
Geology | Lehigh River Watershed Explorations | Weather | Environmental Issues | Data Collection Activities
Why do some passive treatment systems fail?

James J. Gusek, P.E.
Knight Piésold and Co.
1050 17th Street, Suite 500
Denver, CO 80265-0500

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


Curricular Activities | Lehigh River Photojournal | Water Quality | GIS | History | River Exploration
LEO EnviroSci Inquiry is brought to you by the Lehigh Environmental Initiative at Lehigh University.
Copyright ©2000-2011 Lehigh Environmental Initiative at Lehigh University. All rights reserved.