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How are metals removed in passive treatment systems?


Many physical, chemical, and biological mechanisms are known to occur within passive treatment systems to reduce the metal concentrations and neutralize the acidity of the incoming flow streams. Notable mechanisms include the following:

  • Sulfide and carbonate precipitation catalyzed by sulfate reducing bacteria (SRB) in anaerobic zones.
  • Hydroxide and oxide precipitation catalyzed by bacteria in aerobic zones.
  • Filtering of suspended material.
  • Metal uptake into live roots and leaves.
  • Adsorption and exchange with plant, soil and other biological materials. Remarkably, some studies have shown that plant uptake does not contribute significantly to water quality improvements in passive treatment systems (Wildeman, et al., 1993). However, plants can replenish systems with organic material and add aesthetic appeal. In aerobic systems, plant assisted reactions appear to aid overall metal-removal performance, perhaps by increasing oxygen and hydroxide concentrations in the surrounding water through photosynthesis related reactions and respiration in the plant root zone. Plants also appear to provide attachment sites for oxidizing bacteria/algae. Research has shown that microbial processes are a dominant removal mechanism in passive treatment systems (Wildeman, et al., 1993). One anonymous researcher considered a passive treatment system as a "bioreactor with a green toupee," referring to the substrate where most of the bioreactions occur and the collection of plants that grow on top of the treatment cells.

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