Potential use of constructed wetlands for treatment of industrial wastewaters containing metals

Natural wetlands are effective sinks for metals. Processes of metal removal and mobilisation include sedimentation, adsorption, complexation, uptake by plants, and microbially-mediated reactions including oxidation and reduction. Recently, wetlands for wastewater purification have been constructed for treatment of sewage and urban runoff, and also show good potential for concentrating metals from industrial wastewaters and mine seepage. Constructed wetlands include open water pond-like systems, usually sediment-lined, containing floating, submerged and emergent plants. Another design is based on the use of permeable substrata such as gravel, commonly planted with emergent wetland plants such as Typha, Schoenoplectus, Phragmites or Cyperus. The wastewater percolates through the gravel, giving better access to the plant roots and rhizomes and exposure to oxygenated conditions in the rhizosphere. Hydraulic design appears to be critical to the performance of these systems, with a vertical upflow format being preferable. Generally, emergent plants influence metal storage indirectly, by modifying the substratum through oxygenation, buffering pH and adding organic matter. Total plant uptake is generally < 50% of influent metal load, while the proportion translocated to aboveground parts is usually only a few percent of input loading so that harvesting is impractical for metal removal. High metal-removal rates, close to 100%, have been reported in experimental systems for sewage and metal-amended nutrient solutions simulating acid mine drainage water, but only on the small scale or short-term. More information is required on processes of operation and recycling so that the system design can be optimised and more confidence given to predicting long-term performance.