GROUNDWATER AND LEACHATE: EX-SITU BIOLOGICAL TREATMENT
Constructed Wetlands
Typical Constructed Wetlands System
The constructed wetlands-based treatment uses natural geo-chemical and biological processes in an artificial (man-made)wetland ecosystem:
Process:
For highly contaminated metal waters (low pH):
Uses:
Effectively used to accumulate and remove metals, explosives, and other contaminants from influent waters.
Limitations:
- The lake consists of organic soil, microbial fauna, algae, vascular plants & micro-organisms.
- In flowing water passes through algae causing plant uptake, thus microbial oxidation & reduction reactions occur (cause of most of the remediation).
- Constructed wetlands can use a filtration or degradation process.
Process:
For highly contaminated metal waters (low pH):
- Influent waters flow through the aerobic and anaerobic zones of the wetland ecosystem.
- Metals are removed through ion exchange, adsorption, absorption and precipitation with geo-chemical and microbial oxidation and reduction.
- Ion exchange occurs as metals come into contact with the organic substances in the wetland.
- Wetlands constructed for this purpose often have little/no soil but instead have straw, manure or compost. Oxidation and reduction reactions are catalysed by bacteria within the aerobic and anaerobic zones - play a major role in precipitating metals as hydroxides and sulfides.
- Precipitated and adsorbed metals settle or are filtered out as water percolates through the medium/plants.
- In fluent waters flow through and beneath the gravel surface of a gravel-based wetland.
- The wetland (with plants) is a coupled anaerobic-aerobic system.
- The anaerobic cell uses plants with natural microbes to degrade the contaminant.
- The aerobic (reciprocating) cell further improves water quality through continued exposure to the plants and the movement of water between cell compartments.
Uses:
Effectively used to accumulate and remove metals, explosives, and other contaminants from influent waters.
- Most commonly been used in wastewater treatment for organic matter; nutrients (nitrogen and phosphorus), suspended sediments & for control of trace metals and other toxic materials.
- Has been used to treat acid mine drainage - typically acidic with high metal concentrations (from metal or coal mining activity)
- The process can be adapted to treat neutral and basic tailings solutions.
- The gravel-based system has also been effective at degrading TNT and RDX.
Limitations:
- Unsure of long-term effectiveness of constructed wetlands - may decrease in contaminant removal rates over time.
- Possibly very high cost to construct artificial wetland - varies considerably for each site. (Estimated costs for treatment over 1 decade are approximately : $1.36/Kgal)
- Temperature and fluctuations in flow affect wetland function - inconsistent contaminant removal rates.
- Colder conditions slow the rate of degradation.
- A heavy inflow of water can overload the removal mechanisms in a wetland
- A dry spell can damage plants - severely limiting wetland function.
- Wetland treatment is a long-term technology intended to operate continuously for years.