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Surface water has been used by mankind as the main source of water for domestic agriculture and industrial use in numerous countries around the world. However, water quality can be affected by mining activities which influences the natural environment, human health and aquatic ecosystem. The major pollutants in acid mine drainage are high amount of dissolved sulphate and heavy/toxic metals. Constructed wetlands are promising in situ water treatment methods which helps in the remediation process by stimulating microbial growth within the rhizospheric zone of the plants. The sulphate in constructed wetlands are removed by dissimilatory sulphate reduction in strict anaerobic conditions which helps in the removal of metals as metal sulphides.
The objective of this research is to investigate sulphate and metal removal pathways in constructed wetlands treating acid mine drainage. Therefore, several approaches were applied: (a) various types of model wetlands were used to investigate different removal processes (b) intensification of treatment processes with stimulation of the dissimilatory sulfate reduction in an autotrophic way by the use of hydrogen gas.
The application of hydrogen gas (as external electron donor) combined with common physico-chemical and biological parameters helped us to intensify the sulphate reduction process and removal of metals in constructed wetlands.
In the model wetland systems, the removal efficiencies varied with the types of constructed wetlands. The removal efficiencies of sulphate and metals were high in planted constructed wetlands as compared to unplanted constructed wetland. The combination of gravel bed and plants (Phragmites australis) resulted in the highest removal rate in horizontal subsurface flow constructed wetland. The metals mostly precipitated to the bottom of the constructed wetland as metal sulphides.
Though under carbon limited and anoxic conditions, a high mean removal efficiency of sulphate and heavy metals was observed in the experiments with HSSF CW so it is important to take into consideration the importance of electron donor. With hydrogen gas as electron donor the performance efficiency was increased by almost 12% for sulphate removal and almost 6% increase in aluminium removal efficiency.
In conclusion, this research exhibit that the combination of physico-chemical measurements along with hydrogen gas as an electron donor is an efficient tool for investigation of sulphate and metal removal processes in constructed wetlands. Such information is not only beneficial for understanding the processes taking place in these wastewater treatment facilities but also necessary for future technological improvement of constructed wetlands.