The bioremediation of acid mine drainage utilising indigenous South African grass as the organic carbon source for dissimilatory sulfate reduction

Abstract

Acid Mine Drainage (AMD) is an acidic sulfate-rich wastewater that contains a high concentration of heavy metals. Due to historic and ongoing mining activities around the Witwatersrand, large quantities of AMD is generated in this area which seeps into surface and ground water. AMD has negative environmental, health and economic impacts and requires treatment. Passive treatment is a cost-effective option for the remediation of AMD and involves the process of dissimilatory sulfate reduction (DSR) using sulfate reducing bacteria (SRB) and an organic carbon food source. Indigenous South African grass was identified as an inexpensive and suitable organic carbon source for the bioremediation of AMD. The research presented in this thesis identified a one-stage leach bed reactor (LBR), packed with indigenous South African grass, as a suitable and cost-effective reactor design and configuration for the treatment of AMD. A laboratory-scale LBR was constructed and an experimental analysis determined that an optimal ratio of indigenous South African grass to a synthetic AMD leachate was 6 g of grass / L of synthetic AMD. At higher solid/liquid ratios the system had higher sulfate removal efficiencies. The experiment was conducted using a synthetic AMD leachate and an AMD leachate in order to evaluate the performance of the designed process. The bioreactor containing a synthetic AMD leachate had an average sulfate removal efficiency of 59% and a dissolved iron removal efficiency of 65%. A black coating of iron (II) sulphide formed along the sides of the LBR and on the grass. The bioreactor containing an AMD leachate had an average removal efficiency of 28% and 31% of sulfates and dissolved irons respectively. The pH remained acidic throughout the course of the experiment. AMD has a complex chemistry, including a high concentration of various heavy metals, and has a high mineral and total acidity. A required pre-treatment step of alkaline dosing may be required. An investigation should be made into using a sequentially fed LBR coupled with an uplflow anaerobic sludge bed (UASB) in order to increase sulfate and dissolved iron removal efficiencies. A model of the one-stage LBR was developed and tested against the experimental results of synthetic AMD leachate and the AMD leachate. This research used the Anaerobic Digestion Model No.1 (ADM1) as the basis for the model and upgraded it to include the process of DSR and iron transformations. The model was able to qualitatively predict the behaviour of the system (10% < mean average percent error <30%) but there were discrepancies in predicting the dissolved iron concentrations. Further work should be conducted in upgrading the model to account for transformations and the weak acid-base chemistry of various heavy metals present in AMD