Laboratory scale testing and modelling of sub-surface wetlands to reduce sulphate
Water quality impacts related to coal mining are mostly caused by exposing sulphide bearing minerals, such as pyrite, to oxygen and water. The oxidation of sulphide bearing minerals produces acid, metals, trace metals, and sulphate that could impact detrimentally on receiving water courses. Some open cast pits tend to decant if the groundwater level intercepts the surface topography. This study focuses on evaluating the feasibility of biological passive treatment systems, specifically anaerobic bio-substrates, to effectively reduce sulphate from 700 mg/L (decant water) to within acceptable regulatory standards (≤ 250 mg/L), as required by the applicable catchment management agency, the Inkomati Usuthu Catchment Management Agency. The feasibility of using biological passive treatment systems were investigated by means of conducting the following tests on preselected compost and sand substrate mixtures: (1) chemical signature of selected compost, (2) change in dissolved organic carbon concentration over time within 100% compost and 100% sand substrates, (3) constant head permeability testing to measure volumetric flow rate (Q) and hydraulic gradient (i), and calculate hydraulic conductivity (K) as a function of cross-sectional area (A) of flow, (4) calculate dry density (Pb) for each substrate mixture, (5) calculate porosity (n) for each substrate mixture, and (6) conduct tracer testing. These tests were conducted on the following substrate mixtures: 100% compost, 70% compost 30% sand, 50% compost 50% sand, 30% compost 70% sand, and 100% sand. A kinetic model was developed to evaluate sulphate reduction rates as a function of organic carbon (acetate) availability and hydraulic residence time (HRT). The results from the experiments conducted, as well as the kinetic model were used as input to an upscale model, specifically designed for a decommissioned coal mine site that decants. The decant water is characterised by elevated sulphate and metal concentrations. The outcomes from the research conducted indicate that sulphate can effectively be reduced from 700 mg/L to 249,59 mg/L with a minimum HRT of 3,5 days by means of an anaerobic bio-substrate that utilises sulphate reducing bacteria. The lack of readily available acetate can be considered as a limiting factor for optimal sulphate reduction. Anaerobic bio-substrates have a limited lifespan due to the depletion of organic carbon over time. Additional organic carbon will have to be added to ensure sustainable reduction of sulphate. The use of biological treatment systems will require frequent maintenance and monitoring to ensure optimal functionality. Further research is, however, required to validate the findings of the model.