The biological Sulphate removal technology

Mining is implicated as a significant contributor to water pollution, the prime reason being, that pyrites oxidize to sulphuric acid when exposed to air and water. Mine effluents, often containing sulphate, acidity and metals, should be treated to render it suitable for re-use in the mining industry, for irrigation of crops or for discharge in water bodies. This study describes the removal of all three mentioned pollutants in mine effluents, from different origins, containing different concentrations of various metals. The objectives were achieved, applying the biological sulphate removal technology, using ethanol as the carbon and energy source. It was shown that diluting the mine effluent with the effluent from the biological treatment, the pH increased due to the alkalinity in the treated water while the metals precipitated with the produced sulphide. When this treatment regime was changed and the mine water was fed undiluted, it was found that the metals stimulated the methanogenic bacteria (MB) as trace elements. This resulted in a high COD utilization of the MB, such that too little COD was available for the SRB. Metal removal in all three studies was observed and in most instances the metals were eliminated to the required disposal concentration. INTRODUCTION The scarcity of water, due to low rainfall, in South Africa is exacerbated by pollution of the surfaceand groundwater resources due to industrial activities such as mining of minerals. By its very nature and scale, mining makes a marked and visual impact on the environment. Mining is implicated as a significant contributor to water pollution, the prime reason being, that most of the geological formations which are mined contain pyrites which oxidize to form sulphuric acid when exposed to air and water. The combination of auto-oxidation and microbial sulphur and iron oxidation produces large volumes of sulphuric acid, which are highly corrosive and when discharged in the river systems can cause major environmental problems, one of them being the high toxicity level for the aquatic biota. This mining related polluted, often acidic and sulphate rich water is referred to as Acid Mine Drainage (AMD). While a mine is operational, the act of mining (e.g. sinking of shafts or open pits and the excavation of overburden and ore) can have a significant impact on the natural water environment, as mining activities inevitably disrupt pre-existing hydrological pathways (Younger and Wolkersdorfer, 2004). Waste products from both mining and mineral processing operations are often contained in large heaps or in tailings dams. Seepage of contaminated leachate from waste rock piles and tailing dams is a significant cause of surface and ground water pollution in many mining areas. This kind of water pollution often occurs when the mine is in operation and without remediation can persist long after mine closure (Younger and Wolkersdorfer, 2004). Acid mine water should be treated in order to render it suitable for re-use in the mining industry, for irrigation of crops or for discharge in the rivers and dams. Because of the variety of mine waters encountered in nature and because of the familiarity of the mining sector with the physical and chemical processes, necessary to separate metals and water, there is a wide range of conventional treatment methods for mine waters (Younger et al., 2002). Mine waters can be treated chemically applying lime and limestone neutralization technologies (Maree et al, 2003), however the residual sulphate in the form of gypsum (CaSO4) is dependent on the solubility of gypsum, which is measured at about 1500 mg/l as sulphate (SO4). For removal of sulphate below this concentration, the biological sulphate reduction technology can be applied (Greben et al, 2000a, 2000b, Maree et al, 2004a). In order to achieve biological sulphate reduction, anaerobic conditions, favoured by the sulphate reducing bacteria (SRB), and the presence of suitable carbon and energy sources, have to be adhered to. Successful sulphate reduction is typically associated with a pH increase due to the production of alkalinity. Therefore, the biological sulphate reduction technology is particularly beneficial to mining industries experiencing acid mine drainage problems, as it results in removal of sulphate, in a pH increase of the treated water and often in metal removal. The latter occurs as a result of the production of sulphides, followed by metal precipitation to form metal-sulphides. The SRB utilize organic products as the carbon and energy source, providing electrons, while sulphate is used as the terminal electron acceptor with hydrogen sulphide (H2S), CO2, H2O or HCO3 and acetate as the end products (Greben et al., 2002). In this study mine effluents, containing sulphate, acidity and metals, from three different mines are treated biologically, using ethanol as the carbon and energy source. The objective of this study is to show that metals containing acid mine water should be diluted with the effluent of the biological sulphate removal process, so that the metals are precipitated as metal sulphide and that the acidity of the mine water is neutralised with the alkalinity of the biological treated water prior to feeding the mine effluents to the biological sulphate removal reactor. MATERIAL AND METHODS In this paper investigations are described using three different mine waters as feed water in each study. The mine water originated from 1) an active coal mine, 2) a copper and nickel mine and 3) from a passive coal mine, all located in Southern Africa. The treatment of the different mine effluents are presented in three separate studies. 9 INTERNATIONAL MINE WATER CONGRESS 340 Feed water The chemical composition of the three mine effluents is presented in Table 1. The feed water was supplemented with 1.5 ml ethanol/l feed water as the carbon and energy source. This resulted in a COD/SO4 ratio of ≈ 1 (Rinzema & Lettinga, 1988; Greben et al., 2004). Reactor configuration: Single stage completely-mixed reactor The completely-mixed reactor system used in all three studies comprised of a reactor (volume15 l) and a clarifier (volume15 l, Fig. 1). Both reactor and clarifier were open to the atmosphere so that air could partly diffuse into the reactor system. A sludge recycle was in place from the bottom of the clarifier to the top of the reactor. The return sludge flow rate was maintained at 4x the feed flow rate. The operational temperature was maintained at 2022 °C. Biomass The reactors received to sulphate adapted biomass, obtained from the CSIRosure reactor (Witbank, South Africa), of which the VSS was 11.9 g. Analytical Manual determinations of sulphate, sulphide, alkalinity, COD, pH and metals were carried out according to analytical procedures as described in Standard Methods (APHA, 1985). With the exception of the MLSS, VSS, sulphide and feed COD, all analyses were carried out on filtered samples (Whatman #1). The COD samples were pre-treated to eliminate the sulphide contribution to the COD concentration with a few drops of H2SO4 and N2 gas. Figure 1: Completely-mixed reactor Origin Mine Water Parameter (Except for pH in mg/l) Active Coal Mine Copper and Nickel Mine Closed coal mine pH (units) 3.2-3.5 4.3-6.5 3.5 Acidity 600 Alkalinity 100-130 Sulphate 2500-315