Establishing an Total Dissolved Solids: Electrical Conductivity Ratio for Mine Waters

摘要

Total dissolved solids (TDS) expresses the sum of all dissolved ions in water. In a given water, the TDS content in mg/L and electrical conductivity k25 at 25°C in uS/cm are related to each other by the equation TDS = K25 ×f, where/is a constant, commonly referred to be in the range of 0.55 - 0.9. Mine water with elevated TDS contents rapidly causes scaling or corrosion, as high TDS values are often associated with lower pH-values. TDS is also used to calculate the potential amount of sludge in mine water treatment, which makes TDS a crucial parameter.For accurate TDS determinations, filtered water samples are oven dried and the mass of the residue is the TDS-content. As this procedure is time consuming, the TDS is commonly estimated from EC measurements using the above equation and laboratory and field conductometers are manufactured with a programmable conversion factor. Normally TDS is reported without further considering the conversion factor or its way of measurement.In this paper, we describe TDS/EC conversion factors for 45 South African, mining impacted waters. It can be shown that the conversion factor ranges from 0.25 to 1.34 with a mean of 0.86 and a standard deviation of 0.19. No correlation with other on-site parameters was found, though there is a tendency to higher conversion factors at lower pH-values and higher electrical conductivities.Based on our work we conclude that prior to using the TDS/EC conversion equation, the conversion factor for the single mine water, not only site specific but also time depended, must be determined in the lab and the conductometer be programmed with this conversion factor. In addition, the conversion factor 0.65 recommended in the South African Water Quality Guidelines should no longer be used for mining impacted water.