Simultaneously colliers and coal-fired wastewater treatment as well as energy production through reverse electrodialysis
Authors:   , Ngobese Thobeka 
Published: September 2024

One primary sector contributing to the country’s economic development is coal mining. As the country’s primary energy source, coal dominates the country’s energy mix. Conversely, the devastating environmental impact of this industry cannot be ignored. An ever-increasing population and economic growth exacerbate this problem further. This energy resource, “coal”, is mined using large quantities of water, resulting in salted wastewater and further contaminating groundwater. South Africa (SA) is experiencing water shortages because of climate change, which coal directly contributed to, so it has no choice but to implement mitigation plans instead of preventing it to ensure its sustainability. Therefore, it is in this context that led to the motive for this current research. This study uses reverse electrodialysis (RED) technology to mitigate and address this environmental challenge sustainably. A vital advantage of this technology is its ability to produce power while purifying wastewater. This advantage makes it a valuable energy mix that can substantially reduce and alleviate coal-fired emissions. The study aimed to investigate the desalination and power generation process for treating synthesised coal mining and colliers using a RED stack. The study’s hypothesis was tested by using small laboratory-scale RED stacks. This study used synthetic wastewater to mimic SA’s colliery mine and coal power plant wastewater. A performance assessment of the RED stacks was conducted under varying system temperatures, solution concentrations, and flow rates. 20 to 40 °C system temperatures, 1 to 2 mol/L solution concentrations, and 896 to 1550 mL/min flow rates were used. Within the specified experimental ranges, an empirical tool, response surface methodology (RSM), was used to minimise the number of experiments while obtaining sufficient experimental data. The selected process parameters were converted into dimensionless codified data in three levels. A general full factorial design type recommended 18 experimental runs. The influence of each selected parameter was examined individually in the first part of this study. A range of 2.31 to 10.75 W/m2 and 3.94 to 16.13 wt.% power density and salt removal were obtained at the selected experimental ranges. These results corresponded to a membrane flux range used in this study. The membrane flux data was used to assess scale-up feasibility and cost estimation for the RED technology Statistical analysis was performed using the historical data design (HDD) option provided by RSM software to examine the combined influence of the investigated parameters on power density and salt removal. The results recommended that the 2FI model, as the highest order with significant terms, can describe the desalination and power density. A good agreement was found between experimental data and data generated by empirical models, with a less than 3% deviation. A regression (R2) analysis was performed to determine the accuracy and reliability of the developed empirical models. An accuracy level of 95% was obtained in predicting experimental data within the experimental range for these models. Against this brief, the two factor interaction (2FI) acquired by the model elucidates that this model is not recommended since it cannot make accurate predictions, as 0.6949 as well as 0.8704 for salt removal percentage and power density, respectively, indicate a relatively low value for regression. The combined effects and significance of input parameters were assessed with a three-dimensional surface (3-D) and a contour plot. The assessment revealed that power density and salt removal were less affected by the increase in flow rate than by solution concentration and system temperature. The feasibility of the technology was further explored by optimising input variables since the membrane flux data alone cannot provide detailed information on the technology’s potential. Increasing RED parameters, such as pumping at higher flow rates, frequently requires more energy; therefore, pumping costs may increase if the operating parameters necessitate higher solution flow rates, affecting the RED system’s overall running costs. The temperature, concentration, and flow rate were optimal through RSM software at 40℃, 1.93 mol/L and 896 mL/min. This high temperature will accelerate scaling effects, leading to technical failure if the technology is used to treat or produce electricity on a large scale. Consequently, this technology would be more expensive. However, this does not exclude the possibility of replacing current conventional technologies with this technology. After RED technology has been conceptually designed and cost analysis, including sensitivity analysis, has been conducted, a realistic conclusion can be drawn. The effect of divalent ions was investigated in further detail using these optimal conditions. The synthetic coal mine wastewater was prepared by adding Ca2+ and SO4 2- , maintaining NaCl concentration and increasing divalent ions. Deionised water simulating lower concentrations likely affected the overall performance of the RED stack in this study. In this case, further investigation into the effect of lower concentrations should be recommended to examine improving the performance of RED stacks at even lower concentrations. It will also be possible to assess if they are effective on wastewater containing low salt levels. This study does not recommend the HDD method for optimising the experimental results. However, further investigation should be conducted to accurately develop models that would produce a reliable model with acceptable prediction ability. In this manner, similar experimental investigations can be conducted more quickly.