In-Situ Co-Removal of Turbidity and Hardness from Coal Mine Water Using an Enhanced Underground Reservoir: A Pilot-Scale Study

Conventional sequential treatment processes for coal mine water with high turbidity and hardness are often associated with high energy consumption, large land footprint, and high chemical costs. To overcome these limitations, this study proposes a novel in situ technology based on an enhanced underground reservoir for synergistic coremoval of pollutants. A pilot-scale system integrating an automated dosing unit with a simulated underground reservoir was constructed and operated continuously for over 530 h using mine water from the Ordos mining area. The system demonstrated excellent treatment efficiency and stability, achieving average removal rates of 94.9% for Ca²⁺, 83.1% for Mg²⁺, and 90% for turbidity. Mechanistic analysis revealed that suspended coal powder particles, which exhibit a highly negative zeta potential, facilitated calcite precipitation via heterogeneous nucleation, thereby enhancing hardness removal. A reactive transport model developed using PHREEQC was calibrated against experimental data and successfully simulated the spatiotemporal evolution of water chemistry. The model indicated that precipitation and adsorption occurred mainly within the first 5 m of the reservoir, resulting in a porosity reduction of approximately 6% and a more substantial permeability decrease of over 20%. Economic analysis showed that this technology could reduce chemical costs by about 35% by eliminating the need for coagulants. This study confirms that the enhanced underground reservoir is a technically and economically viable sustainable approach for coal mine water reclamation.