Pilot Test of Bio-oxidation Treatment of Acidic Mining Drainage in Guizhou Province

Objective Acid mine drainage (AMD) has strong acidity and high heavy metal content, which poses a significant threat to the ecosystem and human survival. The proper treatment of AMD remains one of the major environmental problems requiring urgent resolution. The biological method has gradually become a research hotspot due to its advantages of low cost and environmental friendliness. However, research on the practical application and resource utilization of biological AMD treatment remains in the small-scale exploration stage. This study proposes a new integrated technology to address the problems of excessive heavy metal ions in acidic wastewater, high sludge disposal costs, and limited comprehensive treatment technologies.Methods A new technology involving bacterial oxidation-pH fractional precipitation-cyclic precipitation for AMD treatment is proposed, and semi-industrial application is conducted. Using this method, a pilot plant is set up at a pollution source site in Guizhou Province to investigate the effects of temperature, aeration intensity, pH value, microbial community structure, and influent Fe²⁺ concentration on AMD treatment efficiency. In addition, the effects of different precipitating agents, precipitating pH value, and cycle times on the grade of iron in the recovered slag are examined.Results and Discussions The research results show that iron and aluminum are the main pollutants of acid mine drainage. The treatment efficiency of the bioreactor is positively correlated with temperature and aeration and negatively correlated with initial Fe²⁺ concentration. Temperature is a principal factor affecting the efficiency of bioreactors. Although the maximum flow rate remains 0.33 m³·L^–1 at a low temperature of 10 ℃ and the oxidation time is 13 h, the oxidation efficiency more than doubles when the temperature rises to 25 ℃. If the discharged wastewater meets the standard, a higher flow rate corresponds to a shorter residence time of the wastewater in the reactor, which means a higher treatment efficiency of the bioreactor. At a low temperature of about 12 ℃, the bioreactor maintains a good oxidation effect and a significant flow rate for Fe²⁺: 100～1 000 mg ·L^–1 AMD, with a Fe²⁺ removal rate as high as 410 g·h^–1. The pH value significantly influences the removal of iron and aluminum in AMD. Fe³⁺ is removed when pH = 3.5, Al³⁺ is released in the range of pH = 4～5, and all indicators meet the emission standard when the solution pH = 5. The comparison test of different termination pH values finds that alkali consumption does not correspond to changes in pH values because the metal ions in the wastewater after oxidation are mainly Fe³⁺ with a small amount of Al³⁺. When the pH value of wastewater is less than 4, OH^–in the alkaline reagent combines particularly with Fe³⁺, while the pH value is primarily connected with Al³⁺ in the wastewater. The remaining OH^–only acts to increase the pH value, so the alkaline reagent is mainly consumed at low pH. The treatment efficiency of the bioreactor increases with the aeration rate. When the aeration rate increases from 16 to 32 Nm³·h^–1, the rise in bioreactor oxidation efficiency is significantly less than when the aeration rate increases from 8 to 16 Nm³·h^–1. Although high aeration intensity can improve the dissolved oxygen level, it also increases the impact force on the filter material, which can influence bacterial attachment to the filter material. This makes it complex for bacteria to attach to the filter material and can cause additional strain loss. Therefore, the aeration intensity cannot be increased blindly. Combining operation cost and treatment efficiency, the optimal reaction condition is 16 Nm³·h^–1 aeration and pH = 5. Lime milk is the best alkaline reagent to regulate pH value, and the removal rates of Fe²⁺ and Al³⁺ in the pilot system are above 99%. In addition, microbial classification and sequencing results show that the relative abundance of Proteobacteria increases under heavy metal pollution and has adaptability to acidic environments. During the experiment, Proteobacteria maintain absolute superiority at the phylum level at all periods. The <italic>Acidophilus</italic> and <italic>Acidophilus</italic> in the original bacterial fluid are replaced by <italic>Unclassified_Callionella</italic>, <italic>Ferredoxin</italic>, and <italic>Acidophilus</italic> at the genus level. The iron-oxidizing bacteria replace Callionella as the final dominant bacterial group in the bioreactor, indicating that the iron-oxidizing bacteria have a competitive advantage and better adaptability to high iron ion concentrations. In the pilot process, the biological method effectively treats the acid mine wastewater and successfully separates the iron in the waste residue, which is conducive to subsequent resource utilization. With the increase in cyclic precipitations, the iron and aluminum content in the waste residue also gradually increases and decreases after reaching maximum values of 0.67% and 20.26% on the 8th cycle. The minimum particle size of the precipitate reaches 258 nm before increasing again, showing a trend of first decreasing and then rising. Excessive circulation can produce a “dissolution” phenomenon, causing some of the target substance to dissolve back into the supernatant or mix with impurities in the precipitation, thus reducing the purity of the target substance. Therefore, the number of cyclic precipitations should be controlled based on the actual situation, and the optimal purity of the filter cake can be achieved with about eight cycles of cyclic precipitation in this test.Conclusions Filter cake with a Fe content of 41.15% can be enriched through process optimization involving the combination of stage and cycle precipitation. It is expected to be used as raw material for applications such as fertilizer, cement, and flocculant, achieving economic benefits in construction, environmental protection, and other fields. This study confirms the feasibility of efficiently treating AMD with oxidizing bacteria as the core and discusses various pilot test parameters. The findings provide a more reliable basis and reference for the future large-scale industrial application of AMD biological treatment.