Treatment of heavy metals (Fe, Mn) in coal mine wastewater by biological conversion method combined with artificial tree planting filter yard

Coal mining is one of the key sectors of the mining industry in Vietnam. However, besides the socio-economic benefits, this activity generates waste and wastewater that have a direct negative impact on human health, landscape and ecosystem change. Coal mine wastewater is acidic and contains high levels of heavy metals (Fe, Mn). These metals are directly related to genetic changes, causing liver cancer as well as seriously affecting the environment. For developing countries like Vietnam, the mining industry in small and medium scale faces many difficulties in treating mine wastewater due to the large amount of wastewater generated, high treatment costs and low investment potential.

Starting from the above fact, Assoc. Prof. Dr. Bui Thi Kim Anh and the research team of the Institute of Environmental Technology conducted the project “Research on treatment of heavy metals (Fe, Mn) in coal mine wastewater by biological conversion method combined with constructed wetland” code UDPTCN04/18-20. The task has been accepted and rated as Excellent.

In order to solve the problem of coal mine pollution, while optimizing costs and not creating secondary pollution, treatment directed by ecological technologies (artificial tree plantations) and biological conversion treatment methods showed a good agreement in terms of both treatment efficiency and cost. This is a potential technology that needs to be developed, researched and applied to treat mine wastewater in Vietnam.

In this study, the team utilizes cheap materials such as rice husks (agricultural by-products) and limestone for initial treatment; heavy metals (Fe, Mn) are significantly reduced after this process. Some substances remaining after the process such as COD, Fe, Mn and residual organic substances, pathogenic microorganisms, etc. will be treated through the artificial tree planting filter.

Constructed wetland (CW) is an eco-friendly and environmentally friendly technology commonly used to treat wastewater. The composition of the filter yard includes: filter material (sand, rock, gravel, ...), aquatic plants, microorganisms in the air and microorganisms in the rhizosphere. The process of pollutant transformation and treatment in the planting filter field is caused by chemical, physical and biological processes that occur among many components that make up the filter field. The interweaving of these components forms different redox conditions (anaerobic, aerobic, anaerobic) producing abundant decomposition and gradual removal of pollutants.

An artificial constructed wetland for the treatment of acidic mine wastewater

Based on the research results in the laboratory, with the desire to evaluate the ability and effectiveness of each parameter in the process of treating coal mine wastewater according to the stages of the system, the research team designed a system which is based on the characteristics of each module, using limestone and hydrolyzed rice husks as filter materials, adsorption and biotransformation. Next is the filter system to plant artificial trees. The proposed research model includes 2 main tanks: tank 1 includes sedimentation compartment and storage compartment for limestone and rice husks; tank 2 is the CW system. The system design calculations of 250l/day are fine-tuned to suit the input wastewater which is coal mine wastewater.

Diagram of the combined system of limestone and rice husk for coal mine wastewater treatment
(1-inlet water tank; 2-limestone treatment tank; 3-rice husk limestone tank; 4-pump; 5-artificial tree filter yard)

At the input of the treatment system, the research team arranged an additional settling compartment, in order to evenly distribute the wastewater horizontally across the tank wall, in addition to have the effect of settling, reducing TSS content, avoiding clogging of the filter system. The constructed wetland is designed to include filter materials of rock, gravel, sand and reeds planted with a density of 15cm × 15cm. Before the experiment, the reeds were planted in natural water for 01 month for the plants to take root and adapt to the growing environment.

The model was tested for 3 months. The processing efficiency of the model reaches from 75% to 99%. The test model operates stably, and the wastewater quality always reaches QCVN 40:2011/BTNMT column B for COD and QCVN 40:2011/BTNMT column A for Fe, Mn. This demonstrates the potential for application of this model in mine wastewater treatment on a larger scale.

In fact, the technology of constructed wetland is suitable for application conditions in Vietnam, with the advantages of cost and efficiency of treatment. The research team has successfully applied the model of using CW technology to treat many different types of wastewater in Vietnam. The key point of the models lies in the design of the filter bed structure, the flow mechanism, in order to save energy and minimize operating costs. In addition, it is completely possible to improve the treatment system as a green space, creating a landscape for the area. The improvement of the environmental landscape, the improvement of the water environment contaminated with heavy metals by this method will make an important contribution to the sustainable socio-economic development and protection of community health. This wastewater treatment system can be applied to treat Fe and Mn in open pit and underground coal mines. After the technology is perfected, the process can be applied on a large scale. In fact, based on the research results of the task, the research team conducted a design consultation for a coal mine wastewater treatment system with a capacity of 7200 m3/day of Tay Lo Tri mine.

The task has been granted the utility solution Patent No. 2541: The process of mine wastewater treatment. In addition, the mission has published 02 scientific articles at Vietnam International Water Week 2019 and the Journal of Mining and Earth Sciences; support to train 01 doctorate and build 01 technological process for Fe and Mn treatment in mine wastewater by limestone and rice husk combined with pilot-scale constructed wetland for technology transfer.

Translated by Phuong Ha
Link to Vietnamese version


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