The weathering of coal mine waste rock releases selenium and other co-contaminants, such as sulfate and iron, into the seepage waters. Additionally, various levels of nitrate are released into these waters from residual blasting compounds. Since nitrate is the preferred electron acceptor in nitrate and selenium containing waters, effective management approaches for water treatment should include
... [Show full abstract] in situ denitrification treatments to reduce nitrate loads. In situ denitrification treatment also holds promise for significant selenium reduction and stabilization within the source materials; both of which could significantly reduce active treatment CAPEX and OPEX costs.
To reduce treatment costs, especially where higher nitrate concentrations are present, INOTEC has proposed a combined nitrate/selenium management strategy for water treatment at coal mining sites. In situ nitrate reduction, has been successfully implemented at full-scale at several US gold mining sites. For example, amended Electro-Biochemical Reactor (EBR) water treatment system effluents have been used to inoculate waste rock in place and waste rock as it is mined and placed in depositories. This approach has a potential for both immediate and long-term contaminant reductions to active treatment systems, along with reduced active treatment costs.
A column testing program for coal mining waste rock was designed based on screening results obtained. Three bench-scale columns were constructed with the following goals: 1) EBR effluent, treating mine waters, was used to produce an amended microbial inoculum for the in-situ column nitrate/selenium reduction tests; 2) a control, down-flow column filled with coal waste rock source materials was used to determine baseline selenium and nitrate elution rates; and 3) down-flow columns filled with coal waste rock source materials was inoculated periodically with EBR amended effluents to evaluate in-situ denitrification and selenium reduction/stabilization.
Average nitrate concentration in the control column effluent was 20 mg/L, while it was below the detection limit in the EBR inoculated column effluent. Average selenium concentration in the control column effluent was 25 ug/L, with initial concentrations as high as 33 ug/L and stabilizing with time at around 20 ug/L. The average Se concentration in the EBR effluent inoculated column was 2.7 ug/L.
The holistic approach proposed by the INOTEC team includes integration of source control measures (in-situ nitrate reduction and selenium stabilization) and active treatment processes for a comprehensive water flow management. Active EBR bench-scale and on-site pilot-scale treatment systems were used to treat five British Columbia coal mine wastewaters; influent Se concentrations ranged from 35 μg/L to 531 μg/L. Se treatment targets for the tested waters ranged from 5 to 10 μg/L. Mean EBR effluent Se concentrations ranged from 0.5 μg/L to 1.4 μg/L. Average influent NO3-N concentrations varied between 11 and 170 mg/L and were removed to below 1 mg/L in all the performed tests.
The test data demonstrate that a holistic approach using the in-situ reduction/stabilization and the active treatment using the EBR technology is an effective nitrate and Se removal option for British Columbia’s coal-mining wastewaters.