Concerns with Passive Anaerobic Bioreactors for Selenium Removal from Coal Slurry Liquid Wastes

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Journal of the International Mine Water Association


Passive anaerobic bioreactors (PABs) are flow-through trenches filled with plant substrates that serve as energy and carbon sources for bacteria that carry out water treatment processes. In this study, the effectiveness of PABs to bioreduce selenate to insoluble Se (0) and improve the water quality of coal slurry impoundment liquid (CSL) was evaluated for seven months in bench-scale PABs containing four types of substrates with elemental carbon: nitrogen ratios (C:N) ranging from 78 to 837. Selenate was rapidly reduced from 0.6 to 0.05 mg/L in 15 days in PABs containing intermediate C:N substrates (Miscanthus and switchgrass (Panicum virgatum)) but not in PABs containing low or high C:N substrates (Timothy-hay (Phleum pratense) and hardwood chips). Nitrate was rapidly reduced from 10 to  <  1 mg N/L in 15 days in all PABs, but nitrate was only partially denitrified to nitrous oxide in PABs containing low and high C:N substrates due to strongly acidic conditions that inhibited nitrous oxide reductase. In all of the PABs, leaching and anaerobic decomposition of substrates released high concentrations of dissolved organic carbon, phosphorus, and several metals, greatly exceeding guideline concentrations. It is strongly recommended that PAB effluent quality be considered before implementing this technology for the treatment of mine water or any other type of wastewater intended for release to natural water bodies.

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