Electrical stimulation for enhanced denitrification in woodchip bioreactors: Opportunities and challenges

Authors: LAW, JI YEOW - Iowa State University; SOUPIR, MICHELLE - Iowa State University; RAMAN, D - Iowa State University; Moorman, Thomas; ONG, SAY - Iowa State University

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2017
Publication Date: 1/1/2018
Citation: Law, J., Soupir, M.L., Raman, D.R., Moorman, T.B., Ong, S.K. 2018. Electrical stimulation for enhanced denitrification in woodchip bioreactors: Opportunities and challenges. Ecological Engineering. 110:38-47. https://doi.org/10.1016/j.ecoleng.2017.10.002.

Interpretive Summary: The Midwestern United States is dominated by agricultural production on lands with artificial subsurface drainage, which is an important source of nitrate pollution. Woodchip bioreactors are being implemented for the removal of nitrate in tile water drainage. One potential approach to improve woodchip bioreactor performance is to provide an alternative and readily available electron source to the denitrifying microorganisms through electrical stimulation. The objective of this study was to evaluate the denitrification efficiency of electrically augmented woodchip bioreactors (BER) and conduct a simple techno-economic analysis (TEA) to understand the possibilities and limitations for full-scale BER implementation for treatment of agricultural drainage. Up-flow column woodchip bioreactors compared two controls (non-energized, and without electrodes), two electrically enhanced bioreactors, each using a single 316 stainless steel anode coupled with graphite cathodes, and two electrically enhanced bioreactors, each with graphite for both anode and cathodes. Both pairs of electrically enhanced bioreactors demonstrated higher denitrification efficiencies than controls when 500 mA of current was applied. While this technology appeared promising, the techno-economic analysis showed that the normalized N removal cost ($/kg N) for BERs was 2 to 10 times higher than the base cost with no electrical stimulation. With our current reactor design, opportunities to make this technology cost effective require denitrification efficiency of 85% at 100 mA. This work informs the process and design of electrically stimulated woodchip bioreactors with optimized performance to achieve lower capital and maintenance costs, and thus lower N removal cost.

Technical Abstract: Woodchip bioreactors are being implemented for the removal of nitrates in groundwater and tile water drainage. However, low nitrate removals in denitrifying woodchip bioreactors have been observed for short hydraulic retention time (HRT) and low water temperature (< 10ºC). One potential approach to improve woodchip bioreactor performance is to provide an alternative and readily available electron source to the denitrifying microorganisms through electrical stimulation. Previous work has demonstrated the capability of bio-electrochemical reactors (BER) to remove a variety of water contaminants, including nitrate, in the presence of a soluble carbon source. The objective of this study was to evaluate the denitrification efficiency of electrically augmented woodchip bioreactors and conduct a simple techno-economic analysis (TEA) to understand the possibilities and limitations for full-scale BER implementation for treatment of agricultural drainage. The up-flow column woodchip bioreactors that were studied included two controls (non-energized, and without electrodes), two electrically enhanced bioreactors, each using a single 316 stainless steel anode coupled with graphite cathodes, and two electrically enhanced bioreactors, each with graphite for 31 both anode and cathodes. Both pairs of electrically enhanced bioreactors demonstrated higher denitrification efficiencies than controls when 500 mA of current was applied. While this technology appeared promising, the techno-economic analysis showed that the normalized N removal cost ($/kg N) for BERs was 2 to 10 times higher than the base cost with no electrical stimulation. With our current reactor design, opportunities to make this technology cost effective require denitrification efficiency of 85% at 100 mA. This work informs the process and design of electrically stimulated woodchip bioreactors with optimized performance to achieve lower capital and maintenance costs, and thus lower N removal cost.