Electrochemical Stimulation of Denitrification In Woodchip Bioreactors and Wetlands

Abstract

Elevated concentrations of nitrate in agricultural runoff can contribute to nutrient enrichment in coastal environments and when ingested can result in methemoglobinemia, a potentially fatal condition in infants. Woodchip bioreactors are ditches constructed at the edge of fields to create anaerobic environments replete with woodchips as carbon and electron sources to promote the growth of denitrifying bacteria to reduce nitrate to inert nitrogen gas. In the Midwest snow melt coincides with fertilizer application, creating a large volume of water with a high nitrate load that limits the efficiency of woodchip bioreactors. The lower temperatures also restrict microbial nitrate-reducing metabolism. One avenue for improving denitrification is the implementation of current-carrying electrodes to supply electron donors to the bacteria. Pyrogenic carbon (Biochar) has been demonstrated to sorb the water-soluble nitrate and acts as an electron shuttle. It is unknown how these two variables in tandem affect denitrification rates, and the microbes associated with these substrates remain uncharacterized. Here, we use batch bioreactors to test the nitrate removal capabilities of woodchip reactors amended with biochar and electrochemical stimulation. Aliquots were collected to measure nitrate removal using a continuous flow analyzer and bacterial communities were characterized based on 16S rRNA gene analysis. Electrode-containing reactors were significantly (p-value < 0.05) less efficient at treating nitrate during the first 8 hours following nutrient injection, but by 24 hours all reactors performed comparably with respect to nitrate removal. Electrode biofilms also had less α-diversity and in terms of β-diversity. Microbiome samples under the influence of electrodes were different from those that were not exposed to any electrochemical stimulation. The difference is bacterial community and the performance lag during the first 8 hours of operation may be a result of increased oxygen concentration in the reactors as a result of O2 evolution during electrolysis in the electrode-containing bioreactors. Biochar had no discernable effect on nitrate removal and did not have a significantly different microbiome from woodchips and water. Electrode biofilm samples were found to enrich Cyanobacteria while the biofilm on the biochar enriched Acidobacteria. Using the results of our experiment we propose the construction of a benchtop-scale electrochemically stimulated constructed wetland reactor.