Amending woodchip bioreactors with corncobs reduces nitrogen removal cost

Authors: LAW, JI YEOW - Iowa State University; SLADE, ALEXIS - Iowa State University; HOOVER, NATASHA - Iowa State University; Feyereisen, Gary; SOUPIR, MICHELLE - Iowa State University

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2022
Publication Date: 12/28/2022
Citation: Law, J.Y., Slade, A., Hoover, N., Feyereisen, G., Soupir, M. 2022. Amending woodchip bioreactors with corncobs reduces nitrogen removal cost. Journal of Environmental Management. 330(15). Article 117135. https://doi.org/10.1016/j.jenvman.2022.117135.
DOI: https://doi.org/10.1016/j.jenvman.2022.117135

Interpretive Summary: Woodchip denitrifying bioreactors are an edge-of-field conservation practice that removes nitrate from tile-drained agriculture by microbial processes that are temperature sensitive. Under cold springtime temperatures in higher latitude growing regions, the nitrate removal rate is low with woodchip media. The current study tested corn cobs in field pilot bioreactors under 3 different flow rates, or hydraulic residence times: 2, 8, and 16 hours. Bioreactors with 75% corn cobs from the inlet end followed by woodchips the remaining 25% of the bed length (CC75) removed nitrate at 1.6 to 10 times the rate of woodchips only (WC100). Removal rates for beds with 25% corn cobs and 75% woodchips (CC25) were from 0.9 times lower to 2.8 times greater than woodchips only. The field results were used to do an economic analysis for each treatment. The most promising treatment was CC75, range of removal cost $2.84 to $3.66/lb N, with WC100 ranging from $3.55 to $27.13/lb, and CC25 $5.95 to $17.99/lb. The abundance of corn cobs in the US Midwest could address the scarce regional woodchip supply needed for broad-scale bioreactor implementation and reduce the transportation distance of bioreactor media. This finding provides insight into how bioreactor performance N removal economics may be improved using corn cobs and is pertinent to researchers, practitioners, academics, and agency personnel with interest in using denitrifying bioreactors to remove nitrate-N from water.

Technical Abstract: Woodchip denitrification bioreactors are an effective agricultural practice to reduce nitrogen (N) export from subsurface drainage, but there are challenges associated with limited woodchip supplies and increasing prices. Previous lab studies indicate that corncobs could be a promising woodchip alternative from the perspectives of N removal rate and cost. This field study aimed to provide early performance and cost assessment of denitrification bioreactors with two woodchip-corncob mixtures. The objectives were to i) compare N removal rate of bioreactors with different carbon and hydraulic retention time (HRT) treatments, ii) compare bioreactor N removal cost, and iii) conduct sensitivity analysis on full-scale bioreactors (FBR) N removal cost with varying corncob lifespan and price. Nine replicated field pilot-scale bioreactors (PBRs) using three carbon treatments and three HRTs were assessed for N removal efficiency. The carbon treatments were woodchip-only (WC100), 25% (by vol.) corncobs + 75% woodchips mixture (CC25) in series, and 75% corncobs + 25% woodchips (CC75) in series set at HRTs of 2, 8, and 16 hours. N concentrations were monitored at each PBR inlet and outlet, and the PBR N removal efficiencies were used to estimate FBR N removal rates and costs. At respective HRTs, the estimated N removal rates of CC75 were 1.6- to 10.1-fold higher than WC100, but CC25 exhibited 0.9-fold lower (at 8-hr HRT) to 2.8-fold higher than WC100. CC75 ($6.26 to $8.06 kg-1 N) was the most cost-efficient treatment, followed by WC100 ($7.83 to $59.83 kg-1 N) and CC25 ($13.11 to $39.66 kg-1 N). This assessment showed CC75 as a promising alternative to WC100 in terms of N removal rate and cost, but CC25 did not provide sufficient N removal rate increase for it to be a cost-efficient option. Nonetheless, bioreactors with corncob media are a relatively new approach, and we encourage more field studies to explore the long-term opportunities and challenges.