Amending woodchip bioreactors with corncobs reduces nitrogen removal cost

Woodchip denitrification bioreactors are an effective agricultural practice to reduce nitrogen (N) export from subsurface drainage via the conversion of nitrate (NO₃^-) to nitrogen gas (N₂), 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 treatments. The objectives were to i) compare N removal rates of bioreactors with different carbon and hydraulic retention time (HRT) treatments, ii) compare bioreactor N removal costs, and iii) conduct sensitivity analysis on full-scale bioreactors (FBR) N removal costs with varying corncob lifespans and prices. 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 media (CC25) in series, and 75% corncobs + 25% woodchips (CC75) in series set at HRTs of 2, 8, and 16 h. 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. A 15-yr cost assessment indicated CC75 ($10.56 to $13.89 kg^-1 N) was the most cost-efficient treatment, followed by WC100 ($13.30 to $88.11 kg^-1 N) and CC25 ($22.41 to $60.13 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, using corncobs as a bioreactor medium is a relatively new approach, and we encourage more field studies to explore the long-term opportunities and challenges.